ABSTRACT - EMAW 2007
A QUARTZ-BEARING ORTHOPYROXENE-RICH WEBSTERITE XENOLITH FROM THE PANNONIAN BASIN, WESTERN HUNGARY: EVIDENCE FOR RELEASE OF SI-OVERSATURATED MELTS FROM THE SUBDUCTED SLAB
BALI ENIKO 1, ZAJACZ ZOLTAN 2, KOVACS ISTVAN 3, SZABO CSABA 4, HALTER WERNER 2, VASELLI ORLANDO 5, TOROK KALMAN 6, BODNAR ROBERT 7
presenter's e-mail: Eniko.Bali@Uni-Bayreuth.DE
1 - BAYERISCHES GEOINSTITUT, BAYREUTH
2 - INSTITUTE OF ISOTOPE CHEMISTRY AND MINERAL RESOURCES, ETH, ZURICH
3 - RESEARCH SCHOOL OF EARTH SCIENCES, ANU, CANBERRA
4 - LITHOSPHERE FLUID RESEARCH GROUP, INSTITUTE OF GEOLOGY AND GEOGRAPHY, EOTVOS UNIVERSITY, BUDAPEST
5 - DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF FLORENCE
6 - EOTVOS LORAND GEOPHYSICAL INSTITUTE, BUDAPEST
7 - DEPARTMENT OF EARTH SCIENCES, VIRGINIA TECH, BLACKSBURG
Keywords: quartz-bearing websterite, silicate melt inclusion, subduction, Carpathian Pannonian Region
A quartz (qz)-bearing orthopyroxene(opx)-rich websterite xenolith was found in Pliocene alkali basaltic tuff of Szigliget, Bakony-Balaton Highland Volcanic Field (western Hungary). The xenolith has cumulate texture and is composed of opx and subordinate interstitial clinopyroxene (cpx). Both cpx and opx contain primary silicate melt inclusions (smi), although only opx contains large (up to 1 mm) rounded or needle shaped qz inclusions. Smi in both pyroxenes is composed of silicate glass and CO2-fluid phase. Smi is large (up to 100 mm), rounded or negative crystal shaped occurring as single inclusions or forming clusters in cores of both host minerals. The glass-fluid ratio is 10:1 – 20:1.
Smi in opx was reheated. At 1 atmosphere the complete dissolution of the CO2 did not happen before decrepitation, thus only the beginning (average: 863°C) and the termination (average: 958°C) of glass melting was observed. The equilibrium temperature of the xenolith is 954ºC, in agreement with the entire melting of glass. The crystallization pressure was calculated from the density of the CO2 inclusions, giving the minimum pressure of 1.1 GPa corresponding to the present day uppermost mantle.
Silicate glass in smi is SiO2- and alkali-rich MgO-, FeO- and CaO-poor. Reheated smi displays lower SiO2 and higher MgO contents as a consequence of remelting of the opx crystallized on the smi walls. The studied smi is richer in SiO2, poorer in Al2O3 and strongly depleted in CaO, MgO and FeO relative either to the high P/T partial melts of peridotite or to those olivine hosted smi that represent partial melts of metasomatized mantle wedge. In contrast, slab derived silicate melts show a similar composition to the studied smi suggesting common origin.
Cpx is strongly enriched in LREE and MREE compared to HREE, with significant negative anomalies of Pb, Sr, Hf, Zr and Ti on PM-normalized trace element diagrams. These cpxs are enriched in LREE and MREE, but depleted in Sr compared to the cpx in local peridotite and pyroxenite and granulite xenoliths. Opx is depleted in LREE and MREE compared to HREE, however shows the same degree of enrichment in incompatible elements compared to opx from BBHVF peridotites that is shown by the coexisting cpx. Both cpx and opx have high concentrations of Cr (4200 and 1770 ppm, respectively) and Ni (855 and 1550 ppm, respectively) the latter being significantly higher than that in the pyroxenes of local peridotite xenoliths.
Trace element composition of smi covers a narrow range independently of the host mineral. They are strongly enriched in incompatible elements displaying negative anomalies of Nb, Ta, and Sr and slight positive anomaly of Pb. The trace element composition of the smi also resembles silicate melts released from subducted slab, but with low Sr-contents compared to those typical for slab melts.
The incompatible element characteristics of both the smi and their host minerals (cpx and opx) and the presence of qz-inclusions in opx suggest a slab-derived origin, whereas the high Ni- and Cr-contents suggest the involvement of peridotite component in the genesis of the studied rock. Model calculations suggest, that:
1) Similar qz-bearing orthopyroxene-rich rocks might form from a hybrid melt developed on the interface between peridotite and a Si-oversaturated slab-melt in case of a high silicate melt/wall-rock ratio ( 5:1).
2) The reaction and fractional crystallization from the hybrid melt did not change significantly the composition of the original slab-melt, giving a possibility to estimate the source and P/T conditions of slab melt formation.
Thus we can conclude that the Si-oversaturated melt was released from the subducted slab at 1.5-2.0 GPa and 1000 °C, leaving behind a restite of cpx+amp+gt+ru+pl/ap(?). The trace element composition of MI in both opx and cpx suggest the involvement of a metasedimentary component during slab melting.
PETROGENESIS OF THE ETHIOPIAN PLATEAU BASALTS AND THEIR BEARING ON MANTLE PLUME COMPONENTS
BECCALUVA LUIGI 1, BIANCHINI GIANLUCA 1, NATALI CLAUDIO 1, SIENA FRANCA 1
presenter's e-mail: bcc@unife.it
1 - Dipartimento di Scienze della Terra – Università di Ferrara
Keywords: Ethiopia, flood basalts, mantle sources, mantle components, metasomatism
The Ethiopian-Yemen continental flood basalts represent an Oligocene Large Igneous Province where some typical features of deep mantle plume, including high 3He/4He ratios, have been documented (Afar plume; Courtillot et al., 2003). In the Ethiopian plateau large volumes (ca. 250000 Km3) of tholeiitic magmas, erupted in a short time span (31-28 Ma), appear to be zonally arranged with low-Ti basalts in the NW part of the province, and high-Ti basalts (and picrites) in the eastern sector neighbouring the Afar-Red Sea region (Pik et al. 1998).
An integrated petrogenetic model based on major element mass balance calculations, phase equilibria, and thermo-barometric evaluations indicates that primary basaltic magmas were generated by ca. 15-20% melting of mantle lherzolite at ca. 1250-1300 °C / 13-16 Kb and ca. 1300-1350 C° / 14-19 Kb for low-Ti and high-Ti tholeiites, respectively; high-Ti picrites by ca. 30% melting at ca. 1400-1450 °C / 20-30 Kb.
The calculated mantle sources invariably require hydrated lherzolite composition with up to 5% and 10% of amphibole for low-Ti and high-Ti magmas respectively, and a parallel increase of 2 – 17 times incompatible element abundances with respect to those observed in the Ethiopian mantle xenoliths. Further Ti-rich metasomatic phases (e.g. rutile, ilmenite, armalcolite) are required in the mantle sources, particularly for the generation of extremely high-Ti magmas (TiO2 up to 5-6%). Therefore, low-Ti basalts may have been generated in the outer zone of the Afar-plume by partial melting of moderately metasomatized lithospheric mantle sources, in connection with the activation of the hotspot and related crustal bulging. High-Ti basalts/picrites could in turn be generated in the inner part of the Afar buoyancy flux from mantle sources significantly more enriched by plume components. These metasomatizing components may correspond to Na-alkali silicate mafic melts enriched in Ba, Th, Nb, Ti, Zr as well as light REE and show compositional analogies with the Ti-rich alkali silicate metasomatism documented in some mantle xenoliths from Kerguelen Islands (Grégoire et al., 2000).
References
Courtillot et al., 2003. Earth Planet. Sci. Lett. 205, 295-308.
Grégoire et al., 2000. Geochim. Cosmochim Acta 64, 673-694.
Pik et al., 1998. J. Volc. Geoth. Res. 81, 91–111.
MANTLE XENOLITHS FROM THE IBERIAN PENINSULA – 20 YEARS LATER
BIANCHINI GIANLUCA 1, BECCALUVA LUIGI 1, BONADIMAN COSTANZA 1, SIENA FRANCA 1
presenter's e-mail: bncglc@unife.it
1 - Dipartimento di Scienze della Terra - Università di Ferrara
Keywords: Iberian lithosphere, mantle xenoliths, metasomatism
After the pioneeristic study of mantle xenoliths from the Neogene volcanic districts of the Iberian Peninsula by Ancochea and Nixon (1987; in P.H. Nixon ed “Mantle xenoliths”, John Wiley and Sons Ltd) extensive sampling of these rocks has been carried out at Olot (north-east Spain), Cofrentes (near Valencia), Calatrava (central Spain) and Tallante (Betic Cordillera).
Mantle xenoliths and host lavas from these localities were systematically investigated for bulk rock major and trace element (XRF, ICP-MS), “in situ” microanalyses of constituent phases (EMPA, LA-ICP-MS), as well as Sr-Nd (Hf) isotopic analyses on mineral separates.
Mantle xenoliths from the OLOT volcanic field comprise a bi-modal suite of protogranular spinel lherzolites (sometimes with pargasitic amphibole), and coarse-grained granular - highly refractory -spinel harzburgites. In these xenoliths distinctive REE patterns and isotopic systematics of clinopyroxenes suggest interaction with metasomatic agents corresponding to a) tholeiitic melts with DM isotopic signature for lherzolites and b) highly alkaline melts with EM1 isotopic signature for harzburgites. Olot lherzolites possibly represent asthenosphere mantle material up-lifted, juxtaposed and accreted to the older subcontinental lithospheric mantle (harzburgites), during the post-Variscan rifting of the Iberian margin.
These genetically different, but adjoining, mantle domains intimately mingled along the northern Iberian margin during subsequent plate convergence processes (Bianchini et al., 2007; Lithos 94, 25-45).
Mantle xenoliths from the single volcanic centre of COFRENTES are mainly lherzolitic in composition and characterized by clinopyroxene REE patterns closely resembling those of the Olot lherzolites. This suggests also for this mantle section the upraise/protrusion of asthenospheric mantle material to shallow levels in the Iberian lithospheric margin, as also indicated by extremely low re-equilibration nominal temperatures (down to 700 °C).
Mantle xenoliths from the TALLANTE volcanic centre in the Betic Cordillera show a more complicated petrogenetic history, which can be attributed to diverse metasomatic events: a) impregnation of lherzolitic domains by tholeitiic melts (up to 5% of modal plagioclase, flat REE patterns of both bulk rocks and clinopyroxene, DM isotopic signature); b) interactions with alkali-silicate basic melts particularly evident in cpx-poor lherzolites and harzburgites (LREE-enriched patterns in both bulk rocks and clinopyroxene, EM1 isotopic signature); c) percolation of silicic melts in the peridotite matrix with formation of veins made up of plagioclase, orthopyroxene, amphibole and phlogopite. This veining appears to be related to silica-oversaturated/alkali-rich hydrous metasomatic melts, inducing progressive conversion of olivine in orthopyroxene or, in extreme cases, crystallization of quartz; particularly high 87Sr/86Sr and low 143Nd/144Nd ratios, coupled with strong negative Sr and Eu anomalies in both bulk-rocks and constituent minerals, indicate a significant contribution of continental crust components in the mantle sources, plausibly related to the Cenozoic subduction processes that ultimately led to the Betic orogeny (Beccaluva et al., 2004; Lithos 75, 67-87).
Mantle xenoliths from the CALATRAVA volcanic district are protogranular lherzolites (and wehrlites) widely overprinted by pyrometamorphic textures, with spongy clinopyroxene and reaction rims around orthopyroxene and spinel with secondary parageneses including feldspars, phlogopite, apatite, carbonate as well as interstitial glassy blebs. Bulk rock and clinopyroxene trace element and isotopic analyses suggest that the sub-lithospheric metasomatic agents were highly-alkaline carbonated basic melts characterized by HIMU isotopic affinity, that seems to be an ubiquitous isotopic fingerprint throughout the European-Mediterranean regions.
CHEMICAL VARIATIONS IN TECTONICALLY-EMPLACED MANTLE ROCKS: SUPERIMPOSED EFFECTS OF PARTIAL MELTING, MELT REDISTRIBUTION AND IGNEOUS REFERTILIZATION
BODINIER JEAN-LOUIS 1, LE ROUX VÉRONIQUE 1, SOUSTELLE VINCENT 1, TOMMASI ANDRÉA 1, GARRIDO CARLOS 2
presenter's e-mail: bodinier@gm.univ-montp2.fr
1 - GÉOSCIENCES MONTPELLIER
2 - UNIVERSIDAD DE GRANADA
Keywords: Tectonically-emplaced mantle rocks, Orogenic peridotites, Igneous refertilization, Rejuvenation of lithospheric mantle
There is a growing body of evidence indicating that chemical variations in tectonically-emplaced mantle rocks mostly reflect melt redistribution and near-solidus reactions at decreasing melt mass superimposed onto previous depletion events. The convergence of the most fertile lherzolite compositions at around 4% Al2O3 in peridotite massifs is not an indication of their pristine origin. This composition rather coincides with a threshold for melt segregation and formation of pyroxenite-peridotite "veined" mantle.
Recent studies also tend to converge on the idea that the orogenic peridotites represent thinned and/or rejuvenated lithospheric mantle, thermally eroded by asthenospheric mantle during continental rifting or early ocean initiation. Lithospheric rejuvenation is marked by annealing of deformation microstructures, overgrowth of mineral grains and km-scale modal/chemical variations resulting from partial melt redistribution. From top to base of eroded lithosphere, melt processes vary from refertilization of aged, refractory (harzburgite) lithosphere to partial melting of previously refertilized material (lherzolite and websterite). In the Ronda and Lherz massifs, both processes are associated with narrow fronts where major microstructural, modal and chemical variations occur within a few meters to tens of meters. Melting and refertilization fronts display striking resemblances (e.g., annealing of textures and small-scale chemical heterogeneities) as well as significant differences. The melting front observed in Ronda is a clear-cut structure that can be followed over 10 km in the massif, suggesting that its formation was thermally-controlled at regional scale. In contrast, the refertilization front of Lherz is extremely convoluted, suggesting its formation by coalescence of relatively narrow ( 10 m) melt infiltration channels. Refertilization and melting fonts are considered as the upper and lower boundaries of transient, moving asthenosphere-lithosphere transition zones during thermo-mechanical erosion of the lithospheric mantle by upwelling asthenosphere. Further complexity arises during the waning stages of lithospheric erosion - upon conductive cooling - when partially molten lithosphere is traversed by the receding melting/refertilization fronts.
An important implication of the origin of fertile orogenic lherzolites by refertilization is that these rocks cannot be straightforwardly used to infer primitive mantle compositions. It is thus logic to wonder whether fertile mantle xenoliths with similar compositions represent refertilized or pristine mantle. Several authors have ascribed the chemical stratification of cratonic lithosphere and/or its temporal evolution to metasomatic refertilization. In this scheme, fertile (predominantly lherzolitic), off-craton lithosphere may be viewed as the ultimate transformation of cratonic lithosphere after one or several cycles of igneous refertilization. Extensive refertilization in the Western Alps (Lanzo) and Betic peridotites (Ronda) is intrinsically related to the lithospheric thinning processes that led to mantle exhumation, which might suggest that large-scale refertilization is specific of tectonically-emplaced, orogenic peridotites. However, the Lherz massif illustrates a situation where refertilization and exhumation are related to distinct events, separated in time by thermal relaxation of subcontinental lithosphere. Several suites of spinel peridotite xenoliths compositionally comparable to the Lherz peridotites (e.g. formed of LREE-depleted lherzolites associated with subordinate, LREE-enriched harzburgites) might also record magmatic refertilization of a previously depleted lithospheric mantle.
HETEROGENEITY OF THE SUBCONTINENTAL MANTLE BENEATH THE UKRAINIAN SHIELD
BOGDANOVA SVETLANA V. 1, TSYMBAL STEPAN N. 2, SHUMLYANSKYY LEONID V. 2, BILLSTROEM KJELL 3, PASHKEVICH INNA K. 4, TSYMBAL YURY S. 2
presenter's e-mail: Svetlana.Bogdanova@geol.lu.se
1 - LUND UNIVERSITY SWEDEN
2 - INSTITUTE OF GEOCHEMISTRY, MINERALOGY & ORE FORMATIONS, KIEV, UKRAINE
3 - LABORATORY FOR ISOTOPE GEOLOGY, SWEDISH MUSEUM OF NATURAL HISTORY, STOCKHOLM
4 - INSTITUTE OF GEOPHYSICS, KIEV, UKRAINE
Keywords: East European Craton, Ukrainian Shield, Subcontinental mantle
The Ukrainian Shield (UkS) comprises the Meso- to Palaeoarchaean (2.8 to 3.7 Ga) Podolian, Bug, Middle Dniepr and Azov crustal blocks of different evolutions. These are separated by Palaeoproterozoic, 2.3-2.1 Ga orogenic belts. All the different units were welded together at ca. 2.1-2.0 Ga. Subsequently, tectonic and/or magmatic reworking occurred at 1.79-1.75 Ga, at the end of Precambrian at ca. 550 Ma, and in the Devonian when the Pripyat-Dniepr-Donets Aulacogen (PDDA) was formed. Accordingly, the subcontinental mantle was developed during several major events, and lateral as well as vertical compositional variation resulted from the degrees of mantle differentiation and depletion during recurrent magmatism and due to metasomatic enrichment of the depleted mantle during its fluidization. Juxtaposition of tectonic terranes of different ages also contributed to mantle heterogeneity.
Geophysically, the lithosphere of the UkS features two large resistant, stable regions corresponding to the principal Archaean crustal blocks. A zone of highly conductive lithosphere and elevated heat flow (more than 40 mWt/m2) coincides, in general, with the main Paleoproterozoic suture zones, where the Moho boundary shows complicated steep offsets. The Vp and densities of the uppermost mantle range from 8.4-8.6 km/s and 3400-3450 kg/m3 along the suture zones to 7.8-8.1 km/s and 3000-3200 kg/m3. A characteristic high-velocity lower crust underlies the 1.79-1.74 Ga gabbro-anorthosite-mangerite-rapakivi plutons.
Mantle-derived alkaline-ultramafic rocks of Palaeoproterozoic and Devonian ages occur in the different blocks of the UkS. They are: Palaeoproterozoic (Kirovograd) and Devonian (Azov and NW Volyn) basaltoid and mica-rich kimberlites, (2) Palaeoproterozoic lamproites (Azov and Kirovograd), (3) Palaeoproterozoic carbonatites, ijolites and other alkaline ultramafic rocks (the Azov, Podolian blocks and Volyn). Numerous placers of minerals (pyrope, Cr-diopside, picroilmenite, Cr-spinels) including diamond are known in the sedimentary cover.
Reconstructions of the compositional variation of the upper mantle with depth by employing xenolith and mineral compositions indicate significant diversity. Garnet-spinel ± phlogopite bearing harzburgites, lherzolites, pyroxenites, dunites and eclogites occur in various proportions. However, without precise rock ages it is difficult to use these data to compare the structure of the mantle at different times.
Isotopic (Nd-Sr-Pb) analyses of mantle-derived rocks in the different crustal blocks are still relatively few and occasionally the isotopic compositions have been modified by post-magmatic processes. Nd-isotope data typically show mildly depleted to slightly enriched compositions for almost all the rocks studied, which range age span from the Archean to the Devonian. Sr isotope data are quite variable, comprising both unrealistically low and very radiogenic ratios when calculated to the times of rock crystallization. Post-magmatic open-system behaviour and crustal assimilation during magma generation may therefore have played a role.
However, a majority of the Sr isotope data appear to yield consistent and reasonable initial 87Sr/86Sr ratios, e.g. between 0.702 and 0.703 for 2.1-1.8 Ga rocks. The Pb isotope data suggest that at least some parts of the mantle were enriched in uranium already in the Archean as evidenced by highly radiogenic Pb compositions for rocks of different ages. Moreover, many whole-rock analyses suggest elevated 207Pb contents, consistent with an enriched source of the protoliths.
In summary, the geophysical, mineralogical and isotopic data demonstrate the mantle heterogeneity beneath the UkS. The most pronounced structural and compositional changes probably occurred at ca. 2.1-2.0, 1.8 and 0.4 Ga.
PRE-MESOZOIC LITHOSPHERIC MANTLE BENEATH SCOTLAND TERRAINS
BONADIMAN COSTANZA 1, COLTORTI MASSIMO 1, DUGGEN SVEND 2, PALUDETTI LAURA 1, SIENA FRANCA 1, THIRWALL MATTHEW 3, UPTON BRIAN G.J. 4
presenter's e-mail: bdc@unife.it
1 - Dip. Scienze Terra, Università di Ferrara, Italy
2 - IFM-GEOMAR, Kiel, Germany
3 - Dept. of Geology, Royal Holloway University of London, U.K
4 - School of GeoSciences, The University of Edinburgh, U.K.
Keywords: mantle xenoliths, pre-mesozoic, mantle metasomatsm, intraplate and subduction metasomatism, Scotland
Mantle xenoliths entrained in late Carboniferous to mid-Permian silica-poor basic magmas from northern Scotland were investigated using major and trace elements of minerals and isotopic compositions of whole-rock and clinopyroxenes. The work concerns peridotites from two localities in the ENE and WSW of the Scottish Northern Highlands Terrane, namely Rinibar (Orkney) and Streap Com’laidh, (near Glenfinnan), but a geochemical comparison with Scottish mantle xenoliths of the same age has been made in order to characterize the Pre-mesozoic lithospheric mantle beneath Scotland terrains. Northern Highland Terrane is one of the five main terranes which constitute Scotland. From South to North, these are: 1) Southern Uplands, 2) Midland Valley, 3) Grampian Highland, 4) Northern Highland and 5) Hebridean to the north-west of the Moine Thrust Zone. The terranes are believed to have been amalgamated into their present relationships by 400 Ma at the latest. Uncertainty exists as to where the Northern Highland Terrane lay in earlier times, but magnetic studies suggest that by around 1.2 Ga ago, it was an integral part of a supercontinent Rodinia located in the southern hemisphere. Break-up of Rodinia led at ~ 600 Ma to the opening of the Iapetus Ocean. This subsequently underwent complex closure resulting in the Caledonian Orogeny. The final ‘docking’ of the Northern Highland Terrane with respect to its neighbouring Grampian and Hebridean Terranes appears to have been effected during the (Silurian) Scandian phase of the orogeny. On the basis of different trace element contents and isotopic ratios clinopyroxenes from Rinibar identified two different metasomatic styles, one carbonatitic and one kimberlite-like, probably derived from the same source for variable degree of partial melting. This is also supported by the similarity of Sr and Nd isotopic ratios at 550+50Ma. This age is crucial for Scotland and for global tectonics. It corresponds to the opening of Iapetus Ocean. At about the same time (550Ma) the Canadian and Finnish Shields were also affected by kimberlitic and carbonatitic magmatism. Late Proterozoic-Early Phanerozoic carbonatite magmatism is also recognized within Scotland (Loch Borralan, Northern Highland Terrane)as also testified by the coherent suites of cpx, amph and exotic megacrysts in the Carbo-Permian basanites & melanephelinites across Scotland from the Hebridean Terrane (Loch Roag) to the Southern Uplands (Burn-Between-the-Laws).At 500 Ma the tectonic regime changed from divergent to convergent as Iapetus began to close and the Baltica continent start approaching Laurentia. It may have been during this convergent stage (~400Ma) that the metasomatism affecting the sub-Streap lithospheric mantle occurred. Clinopyroxenes from this locality show in fact trace element and isotopic features that can be explained by metasomatic fluids coming off a subducted slab. The various terranes that now constitute Scotland came into conjunction at the end of Caledonian Orogeny, and were certainly contiguous by 416 Ma. By comparison, the Rinibar clinopyroxenes record no subduction-related imprinting. This could imply that i) the north easternmost portion of the Highland Terrain lithospheric mantle was unaffected by the subduction or, alternatively, ii) the subduction-related metasomatism recorded in the Streap mantle may be older, when the two lithospheric blocks were far apart.
A detailed analyses of the elemental and isotopic data of the xenoliths from the other Scottish blocks reveal a possible regional distribution of the intraplate metasomatic enrichment from SE (Midland Valley) to NW (Hebridean Craton), whereas subduction-relatated mtasomatism have been clearly detected only in the mantle xenoliths of Northern Terrane (Streap) and Hebridean Terrane (Loch Roag).
POTASSIC AND ULTRAPOTASSIC MAGMATISM IN THE WESTERN MEDITERRANEAN BASIN AND INSIGHTS ON ITS GEODYNAMIC SIGNIFICANCE - 1: EVIDENCE FROM PB, ND, AND SR ISOTOPES AND TRACE ELEMENT DATA ON LAMPROITES, SHOSHONITES AND CALC-ALKALIC ASSOCIATIONS FROM TUSCANY, MURCIA-ALMERIA, CORSICA, AND WESTERN ALPS
CONTICELLI SANDRO 1, GUARNIERI LUISA 1, MATTEI MASSIMO 2, FARINELLI ALICE 1, BIANCHINI GIANLUCA 3, BOARI ELENA 1, AVANZINELLI RICCARDO 4, TIEPOLO MASSIMO 5, TOMMASINI SIMONE 1, VENTURELLI GIAMPIERO 6
presenter's e-mail: sandro.conticelli@unifi.it
1 - Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Firenze, Italy
2 - Dipartimento Scienze Geologiche, Università Roma III, Roma, Italy
3 - Dipartimento di Scienze della Terra, Università degli Studi di Ferrara, Ferrara, Italy
4 - Department of Earth Sciences, University of Bristol, Bristol, U.K.
5 - C.N.R. - Istituto di Geoscienze e Georisorse – Sezione di Pavia, Campus Universitario, Pavia, Italy
6 - Dipartimento di Scienze della Terra, Università degli Studi di Parma, Campus Universitario, Parma, Italy
Keywords: ultrapotassic rocks, lamproites, western Mediterranean, Sr, Nd, Pb isotopes
Lamproitic rocks associated to shoshonitic and calc-alkalic ones are found around the Western Mediterranean. The oldest ones were intruded as dykes into the metamorphic rocks of the Western Alps during the Oligocene. The youngest ones were intruded and poured out in Southern Tuscany from Pliocene to Pleistocene. The most important lamproitic magmatic event is found in South-Eastern Spain, with several dikes, plugs, and lava flows emplaced during the Late Miocene. The smallest lamproitic outcrop is found in Corsica, in the form of dike during the Middle Miocene. Shoshonitic and calc-alkalic rocks are invariably timely and spacely associated to lamproites.
Western Mediterranean lamproites are peculiar ultrapotassic rocks, ranging from slightly silica undersaturated to silica over-saturated, they are extremely enriched in MgO, but low Al2O3, CaO, and Na2O. They are plagioclase-free rocks, but K-feldspar is abundant beside other K-bearing phases. Lamproite-like rocks are also characterised by a strong enrichment in incompatible elements, which sometimes prevent further enrichment due to crustal contamination during ascent to surface.
New trace element beside new and original Sr, Nd and Pb isotopic data are reported. All the sample studied are characterised by strong fractionation of High Field Strength elements with respect to Large Ion Lithophile elements. Extreme enrichment in Th, U, and Pb with respect to Ba, Nb, Ta, Sr, P, and Ti is a common characteristic observed. Shoshonitic, high-K calc-alkalic, and calc-alkalic rocks show similar trace element distribution patterns and elemental fractionation but less enriched then in lamproitic rocks.
Sr isotopic data of lamproite are extremely enriched in radiogenic compositions with initial 87Sr/86Sr in the ranges between 0.716350 and 0.717652 for Western Alps lamproites, between 0.716415 and 0.723595 for Spanish lamproites and between 0.71579 and 0.71672 for Tuscany lamproites. The Corsica lamproite shows the lowest values of initial 87Sr/86Sr (0.71229). Conversely Nd isotopes are extremely low with initial values in the ranges between 0.511990 and 0.512009 for Western Alps lamproites, between 0.511190 and 0.512140 for Spanish lamproites, and between 0.512086 and 0.512123 for Tuscany lamproites. Also in this case Corsica lamproite distinguishes clearly from others Western Mediterranean lamproites showing the highest value of initial 143Nd/144Nd (0.512148). Regarding lead isotopes, no large differences are observed among the 207Pb/204Pb values of lamproites (15.629-15.715) from the different provinces, whereas significant differences are observed in the 206Pb/204Pb and 208Pb/204Pb values, with Tuscan Lamproites showing the lowest values (206Pb/204Pb = 18.653-18.677, 208Pb/204Pb = 38.855-39.026) and Western Alps the highest values (206Pb/204Pb = 18.684-18.904, 208Pb/204Pb = 39.029-39.485). Shoshonitic to calc-alkalic rocks display significantly different isotopic values with dilution of the radiogenic components for Sr and Nd and of the un-radiogenic one for Nd with decreasing alkali contents.
Several evidences indicate that magmas of the different provinces have been generated by a depleted upper mantle subsequently modified by metasomatism. The metasomatic agents were originated by recycling of upper crustal material into the mantle before magma generation. Metasomatism was not patently and not homogeneous, but concentrated in a vein network. Pure vein partial melting produced lamproitic-like magmas, increasing melting of surrounding upper mantle source produced the spectra of magma observed. Deviation from this hypothesis is shown by Murcia-Almeria and Corsica regions. Corsica lamproite bear a consistent withinplate component that indicate a different process of generation and complicate the general geodynamic setting for these rocks. A geodynamic model for the evolution of the Western Mediterranean basin from Miocene is also provided.
MANTLE XENOLITHS EVOLUTION DURING NEOGENE POST-COLLISIONAL TRANSITION FROM CALC-ALKALINE TO ALKALINE VOLCANISME IN ORANIE : A SLAB BREAKOFF
COTTIN JEAN-YVES 1, DELPECH GUILLAME 2, ZERKA M. 3, OREILLY SUZANNE Y 4, LOUNI A. 4, GREGOIRE MICHEL 5, LORAND JEAN-PIERRE 6
presenter's e-mail: jean.yves.cottin@univ-st-etienne.fr
1 - Department of Geology-UMR 6524 "Magmas et Volcans", University of Jean Monnet, Saint-Etienne Cedex, France
2 - "Interactions et Dynamique des Environnements de Surface"
3 - IST, Université d’Oran Es Senia, Oran, Algérie
4 - GEMOC ARC National Key Centre, Earth and Planetary Sciences, Macquarie University, Australia
5 - "Dynamique Terrestre et Planétaire", Observatoire Midi-Pyrénées, Toulouse, France
6 - LAB. Minéralogie, Muséum Paris, France
Keywords: Mantle xenoliths, Slab breakoff, Calc-alkaline and alkaline volcanism
The western Oranie, located at the northern margin of the African plate, is characterized by important plio-quaternary alkali volcanic extrusions. This volcanism shows a change in composition during the Neogene from calco-alkali at the Miocène to alkali at the Plio-Quaternary. These xenoliths are subdivided in three groups : (1) Type I mantle tectonites (porphyroclastic and equigranular lherzolites and protogranular harzburgites) reequilibrated in the spinel and/or plagioclase stability field. (2) type II basaltic cumulates (wehrlites, pyroxenites and hornblendites) and (3) rare composite xenoliths. On the basis of trace element compositions determined by LAM-ICP-MS, three groups of clinopyroxene can be subdivided. (1) Clinopyroxenes with trace element signature depleted in LREE ((La/Yb)N=0.21-0.33) belong to protogranular Sp-bearing harzburgites or Cpx-rich Sp-bearing lherzolite. They show evidence of small degrees of partial melting that affected only LREE. (2) Clinopyroxenes associated with composite xenoliths have the same shape of patterns (enrichments in MREE relative to LREE) but show different level of enrichment in trace elements characteristic of a nearly complete reequilibration with deep alkali segregates. This difference in trace element enrichments is related to the Mg# of the cpx and probably to the depth of crystallisation of the cpx in an alkaline melt. (3) Clinopyroxenes associated with harzburgites or lherzolites showing various degrees of enrichments in LREE are directly related to different stages of incomplete reequilibration during melt-rock reaction processes. Some of these peridotites are enriched in metasomatized cpx and can evolve to a werhlitic composition. Moreover, some of these peridotites are strongly metasomatized and show occurrences of glass in foliation plans underlined by spinel alignements. The trace element composition of the glass has been determined and shows enrichments in LREE (La/YbN=22.4-23.6) but positive anomalies in Nb, Ta ((Nb/La)N=1.82-2.17) without significant other enrichment or depletion in other HFSE. The glass trace element composition is either distinct from the trace element composition of the alkaline host lava nor with the trace element composition of calco-alkaline lavas in the area. The composition of the glass probably results from complex infiltration-reaction-crystallisation processes. The calc- alkaline volcanic products of Oranie would derived from melting of a mantle having preserved an orogenic geochemical, probably inherited of a previous subduction episode. This signature would have dimmed progressively during the time, what would reflect a change of geodynamical context bordering to a contribution more and more marked of the process of partial melt of a subcontinental sheared and more fertile mantle.
MANTLE XENOLITHS IN SPACE AND TIME IN EUROPE
DOWNES HILARY 1
presenter's e-mail: h.downes@ucl.ac.uk
1 - BIRKBECK UNIVERSITY OF LONDON
Keywords: xenolith, mantle, Europe
The European continent is particularly rich in localities in which mantle xenoliths have been brought to the surface, representing lithosphere from beneath regions of crust of Archaean to Phanerozoic age. Cratonic lithosphere is represented by rare xenoliths from a few scattered localities in the Fennnoscandian shield (Finland, Arkhangelsk, Kola) and the East European platform (Belarus, Ukraine), where depleted garnet and spinel peridotites are brought to the surface mostly in kimberlites. More studies need to be undertaken concerning the nature of the lithosphere beneath the cratonic terranes of Europe (Fennoscandia, Ukraine, NW Scotland) and the processes that have formed it. Alkali basalts in Scotland and southern Scandinavia provide samples of somewhat younger (Proterozoic-early Palaeozoic) lithosphere.
In the Tertiary and Quaternary alkaline volcanics erupted in the Hercynian and Alpine regions of western and central Europe, mantle xenoliths are abundant, but the processes that have affected the mantle in these areas are very diverse. Extreme depletion in Zr and Hf in clinopyroxenes associated with unusually high Hf-isotope ratios in mantle xenoliths from the Massif Central indicate that some regions of the lithospheric mantle have been extremely depleted by melting in Hercynian times. More constraints are needed on the age of depletion across different regions of Europe. Enrichment due to subduction-derived fluids has been suggested by various workers for regions of the mantle beneath the Pannonian-Carpathian region, the Dinarides, Spain and central Italy. Carbonatite metasomatism has been detected in xenoliths from the Pannonian-Carpathian region, NE Spain, central Italy, Poland and France. We need to have an agreed methodology to determine the degree of depletion of the mantle in different regions and an agreed set of criteria to distinguish between different types of metasomatism. Silicate melt metasomatism is common beneath most regions, often giving rise to composite peridotite-pyroxenite or peridotite-hornblendite xenoliths.
Metasomatising melts appear to be ubiquitously related to the host Neogene alkali basalt magmas. There are still some fundamental questions about how enrichment really occurs within xenoliths (i.e. to what extent interaction with magma prior to entrainment overprints many samples), and the extent to which the subcontinental lithospheric mantle participates in magmagenesis.
STRUCTURE AND DEFORMATION OF THE SHALLOW LITHOSPHERIC MANTLE IN SUPRA SUBDUCTION (?) SETTING: A XENOLITH STUDY FROM THE EASTERN TRANSYLVANIAN BASIN.
FALUS GYÖRGY 1, TOMMASI ANDREA 2, INGRIN JANNICK 3, SZABÓ CSABA 4
presenter's e-mail: falus@elgi.hu
1 - UM2, ELGI
2 - CNRS, UM2
3 - UPS
4 - ELTE
Keywords: upper mantle, deformation, crystal preferred orientation, subduction
Peridotite mantle xenoliths, with broad textural variation, representing the 35-55 km depth range of the lower lithosphere, provide evidence for consistent microstructural evolution beneath the Eastern Transylvanian Basin, Southeastern Carpathians, Romania, related to ongoing plate convergence in the Carpathian Arc nearby. Recrystallized grain size (indicative of stress), fabric strength and resulting anisotropy and equilibrium temperatures vary continuously from depth to the shallowermost upper mantle. Replacive orthopyroxene structures, mostly consuming olivine, associated with H2O-saturated concentrations and relatively low Al2O3 compositions, are observed in a few of the xenoliths indicating (extremely) limited percolation of subduction-related fluids/melts in the represented upper mantle.
However, despite high stress deformation and high H2O content in some of the studied mantle nodules, fabric analysis in olivine shows the exclusivity of the [100] slip systems and absence of [001] slip, which is attributed to low H2O solubility in olivine at spinel peridotite facies conditions, and questions the overall existence of ‘wet’ deformation of olivine at shallow depths. Seismic anisotropy values, estimated from fabric strength, are in agreement with seismologic measurements in the region. We interpret that strike-parallel fast directions, observed in the region are not the result of ‘wet’ olivine fabrics, but are more likely the consequence of plate convergence-driven mountain-parallel creep of the upper mantle.
TRACE-ELEMENT PATTERNS OF DIAMOND: CLUES TO MANTLE PROCESSES
GRIFFIN WILLIAM L. 1, REGE SONAL 1, ARAUJO DEBORA 1, JACKSON SIMON 1, PEARSON NORMAN 1
presenter's e-mail: bill.griffin@mq.edu.au
1 - GEMOC ARC National Key Centre, Dept. Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
Keywords: Mantle processes, Diamond trace elements, Mantle metasomatism, Lithosphere geochemical signatures, Diamond genesis
Quantitative trace-element analyses of > 40 elements in diamonds have been carried out by LAM-ICPMS, using a multi-element doped-cellulose standard; detection limits range to low-ppb levels for many elements [1]. This new type of data offers new insights into metasomatic processes in the SCLM.
The trace-element patterns of polycrystalline (framesite, diamondite) and fibrous diamonds are consistent with crystallisation from kimberlitic-carbonatitic melts. However, some fibrous diamonds show an abrupt change in trace-element patterns as crystallisation proceeds. Contents of LREE, Sr and Ba decrease sharply, accompanied by decreases in Nb/Ta and Zr/Hf from chondritic to 0.1-0.01 x Chondrites. The fractionation of these pairs of geochemically similar elements is hard to explain by fractional crystallisation, because D(solid/melt) for these elements are very similar. However, such fractionations can be the result of liquid immiscibility: a separation into broadly (?hydrous)-silicate and carbonatite fluids. The ubiquitous development of pronounced negative Y anomalies (relative to Ho-Dy) during the crystallisation of some fibrous diamonds may reflect the separation of fluoride phases or immiscible fluoride melts; microinclusions with positive Y (±Yb) anomalies are observed during ablation of diamondites.
Nearly all monocrystalline diamonds show low LREE/HREE, Ba/MREE and Sr/MREE, as well as low Nb/Ta and Zr/Hf, suggesting that they have crystallised from the hydrous-silicate member of the proposed immiscible-liquid couple. Oscillatory zoning of composition in some diamonds suggests that both fluids were circulating in the system. Preliminary studies show little difference in the trace-element patterns of peridotitic and eclogitic diamonds from single localities. This implies limited interaction between the fluid(s) and wall rocks, which in turn suggests high fluid/rock ratios during diamond crystallisation. Further work is required to reconcile the model described here with the data from single-inclusion studies of fibrous diamonds, which suggest immiscibility between a hydrous fluid (brine) and a continuum of carbonatite-silicate melts. However, it is consistent with observations of carbonate-silicate immiscibility in melt inclusions trapped in Cr-diopside derived from ca 180 km beneath the Slave craton [2].
Modelling of the Mg-rich carbonatitic fluid that develops in the diamond-forming system shows it would have extremely high LREE/HREE, Ba and Sr; similar patterns are observed in secondary coats on monocrystalline diamonds from some localities. The reaction of this fractionated carbonatitic fluid with chromite + olivine + opx can produce subcalcic Cr-pyrope garnets with "sinuous" REE patterns and high Sr contents, which are a characteristic inclusion in diamonds of the peridotitic paragenesis [3]. We therefore suggest that the development of immiscibility during the evolution of methane-rich low-volume melts of the kimberlite-carbonatite spectrum produces conjugate fluids, one of which crystallises most monocrystalline diamonds, and the other of which interacts with mantle harzburgites to produce the most ubiquitous inclusions in peridotitic diamonds. This model provides a genetic link between the diamonds and their most distinctive indicator minerals.
[1] Rege, S., Jackson, S.J., Griffin, W.L., Davies, R.M., Pearson, N.J. and O’Reilly, S.Y. 2005. Quantitative trace element analysis of diamond by laser ablation inductively coupled plasma mass spectrometry. Journal Analytical Atomic Spectroscopy 20, 601-610.
[2] van Achterbergh, E., Griffin, W.L., Ryan, C.G., O'Reilly, S.Y., Pearson, N.J., Kivi, K. and Doyle, B.J. 2002. A subduction signature for quenched carbonatites from the deep lithosphere. Geology 30, 743-746
[3] Malkovets, V., Griffin, W.L., O’Reilly, S.Y. and Wood, B.J. 2007. Diamond, subcalcic garnet and mantle metasomatism: Kimberlite sampling patterns define the link. Geology 35, 339-342.
EVIDENCE OF MELT STAGE REFERTILIZATION AND METASOMATISM IN ABYSSAL PERIDOTITES FROM HESS DEEP (ODP LEG 147)
GRÉAU YOANN 1, GODARD MARGUERITE 2, ALARD OLIVIER 2
presenter's e-mail: ygreau@els.mq.edu.au
1 - GEMOC/GÉOSCIENCES MONTPELLIER
2 - GÉOSCIENCES MONTPELLIER
Keywords: Abyssal peridotites, Mantle metasomatism, Mantle clinopyroxene, Clinopyroxene rare-earth elements
It is commonly accepted that abyssal peridotites (AP) are simple residue of recent partial-melting due to the upwelling of convective and fertile mantle under mid oceanic ridges. Numerous authors have argued that given this straightforward history and their short residence time into the lithosphere, AP should be representative of the Earth’s fertile mantle. This study examines samples from the Hess Deep, the triple junction between the East-Pacific Rise and Cocos-Nazca propagator (ODP Leg 147). Despite extensive alteration (80%) due to serpentinisation and seafloor alteration (a common feature in AP), two assemblages can be recognised in samples from this locality: (1) a primary relict assemblage with large porphyroclasts of orthopyroxene (Opx1) and olivine (Ol1); (2) a secondary assemblage formed of interstitial and vermicular clinopyroxene (Cpx2), which may also occur in symplectitic association with spinel (spl) and sulfide. The textural occurrence of this assemblage 2 suggests that it represents the crystallization of an infiltrated melt Abyssal peridotites from this leg are harzburgites with the average modal composition: 80% Ol, 15% Opx, 3% Cpx and 2% Spl. Major element analyses of primary phases show that they are residual after extensive degrees of fusion and that a chemical disequilibrium between Cpx2 and Ol1-Opx1 has been preserved. Inductively coupled plasma-mass spectrometer (ICP-MS) analyses on whole rocks and Laser Ablation ICP-MS analyses of Cpx2 and Opx1 show that trace-elements concentrations are among the most depleted observed so far in abyssal peridotites. Rare-earth element (REE) patterns of Cpx2 are very depleted in comparison with those from the literature. Moreover, Cpx2 are not in equilibrium with Opx1 concentrations. Enrichment in light rare-earth elements and in large ion lithophile elements (LILE, e.g., U, Th, Pb and Sr), both in Opx1 and Cpx2 indicates a late stage metasomatic event. It is worth noting that U, Pb, Sr enrichments in whole-rock are commonly ascribed to seawater alteration. Here the fact that fresh Cpx2 and Opx1 core show such enrichment, coupled with enrichment in Th and LREE (not affected by alteration processes) suggesting a magmatic rather than an alteration origin.
So we can conclude that:
- Abyssal peridotites from Hess Deep have experienced a primary episode of melting of about 15%, as calculated from the heavy and medium REE composition of Opx1.
- Textures of Cpx2 and disequilibrium between Cpx2 and Opx1 suggest that Cpx2 has crystallised from a percolating melt. The Spl-Cpx2 clusters are reminiscent of a quenching texture and indicate that part of this melt has crystallised under lithospheric (cold) conditions.
- Such high depletion of REE in clinopyroxene indicates that this melt is derived from a very depleted mantle source. Calculated melt in equilibrium with Cpx2 is similar to the ultra depleted melt (UDM) evidenced as melt-inclusion in olivine phenocryst in MORBs by [Sobolev et al., 1993].
This suggests a widespread occurrence of those UDM.
Thus APs are not simple residues of partial melting but record a complex and protracted history of multiple events of melt extraction and percolation-reaction. This recall what have been observed in the sub-continental lithospheric mantle and suggest that the mantle section sampled by APs is not as young as one could expect. UDM giving the Cpx2 are clearly derived from an old and extremely depleted mantle reservoir. The occurrence of such mantle volume/component in such geodynamic setting is at odd with the assumed young and convective nature of the oceanic mantle. However, the occurrence of old and depleted mantle component within the oceanic convective mantle is also suggested by other studies of AP [Seyler et al., 2004] on a worldwide basis and especially by recent Re-Os isotopic investigations [e.g. Alard et al., 2005; Author's unpub data].
LA-ICPMS STUDY OF CLINOPYROXENE-APATITE-K FELDSPAR-PHLOGOPITE METASOMATIC MANTLE XENOLITHS FROM HUNGARIAN LAMPROPHYRES AND THEIR PRIMARY CARBONATITE MELT INCLUSIONS: IMPLICATIONS FOR CARBONATITE MELT METASOMATISM IN THE EARTH'S UPPER MANTLE
GUZMICS TIBOR 1, ZAJACZ ZOLTÁN 2, SZABÓ CSABA 1, HALTER WERNER 2
presenter's e-mail: tibor.guzmics@gmail.com
1 - Lithosphere Fluid Research Lab, Eötvös University, Budapest, Hungary
2 - Department of Earth Sciences, Institute of Isotope Geochemistry and Mineral Resources, ETH, Switzerland
Keywords: carbonatite melt inclusions, carbonatite metasomatism, trace element geochemistry, Hungarian lamprophyres
We studied rare metasomatic mantle xenoliths consisting of clinopyroxene (Cpx), apatite (Ap), K feldspar (Kfs) and phlogopite (phl) from the Alcsútdoboz-2 lamprophyre dikes, Hungary (Szabó et al., 1993). Ap and Kfs contain numerous primary, negative crystal shaped carbonatite melt inclusions (CMI) that were analyzed by LA-ICPMS. Cpx and Phl are barren. The CMI have a phosphorous dolomitic character in Ap and an alkaline-aluminosiliceous dolomitic character in Kfs. Both Ap- and Kfs-hosted CMI show post entrapment crystallization onto the wall of the melt inclusions. The amount of apatite crystallized onto the wall in Ap-hosted CMI and the bulk composition of these CMI show entrapment temperature range of 1090 – 1180 oC, based on the Baker-Wyllie equation (1992). Primitive mantle normalized (PM) trace element distributions of the Ap- and Kfs-hosted CMI (n=60 and 20, respectively) reveal a strong negative Ti anomaly, and an extreme enrichment in the incompatible LILE and LREE relative to HREE and the compatible elements (Sc, V, Ni and Cr). The relative La/Lu to PM is in the range of 107-147, furthermore, PM-normalized U, Th, Sr show ~one million times higher value than those of Ni and Cr. This indicates that the initial melt of the CMI was formed with a very low degree partial melting of a carbonated source. We suggest that the CMI in Ap and in Kfs cannot be differentiation product or residuum of each other because the Kfs-hosted CMI contain small amount of P and most of Ap-hosted CMI contain small amount of K, Al and Si. Therefore, they are more likely to have been formed by liquid-liquid separation, which requires their common origin. This is supported by PM-normalized REE composition of CMI in Ap showing highly similar distribution to that of CMI in Kfs. Addition, apatites rarely enclose unique CMI that show major- and trace-element signature similar to K feldspar-hosted CMI. This confirms that there was a liquid-liquid separation between a P-bearing carbonatite melt and a carbonate-bearing silicate melt before crystallization of their host phases (namely Ap and Kfs). Trace element distributions of the CMI reveal that U, Th, Ba, Pb, Nb, Ta, P, Sr, Y and REE are partitioned into P-bearing carbonatite melt whereas; Cs, Rb, Na, K, Al, Zr and Hf prefer silicate-bearing one.
PM-normalized patterns of the Cpx compositions indicate enrichment in LREE relative to HREE and compatible elements such as Sc, V, Ni and Cr. Furthermore, they show strong negative Ti anomaly. Thus, chemical characteristics of the CMI are clearly reflected in the clinopyroxene compositions. This indicates that formation of the clinopyroxenes was linked to the metasomatic carbonatite melt trapped as CMI in the studied apatite and K feldspar. The studied xenoliths also reveal that modal metasomatism happened, however; clinopyroxenes which occur in the studied xenoliths with high modal proportion of Ap, contain Ti, Sc, Ni, V and especially Cr in much lower concentration than those clinopyroxenes wich occur with low modal proportion of Ap. Coincidentally, distributions of Zr and Hf in Cpx show an opposite pattern. All of the studied CMI reveal that Ti, Sc, V, Cr and Ni were in much lower concentration in the "initial" carbonatite melt than that in PM; contrarily, Zr and Hf were in higher concentration in it than in PM. Consequently, a continuously migrating "initial" carbonatite melt, which crystallized Ap and Kfs, increased Zr and Hf concentration, whereas, decreased amount of Ti, Sc, V, Ni and Cr in the clinopyroxenes. All of these supports that the studied CAKP rocks were formed by carbonatite melt metasomatism, which occurred in an open system in peridotitic environment.
References:
Baker & Wyllie (1992): Geochim Cosmochim Acta, 56, 3409-3422.
Szabó et al. (1993): Miner. Petrol., 47, 127-148.
THE NATURE OF THE SUB-LITHOSPHERIC MANTLE BENEATH THE PANNONIAN BASIN, EASTERN-CENTRAL EUROPE, AS INFERRED FROM THE GEOCHEMISTRY OF THE NEOGENE TO QUATERNARY ALKALINE MAFIC ROCKS
HARANGI SZABOLCS 1, NTAFLOS THEODOROS 2, DOWNES HILARY 3, LENKEY LÁSZLÓ 4
presenter's e-mail: szabolcs.harangi@geology.elte.hu
1 - EÖTVÖS UNIVERSITY, DEPARTMENT OF PETROLOGY AND GEOCHEMISTRY, BUDAPEST, HUNGARY
2 - DEPARTMENT OF LITHOSPHERIC SCIENCES, UNIVERSITY OF VIENNA, VIENNA, AUSTRIA
3 - SCHOOL OF EARTH SCIENCES, BIRKBECK UNIVERSITY OF LONDON, LONDON, U.K.
4 - HUNGARIAN ACADEMY OF SCIENCES, RESEARCH GROUP OF GEOPHYSICS AND ENVIRONMENTAL PHYSICS, BUDAPEST, HUNGARY
Keywords: Pannonian basin, Petrogenetic modelling, Asthenosphere, Melt generation, Geochemistry
The petrology and geochemistry of mafic lavas could provide important information about the thermal state and the composition of the mantle where their parental magmas formed. One of the recent challenging approaches is to constrain the existence or absence of anomalously hot sub-lithospheric mantle, i.e. mantle plumes. This is commonly inferred either by the isotopic composition of mafic lavas (e.g., Pb-isotope values close to the HIMU component) or by the calculated mantle potential temperature (i.e., about 200-300oC higher than MORB-mantle). A strongly heterogeneous mantle model is an alternative explanation of the plume hypothesis.
In Europe, alkaline mafic magmatism has been taking place mostly at the periphery of the Mediterranean region since the Palaeogene. The last eruptions in these volcanic fields (French Massif Central, Eifel, Bohemian area, Pannonian basin) occurred only in some tens or hundreds ka. Understanding the magma generation processes is crucial in order to evaluate the possible continuation of the volcanic eruptions in these areas. The origin of this magmatism is commonly regarded as a response of diapiric upwelling of small-scale, relatively hot upper mantle material (‘plume-fingers’).
Here, we attempt to have an insight into the state of the sub-lithospheric mantle beneath the Pannonian basin. Existence of a ‘plume-finger’ has been supposed also beneath this area. Indeed, Sr-Nd-Pb isotopic compositions of the alkaline basaltic rocks are similar with those shown by the mafic rocks in Europe and the whole area is characterized by high heat flow. However, we demonstrate here that a mantle plume beneath the Pannonian basin is highly unlikely. The trace element composition of the most primitive samples suggests that the primary magmas were formed by different degrees (1-6%) of melting of a garnet-peridotite source region with a moderately enriched composition (1.5- to 4-times primitive mantle values). Another constrain on the mineralogy of the mantle source comes from the presence of negative K-anomaly in the primitive mantle normalized patterns of most of the samples. This requires a K-bearing hydrous phase (amphibole or phlogopite) in the source region or an inherited K-depleted signature of the upper mantle. Trace element modelling appears to be consistent with the first explanation. At low degree (<5%) melting, amphibole or phlogopite could remain in the residuum retaining potassium, whereas they were consumed at higher degree of melting. The inferred depth of the melt generation is between 60 and 120 km, i.e. mostly in the asthenosphere. This should mean that these volatile-bearing mantle rocks could reside in the sub-lithospheric mantle causing mantle heterogeneity in various scales. The long history of orogenic events (Hercynian and Alpine orogenesis) in Europe could supply vast amount of crustal material into the upper mantle resulting in this geochemical heterogeneity. Partial melting of different parts of the shallow asthenospheric mantle such as metasomatized, amphibole/phlogopite-bearing enriched sections with HIMU-like composition at low degree melting and depleted MORB-mantle around them at higher degree of melting and mixing of these melts could also explain the isotopic variation in the alkaline mafic magmas of the Pannonian basin. The high heat flow can be readily explained by the thinning of the lithosphere during the Miocene basin formation and the still shallow asthenosphere. The temperature could be still close to the solidus temperature and local perturbance caused by mantle flow can initiate magma generation. The reason of such a mantle flow is unclear, although a possible explanation for this could be the large gradient in the depth of the lithosphere-asthenosphere boundary at the periphery of the Pannonian basin. This region with an anomalously thin lithosphere could act as a thin spot and its suction force could generate mantle flow from the surrounding areas.
NEW CONSTRAINTS ON THE MESOZOIC EMPLACEMENT OF DIORITIC DYKES IN THE BALDISSERO MANTLE PERIDOTITE MASSIF (IVREA-VERBANO ZONE)
MAZZUCCHELLI MAURIZIO 1, ZANETTI ALBERTO 2, RIVALENTI GIORGIO 1, VANNUCCI RICCARDO 3, CORREIA TEIXEIRA CIRO 4, TASSINARI COLOMBO 4, CELSO GAETA 4
presenter's e-mail: mazzuc@unimore.it
1 - Dipartimento di Scienze della Terra, Università degli Studi di Modena e Reggio Emilia, Largo S. Eufemia 19, 41100 Modena, Italy
2 - IGG-CNR, Sezione di Pavia, via Ferrata 1, 27100 Pavia, Italy
3 - Dipartimento di Scienze della Terra, Università di Pavia, via Ferrata 1, 27100 Pavia, Italy
4 - Instituto de Geociências, Universidade de São Paulo, Rua do Lago 563, Cidade Universitaria, 05508-900 São Paulo, Brazil
Keywords: Mesozoic Magmatism, Diorite Dykes, Baldissero, Ivrea-Verbano
The Baldissero peridotite is the main mantle massif occurring in the south-western part of the Ivrea-Verbano Zone. It is in contact with the gabbros of the Ivrea-Verbano Mafic Complex to the East, while it is sheared by the faults of the Insubric system to the West, being in tectonic contact with the Canavese terrains of the Austroalpine Domain. To the north-west, another Alpine shear zone, the Cremosina line, limits the Baldissero terrains from the rest of the Ivrea-Verbano Mafic Complex. Amphibole-bearing dioritic dykes in the Baldissero mantle peridotite have variable Mg# value in bulk-rock and mafic phases, which positively correlates with the incompatible element concentration. The peridotite at the dyke contact is enriched in modal orthopyroxene, iron and incompatible trace element with respect to the LREE-depleted spinel lherzolites typical of the Baldissero massif. The geochemical characteristics of the diorites are explained by fractional crystallization of a hydrous silica-saturated melt, accompanied by assimilation of peridotite olivine. In this frame, the diorites in equilibrium with the least evolved melts are those with the lowest incompatible trace element concentration and Mg#. The low Mg# argued for the most primitive parent melts of the diorites indicate that they experienced a significant fractionation before the emplacement into the peridotite. New internal Sm-Nd isochrons for two diorites (198±29, with εNdi = +5.6 and 204±31 Ma with εNdi = +6.0) and Re-Os data for peridotites at the dyke contact point to late Triassic - early Jurassic ages. These ages may record subsolidus diffusion on cooling. However, internal Sm-Nd isochrons for hydrous diorites forming the External Gabbro Unit of the Finero Mafic Complex (εNdi from +2.7 to + 5.3; Lu et al., 1997) range from 223 to 231 Ma, and are only slightly different from U-Pb zircon ages (232±2 Ma; Peressini et al., 2005). Therefore, even if the Sm-Nd ages obtained for the Baldissero diorites may be slightly younger with respect to the age of emplacement, the latter should not be, anyway, older than the late Triassic. At the estimated age of the diorite intrusion, the Ivrea-Verbano mantle peridotites were already emplaced in the crust, but the isotopic composition of the diorites does not show any clear evidence of crustal contamination.
The late Triassic - early Jurassic hydrous diorites from the Baldissero and Finero areas represent the first documentation of Mesozoic magmatic activity in the westernmost sector of the Southern Alps. This activity is also supported by the late Triassic zircon ages of peridotites and dykes of the Finero ultramafic mantle sequence, which records a main metasomatic imprint attributed to a hydrous silica-saturated component of crustal origin, deriving from a subducting slab (Zanetti et al., 1999).
The hydrous diorites from the Finero Mafic Complex exhibit, besides the ages, trace element and isotopic compositions similar to those of the Baldissero diorites. The Mafic Complex in the Finero region is in tectonic contact, marked by an ENE high-temperature shear-zone, with the relatively-anhydrous mafic-ultramafic Permian sequences occurring in Val Sesia and Val d'Ossola. Similar high-temperature shear-zones mark the transition between the Baldissero region and the “anhydrous” Ivrea Complex, suggesting that the records of Mesozoic magmatism in the Ivrea-Verbano Zone are presently confined by tectonic lineaments to the southernmost and northernmost portion, respectively.
The evidence of a Mesozoic magmatic activity, its particular location within the Ivrea-Verbano Zone and the hydrous silica-saturated nature of the involved parent melts introduce new complexities in the geodynamic evolution of the westernmost sector of the Southern Alps before the opening of the Jurassic Tethys.
References
Lu et al., 1997. Chem. Geol. 140, 223-235.
Peressini et al., 2005, Epitome, 1, 157.
Zanetti et al., 1999. CMP 58, 345-568.
MAGMATIC ACTIVITY DURING CONTINENTAL BREAKUP AND OCEAN OPENING RECORDED BY THE EXTERNAL LIGURIDE OPHIOLITES (NORTHERN APENNINES, ITALY): IMPLICATIONS FOR A MIXED PERIDOTITE-PYROXENITE MANTLE SOURCE
MONTANINI ALESSANDRA 1, TRIBUZIO RICCARDO 2
presenter's e-mail: alessandra.montanini@unipr.it
1 - DIPARTIMENTO DI SCIENZE DELLA TERRA UNIVERSITA' DI PARMA
2 - DIPARTIMENTO DI SCIENZE DELLA TERRA UNIVERSITA' DI PAVIA
Keywords: MORB petrogenesis, depleted mantle, pyroxenite
Petrogenesis of igneous rocks formed along non-volcanic margins is of special interest because it may shed light on the nature of the magma sources involved during continental break-up and inception of ocean spreading. Previous works have shown that some basalts from the Jurassic Ligurian ophiolites, originated in such a transitional setting, were formed by low partial melting degrees of a MORB-type mantle (e.g. Desmurs et al., 2002). These rocks may be therefore useful candidates to unravel the presence of a non-peridotitic component in the MORB mantle source, which becomes progressively diluted with increasing degree of melting. In particular, remnants of a fossile continent-ocean transition similar to that of the modern non-volcanic continental margins are preserved in the External Liguride (EL) ophiolites, which consist of fertile lherzolites, MOR-type basalts and rare syn-rift gabbros (Marroni et al., 1998; Tribuzio et al., 2004). The mantle peridotites preserve a subcontinental lithospheric origin (Rampone et al., 1995) and are in places characterised by garnet pyroxenite layers (Montanini et al., 2006).
The EL gabbroic rocks include troctolites, olivine-bearing to olivine-free gabbros, Fe-Ti oxide-bearing gabbros and diorites. The gabbros have a cumulus origin and were derived from N-MORB melts, variably evolved through fractional crystallisation. The EL basalts display nearly flat REE patterns (LaN/SmN = 0.8-1.1) and a slight LREE/HREE enrichment (LaN/YbN = 1.0-1.7). They also have high Y/Nb (5-14) and Zr/Nb (20-50), La/Nb > 1 and low Th/Hf (0.05-0.21), as typically observed for N-MORB. Remarkable geochemical feature is the Zr enrichment over neighbouring REE (Zr/Zr* = 1.1-1.6), coupled with a garnet signature represented by high Sm/Yb values (SmDM/YbDM = 1.5-1.8). Nd isotope compositions of both gabbroic rocks and basalts encompass the range of depleted mantle (initial Nd = +7.6 to +10.3). However, the less radiogenic Nd isotope compositions fall at the low end of MORB compositions (Salters & Dick, 2002), thus suggesting the involvement of an enriched component.
Calculations of the aggregated melts formed through low melting degree (5-8%) fractional melting of a mixed spinel peridotite/garnet pyroxenite source, with a small melt fraction (10-15 %) derived from the garnet pyroxenite, are capable to reproduce the trace element fingerprint of EL basalts. A literature data compilation for the MOR-type basalts from the Ligurian Tethys ophiolites, including those formed in a intra-oceanic setting, shows that they are commonly characterised by positive Zr anomaly and high (Sm/Yb)DM values. As a whole, the data are consistent with low to moderate degree of melting of a depleted spinel peridotite source, coupled with a residual garnet signature.
Bodinier & Godard (2003) proposed a mechanism by which relict lithospheric mantle can be entrained into asthenospheric material during thermomechanical erosion of the subcontinental lithosphere by the upwelling asthenosphere. Similarly, recent works on the ophiolitic peridotites from the Alpine-Apennine system have provided evidence for interaction between old lithospheric mantle originally characterized by garnet pyroxenite layers (Piccardo et al., 2004; Tribuzio et al., 2004; Montanini et al., 2006) and the ascending asthenosphere. We can therefore speculate that such a mechanism may have played a role in generating the geochemical features of the MOR-type melts formed during the early phases of ocean opening and subsequent establishment of a slow-spreading ridge system.
Bodinier & Godard (2003) In: Treatise on Geochemistry, Vol. 2, pp. 103-170
Desmurs et al., 2002. Contrib Mineral Petrol 144: 365-382
Marroni et al., 1998. Tectonophysics 292: 43-66
Montanini et al.,2006. J Petrol 47: 1943-1971
Piccardo et al., 2004. Int Geol Rev 46: 1119-1159
Rampone et al., 1995. J Petrol 36: 81-105
Salters & Dick, 2002. Nature 418: 68-72
Tribuzio et al., 2004. J Petrol 45: 1109-1124
METASOMATISM IN THE LITHOSPHERIC MANTLE BENEATH THE CANARY ISLANDS: DOES THIS HAVE BEARING ON CENOZOIC METASOMATISM IN THE CONTINENTAL MANTLE BENEATH WESTERN AND CENTRAL EUROPE?
NEUMANN ELSE-RAGNHILD 1
presenter's e-mail: e.r.neumann@geo.uio.no
1 - UNIVERSITY OF OSLO
Keywords: Canary Islands, Mantle xenoliths, Immiscibility, Carbonaceous silicate melt
In a Nature paper in 1995 Hoernle et al. proposed, on the basis of tomography and radiogenic isotope data, that Cenozoic volcanism in central Europe, the western Mediterranean and eastern Atlantic Ocean is caused by a common, large region of mantle upwelling (a “mega-plume”). In addition to magmatism, this mantle upwelling is believed to have caused metasomatism in the continental lithospheric mantle beneath Europe as well as in the oceanic mantle lithosphere beneath ocean islands in the eastern Atlantic Ocean, including the Canary Islands. This presentation focuses on the effects of this plume on the lithospheric mantle beneath the Canary Islands, hoping that some of the information from that area will be relevant for the upper mantle beneath Europe.
Mantle xenoliths from the Canary Islands are well suited to identify the effects of recent metasomatism. Before metasomatism spinel harzburgite and lherzolite xenoliths from the Canary Islands belonged to a group of ultra-refractory spinel harzburgites which are found in many ocean islands (e.g. Canary Islands, Kerguelen, Cape Verde, Samoa). These rocks are more refractory than average MOR peridotites; their major element and modal compositions are relatively uniform and imply that they have been subjected to a high enough degree of melting to remove clinopyroxene. All cpx presently found in these rocks is secondary, formed by exsolution from orthopyroxene and/or metasomatic melts/fluids.
Metasomatism in the upper mantle beneath the Canary Islands includes formation of minor amounts of phlogopite, melt – wall-rock reactions (e.g. formation of olivine and clinopyroxene at the expense of orthopyroxene), addition of incompatible trace elements (e.g. LREE), CaO, and Na2O, and resetting of the Sr-Nd istopic ratios to values within the range of Canary Islands basalts. Evidence from fluid inclusions indicate that a range of metasomatic fluids formed as the results of immiscible separations, melt – wall-rock reactions, and chromatographic fractionation, from an initial CO2-rich basaltic primary melt, or a carbonaceous silicate melt. The main metasomatic agents appears to be a carbonatitic or carbonaceous melts highly enriched in light REE relative to heavy REE, and depleted in Zr-Hf and Ti relative to REE. The metasomatism mainly acted as open-system processes, meaning that metasomatic melts/fluids ascending through the upper mantle caused reactions and chemical exchange between fluid and wall-rock, leaving a residual fluid which moved to shallower levels. Elements compatible with the wall-rock minerals and/or with phases formed through metasomatic reactions (clinopyroxene and phlogopite) were “trapped” by these minerals, whereas elements incompatible with all phases were preferentially transported away by the residual fluid.
A series of Fe-Al-Ti-rich dunites, wehrlites and clinopyroxenites (and rare Ti-Al-Fe-rich harzburgites/lherzolites) have compositions that imply formation through interaction between peridotite wall-rock and significant proportions of mildly alkaline basaltic melts (similar to Canary Islands magmas); some clinopyroxenites may have formed directly from mildly alkaline magmas. However, interaction between peridotite wall-rocks and basaltic fluids appears to be mainly restricted to magma conduits and the close vicinity of such conduits.
THE SIGNIFICANCE OF THE PHOSPHORAN OLIVINE ON THE PETROGENESIS OF THE GATAIA LAMPROITE, SW ROMANIA
NTAFLOS THEODOROS 1, SEGHEDI IOAN 2
presenter's e-mail: theodoros.ntaflos@univie.ac.at
1 - DEPARTMENT OF LITHOSPHERIC SCIENCES, UNIVERSITY OF VIENNA, AUSTRIA
2 - INSTITUTE OF GEODYNAMICS, STR. JEAN-LUIS CALDERON 19-21, 70201 BUCHAREST, ROMANIA
Keywords: Pannonian-Carpathian region, Lithosphere, Lamproites, Gataia
The Gataia lamproite is a new occurrence located at the southeastern wedge of the Pannonian Basin (Romania). The main petrographical characteristics of the Gataia lamproite are: (1) modal proportions of olivine and leucite are high; (2) relatively abundant are phlogopite, apatite and armalcolite, whereas minor phases include diopside, sanidine, richterite and spinel and (3) Zr-rich glass.
Olivines, as phenocrysts or microcrysts, are zoned with Fo up to 93 in the core and up to 84 in the rim. They have abundant spinel, apatite and melt inclusions suggesting that they are not xenocrysts and the apatite and olivine crystallized simultaneously. Olivine X-ray maps show the presence of phosphorus as fine-scale delicate oscillatory zoning. Occasionally the core is enriched in phosphorus, providing the highest P concentration (P2O5=0.80 wt %). Along the oscillations are growing fine-grained apatites.
The incorporation of phosphorus into silicate minerals is not surprising as from the crystallographic point of view IVP5+ can replace IVSi4+ due to the small differences in their ionic radii. The charge balancing scheme for phosphoran olivine that has been experimentally determined by Boesenberg et al., 2004 is: 4VIM+2 + 2Si +4 2P+5+VI[]. Furthermore they estimated that up to 70% of the tetrahedral sites can be replaced by P. In our case, the incorporation of phosphorus into olivine took place prior to saturation of the lamproitic magma with a phosphate mineral. In addition, compared to other lamproites worldwide, the Gataia lamproite has the lowest CaO content (CaO=3.43 wt %) and is one of the most rich in P2O5 (2.13 wt %). Consequently, the lamproitic magma was oversaturated in P and depleted in Ca preventing the crystallization of apatite and providing the conditions for crystallization of P-rich olivine. The P-oversaturation is continuous up to the groundmass glass generation, which has up to 0.30 wt % P2O5 and only 0.07 wt % CaO.
Whole rock contains 1230 ppm Zr that is concentrated in the glass (ZrO2= 0.26 wt %) explaining the absence of Zr-bearing phases. Zircon has not been observed in lamproites and Zr-bearing minerals as wadeite and/or daylite are recognized as late stage crystallization minerals; their absence in our case suggests a very fast transport to the surface. Such Zr-rich melts can be considered as the metasomatic agent causing anomalous Zr-enrichments in mantle diopsides.
Gataia is located far from the Carpathian subduction system, excluding any relationship to that system. As well, a plume origin for the Gataia lamproite can also be excluded as none of the isotopic compositions shows affinity to HIMU mantle component. The Gataia lamproite appears to be of lithospheric origin related to the extensional processes at the margin of the Pannonian basin. The question whether the source of the potassic volcanism that is common in the Carpathian-Pannonian area could be considered as similar of that of the Gataia lamproite and the Bar leucitite is still open.
Literature
Boesenberg et al., 2004, Lunar and Planetary Science XXV, 1366.pdf
THE MESOZOIC SUBCONTINENTAL LITHOSPHERIC MANTLE BENEATH THE TISZA UNIT (S HUNGARY): UPPER MANTLE XENOLITHS IN LATE CRETACEOUS LAMPROPHYRE FROM VILLÁNY MTS
NÉDLI ZSUZSANNA 1, M.TÓTH TIVADAR 2, SZABÓ CSABA 1
presenter's e-mail: nedlizs@yahoo.com
1 - LITHOSPHERE FLUID RESEARCH LAB, DEPARTMENT OF PETROLOGY AND GEOCHEMISTRY, EOTVOS UNIVERSITY BUDAPEST
2 - DEPARTMENT OF MINERALOGY, GEOCHEMISTRY AND PETROLOGY, UNIVERSITY OF SZEGED
Keywords: upper mantle xenoliths, Late Cretaceous, lamprophyre, Tisza Unit, Carpathian-Pannonian Region
The Mesozoic subcontinental lithosphere beneath the Carpathian-Pannonian Region is poorly known, because upper mantle xenoliths are rare in pre-Neogene volcanics. In contrast, mantle xenoliths from Neogene volcanics have been studied in details in the last decades. Moreover, xenolith studies have been concentrated on localities from the Alcapa Unit (N Hungary), therefore, the subcontinental mantle beneath the Tisza Unit (S Hungary) is quite unknown. Poiana Rusca (Romania) alkali basic rocks and their xenoliths (Downes et al., 1995) held the only direct information about the mantle for the last decade. Therefore, the spinel peridotite xenoliths settled in Upper Cretaceous lamprophyre dykes in the Villány Mts (S Hungary), found in the last decade, offer unique information about the lithospheric subcontinental mantle beneath the Tisza Unit and also make possible to compare it to the mantle beneath the Alcapa Unit.
Villàny Mts xenoliths are porphyritic spinel lherzolites with rare OH-bearing minerals. They display rounded or ovoidal shapes, are of 1 to 5 cm in diameter, consisting of olivine (often altered), enstatite, Cr-diopside and Cr-spinel. They show porphyroclastic texture, ortho- and clinopyroxene are bimodal: they occur as porphyroclasts or fine-grained, equigranular grains. Spherical spinel inclusions are abundant in silicates. Exsolution features also are abundant in pyroxene porphyroclasts.
Porphyroclasts and neoblasts show minute but consequent differences in chemistry indicating that their equilibrium conditions could be different and that the xenoliths suffered cooling and re-equilibration at lower PT conditions. Clinopyroxenes are relatively low in Cr, Al and Na. Compositional tendencies from the porphyroclasts to neoblasts (decrease in Cr, Al, Na and increase of mg#) coincide with the tendencies in the sheared porphyroclastic xenoliths from W Eifel (Witt and Seck, 1987). Orthopyroxene also occur bimodally, porphyroclasts are slightly richer in Cr, Al but lower in Mg, Si and Ti than neoblasts. Compositionally our pyroxenes are very similar to W Eifel sheared porphyroclastic xenoliths and also to mosaic and poikilitic textured xenoliths from the centre of Carpathian-Pannonian Region (Downes et al., 1992; Embey-Isztin et al., 2001). Spinel have low cr# (0.17-0.24) indicating that the mantle suffered low degree of melt extraction (approx. 5-10%).
Thermobarometry also suggests that the xenoliths re-equilibrated at lower PT conditions, probably in the shallower zones of the upper mantle. Porphyroclasts record original temperatures approx. 1000 °C whereas neoblasts indicate a re-equilibration at slightly lower temperatures. Extended exsolution features in porphyroclasts can be explained by the long-time residence and cooling at ambient conditions after emplacement.
These observations provide further evidence of the previously suggested lower temperature for the mantle beneath the Tisza block than the Alcapa (Szabó et al., 1995) moreover hold the first records of metasomatic effects beneath the Tisza block. Strong textural and chemical similarities of studied xenoliths to sheared porphyroclastic xenoliths from the W Eifel and mosaic/poikilitic textured xenoliths from the centre of the Carpathian-Pannonian Region likely suggest their similar evolution, in relation with a diapiric uplift or tectonic emplacement of hot mantle material into a colder upper mantle. Our hypothesis would require to suppose the presence of a mantle upwelling before the Late Mesozoic beneath the Tisza block, not described before.
References
Downes H, Embey-Isztin A, Thirlwall MF (1992) Contrib Mineral Petrol 109: 340-354
Downes H, Vaselli O, Seghedi I, Ingram G, Rex D, Coradossi N, Pécskay Z, Pinarelli L (1995) Acta Vulcanol 7: 209-217
Embey-Isztin A, Dobosi G, Altherr R, Meyer HP (2001) Tectonophysics 331: 285-306
Szabó Cs, Harangi Sz, Vaselli O, Downes H (1995) Acta Vulcanol 7: 231-240
Witt G, Seck HA (1987) J Petrol 28: 475-493
ARCHEAN LITHOSPHERIC MANTLE: THE REFERTILISED REMAINS
O'REILLY SUZANNE Y. 1, GRIFFIN WILLIAM L. 1, BEGG GRAHAM 2
presenter's e-mail: sue.oreilly@mq.edu.au
1 - GEMOC, Dept of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia
2 - GEMOC, Dept of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia and BHP Billiton, Level 34 Central Park, 152-158 St Georges Tce, Perth, WA 6000, Australia
Keywords: Archean lithosphere, Lithosphere refertilisation, Mantle tomography, Mantle composition, Crustal growth rates
The composition of subcontinental lithospheric mantle (SCLM) varies broadly with the age of the last major tectonothermal event in the overlying crust. Archean SCLM is highly depleted, commonly is strongly stratified, and contains rock types absent in younger SCLM. Phanerozoic terrains are generally underlain by fertile mantle, and most Proterozoic SCLM is intermediate between these two extremes. This secular evolution in SCLM composition implies quasi-contemporaneous formation (or modification) of the crust and its underlying mantle root.
The "typical" Archean mantle composition used in geochemical/geophysical modelling is a depleted garnet lherzolite. This composition is derived from peridotite xenoliths in kimberlites, mainly from the SW Kaapvaal Craton, and a few from Siberia. However, most such "typical" Archean xenoliths have experienced repeated metasomatism, leading to a progression from dunite/harzburgite through "depleted" lherzolite to "fertile" lherzolite, mirroring the secular evolution of the SCLM as a whole. Similar refertilisation processes can be studied in situ in peridotite massifs (eg Western Norway, Lherz), showing the lherzolites to be the product of melt infiltration into magnesian dunite/harzburgite protoliths. These rocks are poorly represented in the published xenolith record; the bias partly reflects the collecting of rocks useful for P-T studies, but also has a geological basis. High-resolution seismic tomography of Archean cratons shows high-Vs volumes surrounded and dissected by zones of lower Vs.
The low-Vs parts can be modelled using the "typical" garnet lherzolite compositions, while the higher-Vs volumes require much more depleted rocks. In detail, kimberlites avoid the high-Vs volumes to preferentially follow older zones of fluid passage and metasomatism, hence biasing our "mantle sample" toward the metasomatic products. A revised estimate of the composition of "original" Archean SCLM yields a dunite/harzburgite with 49% MgO, 6.6% FeO, 0.4% Al2O3, 0.34% Cr2O3 and 0.2% CaO. Seismic tomography suggests that this material still underlies the bulk of Archean cratons to depths of 150-200 km, but is poorly sampled by kimberlites. Relict Archean mantle is also imaged as buoyant high Vp blobs in oceanic regions, a likely source for reported "recycled" geochemical signatures in some ocean island basalts, and providing evidence of mechanisms of continental breakup Hf-isotope data on zircons show that much Proterozoic crust, especially in cratonic areas, has Archean protoliths, suggesting that the underlying SCLM also was originally Archean. Seismic tomography shows high-Vs roots, requiring depleted compositions and low geotherms, under many of these areas; clearly juvenile Proterozoic belts (eg SW Scandinavia) do not have such roots. Re-Os isotopic data for the underlying mantle record similar events, indicating a linked tectonic history. These observations suggest that much of the observed secular evolution in SCLM composition reflects progressive reworking of buoyant Archean SCLM, rather than secular changes in the mechanisms of SCLM production. Seismic tomography suggests that 50% of existing continental crust is underlain by relict Archean SCLM, modified to varying degrees. This implies a much larger volume of originally Archean crust than currently accepted, and hence very high early crustal growth rates.
Melt-modelling exercises that treat "typical" Archean peridotites as simple residues are invalid, and cannot be used to support "lithosphere stacking" models for SCLM formation. The "primitive" Archean dunites/harzburgites are best modelled as restites/cumulates from high-degree melting at 3-6 GPa, in ascending plumes/mantle overturns. This uniquely Archean regime may have coexisted with a more modern plate-tectonic regime, which produced weakly depleted residues similar to Phanerozoic SCLM. This "modern" SCLM would be inherently unstable, easily recycled and lost to the modern record.
LINKING CRUSTAL AND MANTLE EVENTS USING IN-SITU U-PB, LU-HF AND RE-OS ISOTOPE ANALYSIS
PEARSON NORMAN 1, O’REILLY SUZANNE Y. 1, GRIFFIN WILLIAM L. 1, ALARD OLIVIER 1, BELOUSOVA ELENA 1
presenter's e-mail: npearson@els.mq.edu.au
1 - Macquarie University
Keywords: geochronolgy, Multi-collector ICP-MS, Lithospheric mantle, sulphide, zircon
Over the past decade significant advances have been made in geochronology and geochemistry with the development of a variety of methods for precise in-situ analysis of trace element compositions and isotope ratios. These advances are primarily the product of coupling a laser ablation microprobe to the induced coupled plasma mass spectrometer (ICP-MS) and the rapid development of the multi-collector (MC-) ICP-MS. In-situ analysis also allows the isotopic data to be interpreted within a microstructural context and in the framework of geochemical data from other microanalytical techniques. The integration of multiple datasets is not only essential to constrain the origin of a sample, but also to unravel the processes that have subsequently modified it.
In-situ analysis of Re-Os isotopes in sulfides in mantle-derived peridotites provides a method for constraining the timing of events within the lithospheric mantle. The sulfides are ‘time-capsules’: like zircons in crustal rocks they record many events in the lithospheric mantle. Apart from dating the depletion events that formed the volume of lithosphere the sulfides provide constraints on a range of processes that might modify the mantle such as the addition of metasomatic fluids during lithosphere reworking. The analysis of individual grains of sulfide indicates that there are multiple generations of sulfides in most mantle peridotites and whole-rock Re-Os ages reflect a mix of these different sulfide populations. In many samples the in-situ data yield older ages for original lithospheric mantle stabilization. The mixtures also reflect the end-product of multiple melting and metasomatic events in the lithospheric mantle. Age-relative probability diagrams can be produced using sulfides that are interpreted to be monosulfide solid solutions that represent residual phases from partial melting or that crystallized from sulfide melts. These ‘age’ spectra for the events in the sub-cratonic lithospheric mantle commonly mirror temporal signatures for thermal and tectonic events in the overlying crust. The correspondence of Re-Os model ages and crustal events in young terranes is heavily influenced by the large populations of sulfides with negative model ages.
Integration of age information from the lithospheric mantle and overlying crust can be used to establish linkages between the two and further our understanding of large-scale geodynamic processes. The importance of zircon as a ‘time-capsule’ has been reinforced by the development of in-situ Hf isotope measurements using the MC-ICP-MS. The combination of U-Pb dating of zircons with their trace element patterns and Hf isotopes is a powerful technique for understanding crustal evolution. TerraneChron® applies this approach to study detrital zircons from modern drainages or sedimentary rocks to construct records of the addition of juvenile material and reworking from the terrane to continent scale. Integrated studies of zircons in lower-crustal xenoliths and of sulfides in mantle-derived xenoliths, in both cratonic and younger off-craton settings, provide new insights into processes of continental generation, tectonism and destruction.
MANTLE STRUCTURE AND COMPOSITION BENEATH THE TYRRHENIAN SEA AREA: EVIDENCE FROM PETROLOGY AND GEOCHEMISTRY OF PLIO-QUATERNARY MAGMATISM
PECCERILLO ANGELO 1
presenter's e-mail: pecceang@unipg.it
1 - Dipartimento di Scienze della Terra, Università di Perugia
Keywords: Upper Mantle, Italy, Quaternary Magmatism, Geochemistry
Plio-Quaternary magmatism in Italy shows an extreme compositional variability, from sub-alkaline to ultra-alkaline, from mafic to felsic and from strongly undersaturated to oversaturated in silica. Trace element and radiogenic isotope (Sr, Nd, Pb, Hf) signatures are also variable and span almost entirely the spectrum of magmatic rocks occurring world-wide. Petrological and geochemical data of mafic rocks allow distinguishing various magmatic provinces, which differ from each other for major and/or trace elements and/or isotopic compositions. The Tuscany Province (14-0.2 Ma) consists of silicic crustal anatectic magmas, of mantle-derived calcalkaline to ultrapotassic mafic rocks and of mixtures between crustal-derived melts and various types of mantle-generated magmas. The Latium Province (or Roman Province s.s., 0.8 to 0.03 Ma) is formed of huge multi-center volcanic complexes (Vulsini, Vico, Sabatini. Albani), which erupted potassic and ultrapotassic magmas showing extreme enrichments in Large Ion Lithophile Elelements (LILE) and high but moderately variable isotopic signatures (e.g. 87Sr/86Sr ~ 0.7100-0.7105). The Umbria-Abruzzi province consist of a few monogentic kamafugitic volcanoes similar trace element and radiogenic isotope signatures as the Latium volcanoes. The Campania Province (0.2 Ma to present) is formed by potassic to ultrapotassic rocks having lower LILE concentrations and Sr isotopic ratios, but higher High Field Strength Elements (HFSE) than Latium volcanoes. Typical calc-alkaline rocks, about 2.0 Ma old, have been found by deep borehole drillings beneath active Campanian volcanoes. Ernici-Roccamonfina rocks (0.7-0.1 Ma) have very variable trace element and isotopic compositions that span both Latium and Campania compositions. The Aeolian Arc Province (1 Ma to present) mainly consists of calcalkaline to shoshonitic rocks. The Sicily Province contains young to active centres (notably Etna) with a tholeiitic to OIB-type Na-alkaline affinity. Finally, volcanoes of variable composition occur in Sardinia and, as seamounts, on the Tyrrhenian Sea floor. Magmas in the Aeolian arc and along the Italian peninsula have high values of LILE/HFSE ratios resembling subduction-related rocks, whereas the Sicily and Sardinia display lower LILE/HFSE, close to intraplate signatures. Intraplate and orogenic volcanics coexist on the Tyrrhenian Sea floor.
The geochemical and isotopic complexities of Plio-Quaternary magmatism reveal that the upper mantle beneath Italy is extremely heterogeneous and consist of a mosaic of compositionally distinct domains, displaying both "orogenic" and "anorogenic" compositions. Such a compositional heterogeneity originated from mixing different types of crustal material and various types of lithospheric and asthenospheric mantle rocks. Geophysical and geochemical evidence does not support a role of deep mantle plumes. Mantle modifications probably occurred during the subduction processes which affected the western Mediterranean area from Oligocene to present .
THE LIGURIA MODE, A MANTLE PETROLOGY – BASED CONCEPTUAL MODEL FOR THE FORMATION OF THE JURASSIC LIGURIAN TETHYS.
PICCARDO GIOVANNI BATTISTA 1
presenter's e-mail: piccardo@dipteris.unige.it
1 - Dipartimento per lo Studio del Territorio e delle sue Risorse, Universita' di Genova
Keywords: Jurassic Ligurian Tethys, Mantle petrology, Tectonic-metamorphic evolution, Asthenosphere partial melting, Lithosphere melt percolation
An ultra-slow spreading ridge class has been recently established on the basis of the investigations of the South-West Indian and Arctic Ridges. It is characterized by intermittent volcanism and continuous sea-floor mantle emplacement over large regions, whereas the spreading rate is approximately lower than 20 mm yr -1 (Dick et al. 2003, and references therein). Distinctive petrologic features of ultra-slow spreading ridges are: (1) the relative abundance of mildly enriched or alkaline basalts, related to melting of garnet-eclogite or veined mantle sources, (2) the strong compositional variability of mantle peridotites, related to interaction with percolating melts (e.g. Hellebrand et al., 2006; von der Handt et al., 2006).
Ophiolites derived from the Jurassic Ligurian Tethys show major structural and petrologic characteristics (i.e. mantle at the sea-floor and alternance of volcanic and a-volcanic segments, coupled with abundance of strongly heterogeneous peridotites and presence of alkaline melts) that allow to interpret the Ligurian Tethys as a Jurassic analogue of modern ultra-slow spreading oceans (Piccardo, 2007, and references therein).
A conceptual model for the tectonic evolution of magma-poor rifted margins have been proposed by Lavier & Manatschal (2006), mostly based on the behaviour of the crust [i.e. (1) stretching and (2) thinning modes]. Exhumation of the lithospheric mantle is referred to the last (3) exhumation mode, and mantle serpentinization is believed to be the key mechanism to allow for mantle thinning and exhumation, in the absence of magmatic activity to weaken the lithosphere.
Present knowledge on Alpine-Apennine ophiolitic peridotites document that mantle lithosphere stretching and break-up were accomodated by inter-dependent tectonic and magmatic processes. In fact, during the pre-oceanic rifting stages of the basin: (1) lithosphere extension caused deformation and tectonic-metamorphic evolution of the subcontinental mantle, starting from spinel-facies conditions, which were relevant to lithosphere thinning and subsolidus exhumation of the lithospheric mantle; (2) lithosphere stretching caused adiabatic upwelling and decompression melting of the underlying asthenosphere; (3) asthenospheric melts migrated through the extending mantle lithosphere; (4) heating by asthenosphere upwelling and reactive melt percolation caused the thermo-chemical erosion of the mantle lithosphere, which was relevant to weaken the mantle lithosphere and to enhance the transition from distributed continental deformation to localised oceanic spreading (Piccardo, 2007; and references therein).
Mantle petrology was significantly different at the different palaeogeographic settings of the Ligurian Tethys, from the ocean-continent transition (OCT) zones to the more internal oceanic (MIO) settings. Across-axis variation of mantle petrology evidences the different evolution stages of the basin: (1) The rifting (continental) stage, characterized by subsolidus evolution of the sub-continental lithospheric mantle, which is recorded by the exhumed subcontinental peridotites cropping out at the ocean-continent transition (OCT) zones; (2) The drifting state, characterized by appearance of MORB melts in the extending lithospheric mantle (i.e. asthenosphere decompression melting and lithosphere melt percolation), which is recorded by the percolated subcontinental peridotites cropping out at more internal oceanic (MIO) settings; (3) The spreading (oceanic) stage, characterized by sea-floor emplacement of refractory peridotites, cogenetic with the Jurassic MORB melts.
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Piccardo, G.B., 2007. Geol. Soc. London, Special Pubblications (in press)
von der Handt, A., Snow, E.J. & Hellebrand, E., 2006. Ofioliti, 31(1), 248-249.
GEOCHEMICAL FINGERPRINTING: VARIATIONS IN LITHOSPHERIC MANTLE SIGNATURES WITHIN AND BETWEEN TERRANES
POWELL WILLIAM 1, O’REILLY SUZANNE Y. 1
presenter's e-mail: wpowell@els.mq.edu.au
1 - GEMOC - MACQUARIE UNIVERSITY
Keywords: Mantle metasomatism, Lithospheric mantle composition, Mantle peridotite, Re-Os isotopes
Geochemical signatures in mantle xenoliths reflect a range of processes including initial formation, melting history, and the type and extent of metasomatic processes affecting a specific mantle volume. Geochemical signatures may vary both within and between domains of different tectonic style mapped in the overlying crust, and between lithospheric sections affected by different geodynamic regimes. Detection and recognition of geochemical signatures in mantle samples, including whole-rock and mineral major, trace-element and isotopic chemistry is an essential part of unravelling the history of different mantle processes through time on a variety of scales.
A suite of mantle xenoliths from the easternmost structural element of central eastern Australia (the New England Orogen) has been studied in order to characterise the composition, architecture and evolution of sub-continental lithospheric mantle underlying the region. The New England Orogen represents a Palaeozoic-Early Mesozoic convergent margin setting, and is typically divided into two crustal domains separated by a fault system and delineated by a serpentinite belt, interpreted to represent the suture along which subduction took place. The eastern domain is considered to represent a microcontinent accreted to the Australian margin during subduction, and the western domain consists of continental shelf material inboard of the subduction zone. Mantle-derived peridotite xenoliths from six widely spaced localities across the orogen have been used to help understand the composition, architecture and evolution of sub-continental lithospheric mantle beneath the region, to assess regional mantle heterogeneity, and to determine if there are systematic differences in the lithospheric mantle underlying the two crustal domains.
Geochemical signatures of clinopyroxenes in the xenoliths indicate variations in the melting history and the type and extent of metasomatism in the lithospheric mantle both within and between the two crustal domains. Xenoliths from one locality in the western domain can be divided into two groups showing distinctly different metasomatic characteristics – one has been metasomatised by a dominantly silicate agent, the other a carbonatitic agent. Samples from another locality < 80 km distant, also in the western domain, have clinopyroxene trace-element signatures indicating progressive chromatographic metasomatism by an evolving silicate fluid at low fluid-rock ratios. The presence of amphibole in xenoliths from a third locality (~200 km away, in the eastern domain) provides evidence for modal metasomatism. In these samples, amphibole is a significant ‘other’ host phase for many trace-elements (other than clinopyroxene), allowing the bulk rock to contain higher concentrations of (for example) high field-strength elements than the amphibole-free mantle wall-rock peridotites, and affecting the geochemical signature of the coexisting clinopyroxene due to its strong partitioning of certain elements.
In situ and whole-rock Re-Os isotopic data have been used to provide age constraints on the New England sub continental lithospheric mantle. Rhenium depletion model ages based on in situ analysis of sulfide grains suggest different localities have different age signatures, and show that lithospheric mantle beneath the New England region contains material of at least Proterozoic age. Comparisons between whole-rock and sulfide in situ Re-Os data show that multiple populations of sulfides may be present in a given sample, and consequently, that whole-rock Re-Os data represent a mixture of these populations and must be regarded as giving only minimum age constraints.
NEW FINDINGS OF LAMPROPHYRIC ACTIVITY WITHIN THE SUBBETIC ZONE OF THE BETIC CORDILLERA AS EVIDENCED BY DRILLINGS IN THE CERRO PRIETO LOCALITY
PUGA RODRIGUEZ ENCARNACION 1, BECCALUVA LUIGI 2, BIANCHINI GIANLUCA 2, DÍAZ PUGA MIGUEL ANGEL 3, GALINDO-ZALDÍVAR JESÚS 4, A. DÍAZ DE FEDERICO ANTONIO 1, WIJBRANS JAN 5
presenter's e-mail: epuga@ugr.es
1 - Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR)
2 - Università di Ferrara
3 - Universidad de Almeria
4 - Universidad de Granada
5 - Vrije Universiteit Amsterdam
Keywords: Lamprophyres, Mantle metasomatism, Ultramafic xenoliths, Betic Cordilleras, Subbetic Zone
Two drillings in the Cerro Prieto locality (Málaga province) highlight the existence of a magmatic body formed by rock-types previously unknown in the Subbetic zone. This body, according to geophysical data, is formed by basic rocks and have a lenticular form with about 200 m of lateral extension and 20 m of thickness.
The recovered rocks show a predominant porphyric texture, formed by olivine, clinopyroxene and phlogopite phenocrysts, up to 1-2 mm in size, in a microcrystalline to hypocrystalline groundmass composed by alkali-feldspar, clinopyroxene, phlogopite and abundant magnetite. According to the observed textures and modal composition, these rocks, emplaced in hypoabyssal conditions, could be classified as potassic lamprophyres. Major element composition reveals high-Mg (Mgv 0.67-0.70) trachybasaltic composition, with Na2O/K2O ranging between 1.0 and 1.4. Primordial mantle normalized trace element patterns are typical of intra-plate alkali-basalts, and closely approach some of the Permian lamprophyres recognised in other parts of the Iberian Peninsula (Villaseca et al., 2004; Perini et al., 2004). Their enrichment in the most incompatible elements coupled with their low HREE contents (LuN=6-8.6) suggest that these magmas were generated by low melting degree (< 10%) of deep (garnet-bearing) highly metasomatized portions of the lithospheric mantle.
Radiometric dating of these rocks is problematic, as Ar/Ar on phlogopite separate indicate an age of 217 ± 2.5 Ma (with an MSWD value of 8.4), whereas Ar/Ar and K/Ar dating of the groundmass gave a series of ages ranging from 160 to 29 Ma, thus indicating that the Ar system has been profoundly disturbed.
The Cerro Prieto lamprophyres contain numerous centimetric xenocrysts of sericitized sodic plagioclase corroded by the magma, and centimetric xenoliths formed by diopside aggregates. Millimetric inclusions of polycrystalline calcite are also common. The latter display reactions with the host rock, with formation of hydro-andradite garnet, diopside and phlogopite crystals.
It has to be noted that the same drillings also exhumed some enigmatic lithotypes constituted by a prevalent carbonate matrix that surround some millimetric crystals of mantle provenance such as Cr-spinel and Cr-diopside, and scarce Opx, Ol, as well as corrensite and hydro-andradite. The hypothesized pre-existence of garnet in these mantle parageneses is suggested by the presence of rounded aggregates, up to one centimetre in diameter, formed by symplectitic textures, consisting of an intimate association of vermicular crystals of chromian spinel and pyroxenes. Similar symplectitic intergrowths have been interpreted by different authors as the result of garnet destabilization in mantle peridotites (Mercier & Nicolas, 1974; Piccardo et al., 2004).
References
- Mercier J-C. C., Nicolas, A. (1975) Textures and fabrics of Upper-Mantle peridotites as illustrated by xenoliths from basalts. Journal of Petrology 16, 2, 454-487.
- Perini G., Cebria J.M., Lopez-Ruiz J., Doblas M. (2004). Carboniferous-Permian mafic volcanism in the variscan belt of Spain and France: implication for mantle sources. In “Wilson et al. Eds. Permo-Carboniferous Magmatism and Rifting in Europe. Geological Society, London. Special Publication 223, 415-438.
- Piccardo G.B., Müntener O., Zanetti A., Romairone A., Bruzzone S., Poggi E., Spagnolo G. (2004) The Lanzo South peridotite: Melt/peridotite interaction in the mantle lithosphere of the Jurassic Ligurian Tethys. Ofioliti 29, 1, 37-62.
- Villaseca, C., Orejana, D., Pin, Ch., López García, J.A., Andonaegui, P. (2004). Le magmatisme basique hercynien et post-hercynien du Système central espagnol : essai de caractérisation des sources mantelliques. Comptes Rendus Geoscience 336, 877-888.
MANTLE AND ERUPTIVE METASOMATISM OF PERIDOTITE XENOLITHS BY HOST NEPHELINITE – AN EXAMPLE FROM KSIEGINKI (SW POLAND)
PUZIEWICZ JACEK 1, KOEPKE JÜRGEN 2
presenter's e-mail: jpuz@ing.uni.wroc.pl
1 - UNIVERSITY OF WROCLAW, POLAND
2 - UNIVERSITY OF HANNOVER, GERMANY
Keywords: mantle xenoliths, SW Poland, phase relationships, metasomatism
The Lower Silesian part of the Central European Tertiary Volcanic Province in SW Poland erupted basanites, nephelinites and basalts between 30 and 20 My (Oligocene-Pliocene), forming ca. 300 isolated occurrences. Mantle xenoliths are numerous in some places; they are mostly anhydrous (only one amphibole-bearing xenolith locality has been documented). The Księginki nephelinite contains the greatest abundance of mantle xenoliths in Lower Silesia. Their sizes reach a few decimetres, and their compositions range from dunites to websterites and clinopyroxenites.
The Księginki peridotites typically consist of protogranular olivine ± orthopyroxene ± sparse clinopyroxene domains and interstitial clinopyroxene ± spinel. Some xenoliths are deformed with reduced grain sizes and foliation due to alternating layers a few mm thick showing different grain sizes. Droplets of devitrified glass are common and glass pools a few mm in diameter occur in some xenoliths.
The interstitial clinopyroxene consists usually of low-calcic cores (0.7-0.8 atoms Ca pfu) surrounded by thin, spongy rims of higher Ca (0.8-0.9 a pfu). The spongy rims correspond compositionally to the cores of nephelinite phenocrysts, suggesting that they originated due to contact between the peridotite and nephelinite magma, concomitantly with crystallization of clinopyroxene phenocrysts in the nephelinite. In some xenoliths the spongy rim shows an overgrowth of even higher Ca (> 0.9 a pfu) clinopyroxene, identical to that in the groundmass of the host nephelinite. Its appearance correlates with a change of the crystallizing spinel phase from Mg-Al-Cr spinel to magnetite-ulvöspinel. Therefore, the high-Ca overgrowths crystallized during or after eruption of the nephelinite host. Only the low-Ca cores are remnants of the mantle from before contact with nephelinite magma. Parts of peridotites with significant spongy and high-Ca clinopyroxene are usually devoid of orthopyroxene and contain low-Mg olivine (Fo86.0-88.6). Orthopyroxene, if present, is slightly less magnesian and more aluminous (#mg = 0.89, Al 0.16 a pfu, O2- = 6) relative to that in peridotites with more forsteritic olivine (Fo89.5-92.0), characterized by #mg = 0.91 and 0.10 – 0.14 atoms Al pfu.
The cores of large olivine grains in protogranular domains contain 400 – 600 ppm Ca. Olivine which is recrystallized due to deformation contains 900 – 1100 ppm Ca. Small grains which originated contemporaneously with high-Ca clinopyroxene contain 1400 – 2000 ppm of Ca, values typical for olivine phenocrysts in the host nephelinite. The amount of Ca in olivine is not related to its forsterite content. Cr/(Cr+Al) in spinel is very variable (0.24-0.62) and positively correlated with forsterite content in associated olivine. Temperatures of equilibration between ortho- and clinopyroxene (Brey and Köhler algorithm) cluster around 1100 ºC.
The peridotites sampled by the Księginki volcano were affected by interaction with nephelinite magma both at mantle depths and during eruption. At mantle depths, this led to metasomatism, manifested by (1) lowering of Fo content in olivine down to 86 %; (2) crystallization of spongy clinopyroxene rims enriched in Ca and (3) enrichment of orthopyroxene and spinel in Al. During eruption the peridotite xenoliths contained drops or small pools of melt, from which interstitial low-pressure clinopyroxene crystallized. The mineral assemblages older than the nephelinite interaction are preserved as protogranular domains of low-Ca ( 600 ppm) and high Fo (89.5-92.0) olivine, low-Ca clinopyroxene and low-Al high-Mg orthopyroxene.
CHARACTERISATION OF THE METASOMATIC AGENT IN MANTLE XENOLITHS FROM DEVES, MASSIF CENTRAL (FRANCE) USING COUPLED IN-SITU TRACE- ELEMENT AND O, SR, ND ISOTOPIC COMPOSITIONS
RENAC CHRISTOPHE 1, TOURON STEPHANIE 1, COTTIN JEAN-YVES 1, O' REILLY SUZANNE Y. 2, GRIFFIN WILLIAM L. 2
presenter's e-mail: christophe.renac@univ-st-etienne.fr
1 - Department of Geology-UMR 6524 "Magmas et Volcans", University of Jean Monnet, Saint-Etienne Cedex, France
2 - GEMOC ARC National Key Centre, Earth and Planetary Sciences, Macquarie University, Australia
Keywords: Mantlemetasomatism, Massif Central, In situ trace element, Stable and radiogenic isotopes
Spinel lherzolites and harzburgites from Mont-Briançon and Marais de Limagne in the Devès volcanic district display coarse-grained to porphyroclastic microstructures and the modal content of volatile-bearing phases increases with the degree of deformation. Clinopyroxene and/or spinel are partly or totally reacted to amphibole. The coupled interpretations of traces, REE and O-Sr-Nd data on clinopyroxene and amphibole indicate that the metasomatised mantle beneath the Devès is a mixture of depleted and enriched mantle associated with an alkaline, HFSE-poor, LREE-, U- and Th- rich compositions of carbonate-rich silicate fluid/melt metasomatic agent. Oxygen isotopes and REE data of clinopyroxene-amphibole pairs indicate an [La/Yb]N enrichment related to an increasing metasomatic agent/rock ratios.
OROGENIC GARNET PERIDOTITES AS TRACERS OF MANTLE WEDGE TRANSFORMATIONS AND METASOMATISM BY SUBDUCTION FLUIDS
SCAMBELLURI MARCO 1, HERMAN JOERG 2, MORTEN LAURO 3, PETTKE THOMAS 4, RAMPONE ELISABETTA 1, VAN ROERMUND HLM 5
presenter's e-mail: marco.scambelluri@dipteris.unige.it
1 - Dipartimento per lo Studio del Territorio e delle sue Risorse, Università di Genova, Italy
2 - Research School of Earth Sciences, Australian National University, Canberra, Australia
3 - Dipartimento di Scienze della Terra, Università di Bologna, Italy
4 - Institute of Geological Sciences University of Bern, Switzerland
5 - Faculty of Earth Sciences, University of Utrecht, Netherlands
Keywords: Subduction metasomatism, Garnet peridotites, fluids
At subduction zones, the fluid phase released by the dehydrating subducting plates is the viable agent for slab-to-mantle element transfer, mantle metasomatism and melting. So far, much work has been focussed on the slabs and still few are the observations of mantle wedge peridotites, which are the least known pieces of the subduction factory. For this purpose, information can be gained by the study of orogenic garnet peridotites which, in several occurrences, are slices of mantle wedge tectonically sampled by the subducted continental slabs and which disclose the physical and chemical changes affecting the mantle in a 100-200 km depth range. Here we show that orogenic garnet peridotites record influx of incompatible element-rich fluid phases sourced from the continental crust.
A key case-study is represented by the Ulten Zone (Italian Eastern Alps) peridotite bodies, enclosed in Variscan high-pressure migmatites and recording transformation of porphyroclastic spinel peridotites (T=1200 °C; P=1.5 GPa) into fine-grained garnet + amphibole peridotites (T=850 °C; P=3 GPa). This occurred in response to corner-flow inside a mantle wedge and to slicing of the wedge peridotites into a subducted continental slab. In this frame, the garnet + amphibole peridotites fromed by fluid phase infiltration once the wedge peridotites became close to the subducting slab. However, the spinel and the garnet + amphibole peridotites show similar LREE, LILE and light element- enriched signatures, which imply the recycling of crustal components in various wedge domains. The garnet + amphibole peridotites display high LILE-HFSE fractionation (cpx Pb/Nb from 391 to 443), and variable LILE and LREE enrichments, clearly reflecting addition of a crustal component to formerly depleted mantle rocks.
The coupled increase of water and incompatible elements in the garnet + amphibole peridotites indicate that metasomatism was caused by crust-derived aqueous fluids. Since crustal rocks were undergoing high-pressure partial melting, a mechanism to extract water from hydrous granitic melts must be envisaged.
The ultradeep garnet peridotites and websterites from the Western Gneiss Region of Norway record an old stage of upwelling and accretion to a cratonic lithosphere of depleted Archean transition-zone mantle (350 km depth). This ultradeep mantle was involved in a 430 Ma-old Scandian subduction causing formation of a new clinopyroxene + orthopyroxene + phlogopite + garnet + spinel + carbonate assemblage hosting microdiamond inclusions precipitated by circulating fluids.
New observations show that majoritic garnet was precipitated at grain boundaries and in microveins at 7 Gpa and 900-100 °C by COH silicate fluids during the Scandian subduction. The trace element patterns of the UHP subduction minerals are significantly incompatible element-enriched: the main repository being phlogopite. These features indicate an influx of crust-derived fluid phases at diamond- to majorite-facies conditions. The UHP cpx stable with majorite has trace element patterns well comparable with those of cpx from the lower pressure Ulten Zone garnet + amphibole peridotites. Although preliminary, this comparison shows that mantle re-fertilization by subduction of continental crust is an effective mechanism working on a large depth range. Despite partial melting processes in the crust, fluids appear as the major mobile agents for mantle wedge metasomatism close to subducting slabs over a 100-200 km interval.
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EVOLUTION OF LITHOSPHERE BENEATH THE EASTERN TRANSYLVANIAN BASIN (CARPATHIAN-PANNONIAN REGION): IMPRINT IN GARNET PYROXENITE XENOLITHS
TÓTH ATTILA 1, DOBOSI GÁBOR 2, BALI ENIKO 3, DOWNES HILARY 4, SZABÓ CSABA 1, NTAFLOS THEODOROS 5
presenter's e-mail: totattila@yahoo.com
1 - Lithosphere Fluid Research Lab, Eötvös University Budapest, Hungary
2 - Hungarian Academy of Science, Institute for Geochemical Research, Hungary
3 - Bayerisches Geoinstitut, Universität Bayeruth, Germany
4 - School of Earth Sciences, Birkbeck College, University of London, U.K.
5 - Department of Petrology, Geozentrum, Universität Wien, Austria
Keywords: garnet pyroxenite xenoliths, symplectite, lithospheric upwelling, Transylvanian Basin, Carpathian-Pannonian Region
Ultramafic xenoliths hosted in the Neogene-Quaternary alkaline basalts of the Carpathian-Pannonian Region (CPR) have already been studied by several authors (e.g., Szabó et al., 2004), however most of these xenoliths are lithologically peridotites. Vaselli et al. (1995) have reported garnet pyroxenite xenoliths from the Eastern Transylvanian Basin (ETB) but none of them have carried out a detailed study on these rocks. The Perºani Mts are situated in the ETB, which is the easternmost Plio-Pleistocene alkaline basaltic volcanic field in the CPR and is nearby to the East Carpathian subduction zone.
Pyroxenite xenoliths are composed of orthopyroxene (opx), clinopyroxene (cpx), garnet (grt) ± spinel (spl). The studied xenoliths have been identified as garnet websterites and one garnet clinopyroxenite. Garnet websterites have deformation features such as porphyroclastic texture; cracks, sub-grain formation and exolution lamellae in ortho- and clinopyroxene, whereas the garnet clinopyroxenite has cumulate texture. In each xenolith garnets are surrounded by different generations of symplectites composed of opx+cpx+spl±plagioclase (plg).
Symplectite type-I is situated furthest from the garnet, and could be observed only in garnet websterites. In these symplectites the pyroxene grains are rounded and have a size between 200-500 m, the spinels have vermicular, rounded to euhedral shape. This is the oldest symplectite generation. Symplectite type-2 is situated between the outer smp1 and the garnet.The shape of the pyroxene grains in this generation is vermicular and they contain vermicular spinel inclusions. The size of the grains of smp2 ranges between 20-200 m. These vermicular grains are perpendicular to the garnet surface. Symplectite type-3 is situated along the cracks in the garnets and at the margin of the garnets. This type forms ~100 m thick bands with fibrous textured. Symplectites have not usually consumed more than 10% of the garnet.
The primitive mantle normalized REE-patterns of bulk xenoliths are similar to that of N-MORB with slight depletion in LREE and elevated HREE concentration. Our model calculations suggest that the melt from which the garnet pyroxenites crystallized were generated by a > 5% partial melting of garnet peridotite source. In contrast, the host alkaline basalts could have formed by a 1-3% partial melting of a similar source rock. Such high degree (>5%) partial melting is characteristic for rifting environments, which is consistent with the MORB-like trace element character of the bulk xenoliths.
Based on the composition of rock forming minerals and the widely accepted phase equilibria for pyroxenites, we can estimate the formation conditions of the studied rocks. The estimated equilibrium P-T range is 1.4-1.7 GPa and 1030-1140°C. The obtained P and T reflect that the rocks formed at 45-50 km depth.
References:
Falus et al. (2000): Terra Nova, 12, 295-302.
Szabó et al. (2004): Tectonophysics. 393, 119-137.
Vaselli et al. (1995): J. Petrol., 36, 23-55.
The presence of the different symplectite generations suggests a multistage evolution of the studied mantle section. The chemical composition of the three different symplectite types reveals significant Na- and Ba- and slight LREE-enrichment compared to the unaltered garnet composition. This indicates that the symplectite formation happened in an open system. Those symplectites that contain only opx+cpx+spl were formed at a depth of 40-45 km in equilibrium with cpx, opx, spl in the surrounded area of the whole rock. Whereas, the opx+cpx+spl+plg-bearing symplectites formed in the plagioclase stability field, at around 30-40 km depth.
Based on the formation depth of the garnet pyroxenites and the presence of plagioclase in the symplectites, we estimated that the upwelling of the studied mantle segment was approx. 15-20 km. It is in agreement with results of Falus et al. (2000) who proposed ~50-60 km upwelling for the astenopheric mantle beneath the studied area.
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PETROLOGY OF THE SUB-SCOTTISH LITHOSPHERE
UPTON BRIAN G. J. 1
presenter's e-mail: Brian.Upton@glg.ed.ac.uk
1 - School of GeoSciences, University of Edinburgh, U.K.
Keywords: Mantle metasomatism, Scotland, Lithospheric mantle, Carboniferous and Permian volcanism
The Scottish terranes have had complex evolutions as a result of their proximity to active continental plate margins. Knowledge of their lithospheric constitution accrues from seismological and xenolith studies. Since the xenoliths were entrained by silica-poor mafic magmas in the Carboniferous and Permian, the compositional data generally concern the lithosphere as it was in the late Palaeozoic. The lithospheric thickness is inferentially 70-80 km thick, of which the lower half is held to be composed principally of protogranular to porphyroclastic spinel lherzolite, with subordinate amounts of other rock types including garnet pyroxenite. Olivines in the lherzolites range Fo91.6 – 87, typically ~Fo89. Cr# values, (100.Cr/(Al + Cr)), for the spinels are mainly less than 16 but range up to ~ 60.
‘Pyroxenitic’ xenoliths (wehrlites, olivine pyroxenites, websterites, orthopyroxenites, clinopyroxenites, biotite- and pargasite-pyroxenites) are inferred to have originated as cumulates from underplated basaltic magmas: their very wide textural and compositional variety suggests that they come from an ultramafic layer of substantial thickness (several km?) overlying the peridotitic mantle.
This hypothetical layer comprises less magnesian ultramafic rocks typified by aluminous pyroxenes and hercynitic spinels. Where olivines occur, they are typically in the range Fo87-72. Whereas ages may vary from Proterozoic to Palaeozoic, textures in some imply contemporaneity with their host magmas.
Apart from an inferred discontinuity between the mantle peridotites and pyroxenitic cumulates, a second, relatively abrupt, discontinuity is deduced, indicated by the appearance of meta-gabbroic and dioritic rocks containing abundant plagioclase. Such rocks (‘basic granulites’) are also believed to be of cumulate origin, and to be broadly cogenetic with the ultramafic cumulates. All the lower crustal cumulates are regarded as products of repetitive underplating events that utilised the density break represented by the incoming of plagioclase. These two discontinuities may define petrological and seismic Mohorovicic Discontinuities respectively.
High-temperature alkali feldspar (commonly anorthoclase) occurs with Fe-rich clinopyroxene, biotite, magnetite, apatite and zircon as megacrysts, composite megacrysts and (rare) salic xenoliths. Some peraluminous associations, as indicated by the presence of corundum (sapphire), are also distinguished by their enrichment in LREE, Y, Nb, Th, and other incompatible elements. Crystallisation of these highly evolved megacryst suites appears to have been contemporaneous with the host magmatism. The megacryst protoliths are inferred to be pegmatitic salic sheets in the shallow mantle and deepest crust. Whilst their petrogenesis remains enigmatic, such sheets may represent aggregations of the volatile-rich, salic ‘melt pockets’, reported globally within spinel lherzolite xenoliths.
TERTIARY-QUATERNARY MAGMATISM IN EUROPE: HOW HAS IT INFLUENCED OR BEEN INFLUENCED BY THE EVOLUTION OF THE LITHOSPHERE?
WILSON MARJORIE 1
presenter's e-mail: M.Wilson@earth.leeds.ac.uk
1 - School of Earth & Environment, Leeds University
Keywords: basalt, lithosphere, Cenozoic, geodynamics
Within-plate (anorogenic) magmatism was widespread in Europe and the Mediterranean region from early Tertiary to Recent times; contemporaneous magmatic activity also occurs in north Africa. There is a spatial and temporal association with Alpine-Pyrenean collisional tectonics, the development of lithospheric rift systems and, locally, with uplift of Variscan basement massifs (e.g. Massif Central, Rhenish Massif, Bohemian Massif). Magmatism locally post-dates an earlier phase of subduction-related magmatism within the Mediterranean region, the Dinarides, the Pannonian Basin and Bulgaria. There appears to be a complex relationship between extensional tectonics and the presence of discontinuities in lithospheric thickness between adjacent terrane blocks (e.g. Massif Central, Rhenish Massif); extension of heterogeneous lithosphere, consisting of a collage of accreted Variscan terranes, might have induced localised convective instabilities in the upper mantle and resultant decompression melting. Wilson & Downes (2006) and Lustrino & Wilson (2007) have recently provided comprehensive reviews of the geochemical characteristics, petrogenesis and geodynamic setting of the magmatism.
In order to understand the processes which have caused both enrichment and depletion of the European lithospheric mantle during the Phanerozoic we need to understand the petrogenesis of the Tertiary-Quaternary magmas - in particular the relative roles of lithospheric and asthenospheric mantle source components. The major and trace element and Sr-Nd-Pb isotope characteristics of the most primitive mafic magmatic rocks (MgO > 6 wt %) provide important constraints on the nature of the mantle source and the conditions of partial melting. These are predominantly sodic magma types (melilitites, nephelinites, basanites and alkali olivine basalts); however, locally, potassic magmas (olivine leucitites, leucite nephelinites) also occur. In several localities (e.g., Sicily; Vogelsberg and the Rhine Graben, Germany; Calatrava, central Spain) olivine- and quartz-tholeiites form a significant component of the magmatism. The sodic magmas were derived by variable degrees of partial melting (~ 0.5 - 5 %) within a transitional zone between garnet-peridotite and spinel-peridotite mantle facies, close to the base of the lithosphere. The potassic magma types are interpreted as partial melts of enriched domains within the lithospheric mantle; these enriched domains could have originated in supra-subduction zone tectonic settings during the Hercynian orogeny (~ 300 Ma).
The distribution of the major volcanic provinces within Europe is broadly anti-correlated with the location of a zone of high-velocity, presumed subducted slab material, in the base of the upper mantle (500-600 km depth). Many of the major volcanic fields are located around the periphery of this velocity anomaly, coincident with the distribution of finger-like slow velocity anomalies in the upper mantle (imaged by local seismic tomography experiments). These anomalies could be a dynamic response to subduction and slab detachment during the Alpine collision (“splash” plumes). Many authors have proposed that they are thermally anomalous “hotspots” or plumes, 100-150 ˚C hotter than ambient upper mantle; however the absence of any evidence for a positive thermal anomaly at Transition Zone depths (400- 600 km) argues against this. An alternative explanation for the “diapirs” is that they are the products of localised fluid streaming from the top of the Transition Zone.
References
WILSON, M. & DOWNES, H. (2006) Tertiary-Quaternary intra-plate magmatism in Europe and its relationship to mantle dynamics. In: Gee, D. & Stephenson, R. (eds.) Geological Society, London, Memoirs 32, pp 147-166.
LUSTRINO, M. & WILSON, M. (2007) The Circum-Mediterranean anorogenic Cenozoic igneous province. Earth Science Reviews 81: 1-65.
THE THERMAL AND GEOCHEMICAL EVOLUTION OF THE LITHOSPHERIC MANTLE BENEATH THE EIFEL (GERMANY): CONSTRAINTS FROM MANTLE XENOLITHS, A REVIEW
WITT-EICKSCHEN GUDRUN 1
presenter's e-mail: Gudrun.Witt-Eickschen@uni-koeln.de
1 - UNIVERSITÄT KÖLN
Keywords: Eifel mantle lithosphere, Mantle xenoliths, Mantle metasomatism
Different seismological models suggest the presence of a small-scale (50-60 km radius) low-velocity anomaly beneath the West Eifel extending from a depth of about 50-60 km in the shallow upper mantle down to the 410 km transition zone. The seismic anomaly is interpreted as an upper mantle plume, which may serve as the prototype of a moderately weak hotspot in a continental environment (Ritter & Christensen, 2007).
Spinel peridotite xenoliths entrained in Cenozoic mafic alkaline magmas provide insights into the thermal, dynamic, and geochemical evolution of the shallow part of the lithospheric mantle beneath the Eifel. The mantle xenoliths equilibrated at depths from about 30 km down to 80 km and reflect conditions of at least three distinct thermal evolutions: (1) Well-equilibrated, coarse-grained and equigranular recrystallized spinel peridotites plot along a 80-90 mWm-2 conductive model steady-state geotherm. The xenoliths from the 80 km depth range approach the adiabatic upwelling curve of asthenosphere and may reflect a young convective heat transport at the base of the lithosphere. (2) Porphyroclastic xenoliths from shallow depths (~30 km) record a long-term (50 to 130 Ma) cooling in strong conjunction with a deformation process. Evidence for a re-heating of these xenoliths is totally lacking. Hence, a Tertiary to recent conductive heat transport into this shallow part of the lithospheric mantle near to the crust/mantle boundary can be excluded. (3) Zoning patterns in pyroxenes recognized in some xenoliths from mantle depths of about 40-50 km point to rapid (< 0.5 Ma) re-heating probably as consequence of the Cenozoic volcanism.
Systematic changes in modal compositions with major element chemistry are consistent with an origin of the spinel peridotite xenoliths as residues of partial melting. The Re-Os and Sm-Nd isotope systematics of the xenoliths yield a model age of ~1.6 Ga and ~2.0 Ga, respectively, for an ancient initial melt extraction. Subsequently, the majority of the Eifel mantle xenoliths were metasomatized by fluids from distinct mantle sources: (1) Volatile-rich fluids from an isotopically enriched (EM-like) mantle reservoir caused amphibole formation during deformation in the shallow upper mantle and generated trace element patterns similar to those observed in island arc lavas. The fluids appear to be associated with subduction processes under the Eifel in Paleozoic time. (2) During a second phase the EM-like lithospheric mantle was affected by melts from a HIMU-like mantle source. The HIMU-dominated xenoliths have Pb isotope ratios even more radiogenic than in the uniform asthenospheric source proposed for the Cenozoic European magmatism. Cretaceous Eifel lavas show Sr-Nd-Pb isotope ratios nearly identical to these xenoliths. Thus, the HIMU-like signature may be linked to reactivation of ancient subducted crustal domains during early Cretaceous plume activity. (3) During a brief final stage the heterogeneous enriched EM-HIMU subcontinental lithosphere was locally modified by basaltic melts with isotopic signatures equivalent to the Tertiary and Quaternary Eifel lavas. The melts migrated along fractures through the upper mantle and generated magmatic pyroxenite and hornblendite veins as well as small-scale compositional gradients in the peridotite wall rocks. Trace element modelling of the zoning profiles shows that this event was rapid. Thus this last stage of metasomatism is a consequence of the Cenozoic Eifel volcanism.
Ref.: J.R.R. Ritter & U.R. Christensen (eds.) (2007): Mantle plumes; Springer Verlag Berlin; ISBN-13 978-3-540-68045-1
UTILISING “NEW” LITHOPHILE ISOTOPE SYSTEMS TO THEIR FULL POTENTIAL: U-TH-PB AND LU-HF ISOTOPE TECHNIQUES IN DECIPHERING MANTLE MELTING, METASOMATISM AND EXOGENEOUS CONTAMINATION
WITTIG NADINE 1, DOWNES HILARY 2, BAKER JOEL 3, PEARSON GRAHAM 1
presenter's e-mail: Nadine.wittig@durham.ac.uk
1 - DURHAM UNIVERSITY, UK
2 - UNIVERSITY OF LONDON, UK
3 - VICTORIA UNIVERSITY/WELLINGTON, NZ
Keywords: U-Th-Pb isotopes, Lu-Hf isotopes, French Massif Central, Hf model ages
I explore the potential of Lu-Hf and U-Th-Pb isotope techniques in dating mantle depletion events and subsequent metasomatic enrichment and present a case study from the French Massif Central (FMC). In addition, we have examined in some detail the role of late-stage geological and anthropogenic grain-boundary contamination. For the recovery of pristine U-Th-Pb isotope systematics in mantle minerals rigorous leaching protocols are required. Pb isotope data available from the literature for SCLM clinopyroxene highlight that the lack of unradiogenic 206Pb/204Pb (1st Pb paradox), scarcity of HIMU-like SCLM and the alleged dominance of EM in continental roots, but these observations may be reconciled if the present-day Pb isotope database of SCLM contains samples that were insufficiently leached prior to digestion and thus are contaminated by anthropogenic Pb.
The intraplate volcanic province of the FMC hosts one of the most extensively studied peridotites xenoliths suites. Notably, there are distinct compositional differences between the northern and southern parts of the FMC. The continental roots beneath the northern FMC have clinopyroxenes with extreme Lu/Hf ratios and ultra-radiogenic Hf ( Hf = +39.6 to +2586) that reflect ~15-25% partial melting in Variscan times (depleted mantle model ages ~360 Ma). Zr, Hf and Th abundances in these clinopyroxenes are low and unaffected by hydrous/carbonatitic metasomatism that overprinted LILE and light REE abundances and caused decoupling of Lu/Hf-Sm/Nd ratios and Nd-Hf isotopes ( Nd = +2.1 to +91.2). Pb isotopes of northern FMC clinopyroxenes are radiogenic (206Pb/204Pb > 19), and typically more so than the host intraplate volcanic rocks. 238U/204Pb ratios range from 17 to 68, and most samples have distinctively low 232Th/238U (< 1) and 232Th/204Pb (3 to 22).
Clinopyroxenes from southern FMC lherzolites are generally marked by overall incompatible trace element enrichment including Zr, Hf and Th abundances, and have Pb isotopes that are similar to or less radiogenic than the host volcanic rocks. Hf isotope ratios are less radiogenic ( Hf = +5.4 to +41.5) than northern FMC mantle and have been overprinted by silicate-melt-dominated metasomatism that affected this part of FMC mantle. Major element and Lu concentrations of clinopyroxenes from southern FMC harzburgites are broadly similar to northern FMC clinopyroxenes and suggest they experienced similar degrees of melt extraction as northern FMC mantle. 238U/204Pb (53 to 111) and 232Th/204Pb ratios (157 to 355) of enriched clinopyroxenes from the southern FMC are extreme and significantly higher than the intraplate volcanic rocks.
The extreme mantle 238U/204Pb (northern and southern FMC), 232Th/238U (northern FMC) and 232Th/204Pb ratios (southern FMC), coupled with unremarkable present-day Pb isotope ratios, constrain the timing of enrichment. Mantle metasomatism is a young feature related to melting of the upwelling mantle responsible for Cenozoic FMC volcanism, rather than subduction-related metasomatism intimately associated with mantle depletion during the Variscan orogeny. The varying metasomatic styles relate to pre-existing variations in the thickness of the continental lithospheric lid, which controlled the extent to which upwelling mantle could ascend and melt. In the northern FMC, a thicker and more refractory lithospheric lid ( 80 km) only allowed incipient degrees of melting resulting in fluid/carbonatitic metasomatism of the overlying sub-continental lithospheric mantle. The thinner lithospheric lid of the southern FMC ( 70 km) allowed larger degrees of melting and resulted in silicate-melt-dominated metasomatism, and also focused the location of the volcanic fields of the FMC above this region.
P-T CONDITIONS, ND-SR ISOTOPIC COMPOSITIONS AND THE TIMING OF METASOMATIZM RECORDED IN PERIDOTITE XENOLITHS OF FRENCH MASSIF CENTRAL
YOSHIKAWA MASAKO 1, KAWAMOTO TATSUHIKO 1, YAMAMOTO JUNJI 1
presenter's e-mail: masako@bep.vgs.kyoto-u.ac.jp
1 - Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Japan
Keywords: Peridotite xenoliths, Massif Central, Nd-Sr isotopic compositions, Mantle metasomatism
Cenozoic volcanoes in French Central Massif brought many peridotite xenoliths and extensive studies were carried out with those peridotite samples by the use of petrography, trace element and isotopic compositions. We examined the P-T conditions of 10 peridotite xenoliths in scoria deposits from Puy Beaunit and Mont Briançon, and in lava flow from Ray Pic (Bruzet), and describe textural and chemical features of the partial melt texture of phlogopite vein in a Puy Beaunit xenolith and discuss the timing of the metasomatism forming the phlogopite using the Rb-Sr systematics of clinopyroxene and phlogopite.
PT conditions
We determined the equilibrium temperatures based on the two-pyroxene geothermometer using chemical compositions of Ca-rich and Ca-poor pyroxenes (Wells, Contrib. Mineral. Petrol., 1977). The density, and therefore the pressure, of CO2 fluid inclusions in host minerals can be estimated from the Fermi diad splitting of Raman spectra of CO2. If the pressure of CO2 fluid inclusions was determined, the depth where the CO2 inclusions were entrained can be obtained (Yamamoto and Kagi, Chem. Lett., 2006). The estimated ranges of equilibrium temperatures and pressures are ~900 ˚C and 0.5-0.7 GPa for Puy Beaunit, ~900 ˚C and ~1 GPa for Mont Briançon and ~1000 ˚C and 0.9-1 GPa for Ray Pic (Bruzet), respectively. Puy Beaunit xenoliths were derived from obviously shallower depths (~25 km) than the other localities (~30-40 km). We conclude that the peridotite xenoliths of Puy Beaunit were brought up from the present Moho depth (~25km, Zanga et al., Contrib. Mineral. Petrol., 1997). The xenoliths from the Puy Beaunit are characterized by the followings: (1) recrystallization texture, (2) existence of hydrous minerals and interstitial glasses, and (3) enriched Sr-Nd isotopic ratios (Mercier and Nicolas, J. Petrol., 1975; Downes and Dupuy, Earth Planet. Sci. Lett., 1987). On the basis of isotopic and textural signatures, it has been inferred that Beaunit xenoliths were from the depth for the Moho discontinuity (e.g. Mercier and Nicolas, 1975; Downes and Dupuy, 1987). Furthermore, mixing relationship of major element compositions between interstitial glasses and Cenozoic magmas has been suggested (Wilson and Downes, J. Petrol., 1991).
Vesiculated glass through partial melting of phlogopite veins.
One peridotite xenolith in Puy Beaunit has rounded shape phlogopite surrounded by vesiculated glass. Chemical compositions of glasses have high K2O, Al2O3 and CaO contents and enriched isotopic ratios of Sr and Nd than previously reported (Wilson and Downes, 1991; Downes and Dupuy, 1987; Downes et al., Chem. Geol., 2003). Sr-Nd isotopic and mineral compositions of the Cenozoic magmas are plotted between primitive magma and clinopyroxenes or glasses in this sample. This suggest that metasomatism forming phlogopite vein plays a role in basaltic genesis in this region as suggested by previous studies. The chemical compositions of the glasses and the crystalline phases allow us to carry out mass balance calculations among them. The glass chemistry can be formed by the following reaction in nine components of SiO2-TiO2-Al2O3-Cr2O3-MgO-FeO-CaO-Na2O-K2O system:
1.1 opx + 0.25 cpx +0.79 phl = 1.0 ol + 1.0 gl + 0.01 sp
opx = enstatite, cpx = diopside, phl = phlogopite, sp = Cr-spinel, ol = forsterite, gl = glass. From these observations, we suggest that a partial melting of phlogopite and pyroxenes formed the glass and olivine + spinel crystallization.
Timing of metasomatism, the formation of phlogopite vein.
The Rb-Sr isotopic systematics of phlogopite and clinopyroxene give a reference age of around 60 Ma.This age is younger compared than previous obtained melt extraction age of the Massif Central xenoliths (ca. 360Ma; Witting et al., Geology, 2005) and metasomatized age during Variscan orogeny inferred from model ages (e.g. Dunai and Baur, Geochim. Cosmochim. Acta., 1995). We propose that this age represent the modal metasomatism beneath the French Massive Central.
PETROLOGICAL FEATURES OF MANTLE XENOLITHS FROM EASTERN TRANSYLVANIAN BASIN: A VERY FERTILE MANTLE OR RE-FERTILISATION PROCESSES?
COLTORTI MASSIMO 1, BONADIMAN COSTANZA 1, FACCINI BARBARA 1, HARANGI SZABOLCS 2, NTAFLOS THEODOROS 3, SEGHEDI IOAN 4
presenter's e-mail: barbara.faccini@unife.it
1 - Earth Science Department, Ferrara University, Italy
2 - ELTE Department of Petrology and Geochemistry, Hungary
3 - Department of Lithospheric Sciences, University of Vienna, Austria
4 - Institute of Geodynamics, Bucharest, Romania
Keywords: Transylvanian basin, Mantle metasomatism, Amphibole
Mantle xenoliths from Eastern Transylvanian Basin have been collected in the Perşani Mts. area from tuff cones and a basaltic quarry. The rocks are mainly amphibole-bearing lherzolites with subordinate ol-websterites and anhydrous lherzolites. Most peridotites have porphyroclastic textures with aligned spinels and variable grain size of olivine and orthopyroxene (opx) porphyroclasts (5-2 mm and 1 cm-3 mm, respectively). Amphibole can be found mostly as disseminated crystals around spinel (sp), usually associated with opx and clinopyroxene (cpx), and/or as selvages at the contact with peridotite, and/or as discontinuous veinlets intruding the peridotite matrix. Glassy patches and veinlets, not related to host basalt infiltration, occur close to amphibole (amph) and cpx; secondary olivine, cpx and sp are found in the glassy pools. Xenoliths present cpx modal content varying between 6.5 and 27%, together with high opx and sp modal content (up to 33% and 12% respectively).
Primary olivine (Ol1) have mg# and CaO varying from 88.9 to 91.8 and 0.02 to 0.13 respectively, whereas secondary idiomorphic crystals (Ol2) reach higher values, from 90.7 to 94.1. In Ol2 CaO content ranges between 0.10 and 0.29. Orthopyroxenes mg#, SiO2 and Al2O3 vary from 89.14 to 92.12, from 53.10 to 55.88 and from 2.84 to 5.90, respectively. Primary clinopyroxenes (Cpx1) have mg# going from 89.01 to 92.99, SiO2 from 49.92 to 52.23, TiO2 from 0.11 to 0.71, Al2O3 from 3.45 to 8.16, CaO from 19.52 to 23.25, Na2O from 0.49 to 1.80 and Cr2O3 from 0.47 to 1.37. Cpx1 rims and secondary crystals have variable mg#, 85.40-94.51, probably in relation with the presence of amphibole. On the whole they show an increase of TiO2, Al2O3 and Cr2O3 and a decrease of SiO2 and Na2O. Primary spinels are alumina-rich, whith cr# varying between 6.64 and 34.60. Secondary tiny spinels in reaction zones tend to increase their Cr2O3 content. Amphiboles major element contents are: mg# (86.55-89.50), SiO2 (41.59-43.99), TiO2 (0.64-2.34), Al2O3 (14.61-16.28), CaO (10.37-12.06), Na2O (2.91-3.81), K2O (0.01-1.34), Cr2O3 (0.73-1.57). Glasses in anhydrous samples are SiO2-richer and FeO-, MgO-, CaO- and Na2O-poorer than those in amphibole-bearing xenoliths. All glasses are quite rich in Al2O3 (20.70-26.65).
Based on trace element contents three different groups of cpx1 can be recognized. The first group is characterized by slight depletion in LREE [(La/Yb)N, 0.42-1.03] with HREE at 6-9 x Ch, a negative Zr-Hf anomaly, a diffuse Sr positive anomaly (Sr/Sr*, 0.91-2.00) and a highly variable Th and U contents. The second group is equally depleted in LREE [(La/Yb)N, 0.34-1.37] but at higher HREE content (11-13 x Ch). It has negative to positive Hf and Sr anomalies, and less variable Th and U contents. The third group is LREE enriched [(La/Yb)N, 1.36-5.45] with HREE at 9-11 x Ch and a Sr negative anomaly (Sr/Sr*, 0.65-0.96). All cpx1 share a slight but peculiar Eu positive anomaly. Amphiboles have patterns identical to their associated cpx except for those elements which are better partitioned into amphibole, such as Nb, Ta, Sr and Ti.
Accurate measurements of Ca contents in Ol1 allowed the application of the Ca-exchange barometer coupled with the two-pyroxene thermomether, which revealed equilibrium temperatures and pressures of 920-1010°C and 13.0-17.7 Kbar, consistent with the presence of amphiboles. The high modal percentage of opx, cpx and spinel, coupled with their high Al2O3 content, the high HREE and the positive anomalies in Sr and Eu in cpx trace element patterns seem to rule out that we are simply facing a very fertile mantle portion, but point toward a re-fertilisation process involving an hydrate LREE-poor, HREE-rich, silica-saturated melt with high Sr contents.
GEOCHEMICAL CHARACTERISTICS OF APATITE-RICH LAYERS IN THE FINERO PHLOGOPITE-PERIDOTITE MASSIF
MORISHITA TOMOAKI 1, HATTORI KEIKO 2, TERADA KENTARO 3, MATSUMOTO TAKUYA 4, YAMAMOTO KOSHI 5, TAKEBE MASAMICHI 5, TAMURA AKIHIRO 1, ARAI SHOJI 1
presenter's e-mail: moripta@kenroku.kanazawa-u.ac.jp
1 - KANAZAWA UNIVERSITY
2 - UNIVERSITY OF OTTAWA
3 - HIROSHIMA UNIVERSITY
4 - OSAKA UNIVERSITY
5 - NAGOYA UNIVERSITY
Keywords: Finero phlogopite-peridotite, Apatite-rich rock, Metasomatism
Mantle metasomatism is caused by infiltration of metasomatising agents (fluids or melts) in diverse tectonic settings under a wide range of P-T conditions (e.g., Zinngrebe and Foley 1995; Vannucci et al., 1998; Wulff-Pedersen et al. 1999; Arai et al., 2003, 2004; Morishita et al., 2003a). Apatite in mantle xenoliths has been reported from intra-plate rifts (e.g., O’Reilly and Griffin, 1988; Yaxley et al., 1991; Hauri et al., 1993; Rudnick et al., 1993; Ionov et al., 1996) as well as mantle wedges (e.g., McInnes and Cameron, 1994; Laurora et al., 2001). Apatite can contain high concentrations of rare earth elements (REE), Cl, F, U, Th, Sr and it would play an important role in the behavior of these trace-elements in the upper mantle (e.g., Watson, 1980; Exley and Smith, 1982; O’Reilly and Griffin, 2000). Therefore, the study of mantle-derived apatite-rich lithology is important in understanding the behavior of P-rich metasomatising agent locally present in the mantle.
The Finero phlogopite-peridotite massif in the western Italian Alps is characterized by intense metasomatism forming apatite and carbonate minerals after partial melting (Exley et al., 1982; Cummings et al., 1987; Voshage et al., 1987; Hartmann & Wedepohl, 1993; Zanetti et al., 1999; Grieco et al., 2001, 2004; Prouteau et al., 2001; Morishita et al., 2003b; Zaccarini et al., 2004; Raffone et al., 2006). The occurrence of apatite- and carbonate-bearing domains in the area was first documented around pyroxenite layers by Zanetti et al. (1999). Morishita et al. (2003b) found a fine-grained apatite-rich peridotite layer (AP-layer hereafter) Metasomatising agents in the Finero peridotites are, however, still in debate. We present major-element, trace-element and isotopic compositions of whole-rock and metasomatic minerals of the AP-layer, and ages of apatite using a sensitive high-resolution ion microprobe (SHRIMP) and discusses the origin and evolution of the metasomatising agent responsible for the formation of the AP-layer in the Finero massif.
Strontium and Nd isotope compositions of the AP-layer and its host peridotite show the bulk silicate Earth signature which is distinctive from the previous data obtained from apatite-free peridotites except for one carbonate aggregate-bearing sample with high 87Sr/86Sr and low εNd. The carbonate aggregate restrictedly occurs in an altered serpentine-talc vein cutting both the AP-layer and its host. These data coupled with C-O-isotope compositions from carbonate in the carbonate aggregate-bearing sample suggest that the some carbonates were formed or recrystallized by infiltration of fluids with ancient crust signature after the formation of the apatite-rich layer.
The AP-layer and the host peridotite are geochemically characterized by extremely high REE and LILE contents with high LREE/HREE ratio, and low HFSE concentrations compared with other apatite-bearing lithologies in the Finero massif. Incompatible trace elements in whole rock and amphiboles increase in concentration toward the apatite-rich peridotite layer. The AP-layer was formed by infiltration of highly evolved SiO2-CO2-bearing hydrous metasomatising agents derived from parent metasomatising agents having carbonatitic geochemical signatures at the late stage of the metasomatism. Sensitive high-resolution ion microprobe (SHRIMP) analyses of apatite grains yielded a Tera-Wasserburg concordia three-dimensional isochron age of 215 ± 35 Ma in the 238U/206Pb-207Pb/206Pb-204Pb/207Pb diagram.
ISOTOPIC EVIDENCE FOR CHAOTIC IMPRINT IN UPPER MANTLE HETEROGENEITY
ARMIENTI PIETRO 1, GASPERINI DANIELA 1
presenter's e-mail: armienti@dst.unipi.it
1 - DST – Pisa University
Keywords: Mantle heterogeneity, Mantle convection, Scale invariance
A systematic check of the structure of the data set of geochemical composition of samples from the Mid-Atlantic Ridge (MAR) and East Pacific Rise (EPR) reveals an isomorphism between geometric structures and chemical features. We observe that the distribution of isotopic heterogeneity in lavas sampled at these ridge crests, believed to represent the isotopic composition of their mantle source, is self-similar in the range of about 7000 km. Self-similarity is the imprint of chaotic mantle processes, induced by recursion and favored by "high-turbulent" behavior of the mantle. The size of the identified fractal region reflects the large length scale of upper mantle chemical variability, and it is likely frozen since the Early Proterozoic. The geochemical heterogeneity of the asthenosphere along the ridges would record a fundamental transition in the thermal conditions of the Earth’s mantle, marking the shift towards a "soft-turbulent" regime.
SIGNIFICANT WATER SOLUBILITY REDUCTION IN FORSTERITE DUE TO THE VARIATION OF WATER ACTIVITY IN THE EARTH'S MANTLE
BALI ENIKO 1, BOLFAN-CASANOVA NATHALIE 2, KOGA KENNETH 2
presenter's e-mail: Eniko.Bali@Uni-Bayreuth.DE
1 - BAYERISCHES GEOINSTITUT, BAYREUTH
2 - LABORATOIRE MAGMAS ET VOLCANS, CNRS, UNIVERSITÉ BLAISE PASCAL, CLERMONT-FERRAND
Keywords: Water solubility in forsterite, mantle, FTIR
The storage capacity of water in the upper mantle largely depends on water solubility in mantle olivine, and provides a fundamental information required for petrological, geophysical and geochemical models of the mantle. An accurate assessment of water storage capacity of minerals of the upper mantle must account for simultaneous effects of variables such as pressure, temperature, iron content and silica activity. Previous experimental studies have shown that the water solubility in olivine increases with increasing water fugacity up to 12 GPa at 1100°C (Kohsltedt et al., 1996). Water incorporation in olivine was also observed to increase with increasing temperature (from 1000˚C to 1300˚C) at 0.3 GPa (Zhao et al., 2004). This contrasts with the incorporation of water in transition zone phases such as wadsleyite and ringwoodite for which water solubility decreases with increasing temperature.
We performed experiments at 2.5, 6 and 9 GPa, and temperatures ranging from 1000 to 1400°C in the MgO-SiO2-H2O system using a multi-anvil apparatus. The starting material consisted of forsterite and enstatite in the 1:1 molar ratio with 1 to 5 wt% H2O. The samples were analysed using scanning electron microscopy, electron microprobe and polarized Fourier transform infrared spectroscopy.
We report the dependence of water maximum concentration on temperature at pressures higher than 0.3 GPa. The results show that water solubility in olivine increases with temperature only at 2.5 GPa. At 6 and 9 GPa, the water solubility reaches a maximum at temperatures of 1175 to 1250˚C, depending on pressure, then decreases at higher temperatures. These results agree with recent observations at 12 GPa in the MgO-SiO2-H2O system (Smyth et al., 2006).
Such behaviour is explained by the change in water fugacity due to dissolution of silicate component in the fluid. Coincidentally, these drastic reductions of water activities are observed at the conditions above the second critical point of silicate-water system. The solubilities determined in this study allow estimating the water storage capacity in the mantle. Furthermore, because of the variation of the geothermal gradient, the continental lithosphere is expected to have higher water storage capacity than oceanic mantle at depths ranging from 100 to 400 km.
References
Kohlstedt, D.L., Keppler, H. & Rubie, D.C. (1996): Solubility of phases of (Mg,Fe)2SiO4. Contributions tog and b, awater in the Mineralogy and Petrology, 123, 345–357
Smyth, J.R., Frost, D.J., Nestola, F., Holl, M., Bromiley, G. (2006): Olivine hydration in the deep upper mantle: effects of temperature and silica activity.
Zhao, Y.H., Ginsberg, S.B. & Kohlstedt, D.L. (2004): Solubility of hydrogen in olivine: dependence on temperature and iron content. Contributions to Mineralogy and Petrology, 147, 155–161.
PRESSURE PRESERVED BY CO2-RICH FLUID INCLUSIONS: A CASE STUDY FROM TIHANY PERIDOTITES, WESTERN HUNGARY
BERKESI MARTA 1, HIDAS KAROLY 1, SZABÓ CSABA 1
presenter's e-mail: marta.berkesi@gmail.com
1 - Lithosphere Fluid Research Lab, Eötvös University, Budapest, Hungary
Keywords: CO2-rich fluid inclusions, upper mantle peridotite xenoliths, minimum trapping pressure, Pannonian Basin
Detailed fluid inclusion study hosted in upper mantle peridotite xenoliths from Tihany (western Hungary, central part of the Pannonian Basin) were carried out in this work. The peridotite xenoliths were brought to the surface by post-extension-related alkali basalt 8 million years ago (Balogh & Németh, 2005), which is the oldest volcano of the Bakony-Balaton Highland Volcanic Field hosting subcontinental upper mantle xenoliths (Szabó et al., 2004; Falus & Szabó, 2004).
In previously study, using geothermometer based on the orthopyroxene/clinopyroxene equilibrium in the host xenoliths by Brey and Köhler (1990), two domains in the mantle lithosphere beneath Tihany have been distinguished (Hidas, 2006). We applied the given temperature values to be able to estimate pressure, whilst the individual CO2-rich fluid inclusions, occurring particularly in orthopyroxene and clinopyroxene, have preserved their high density representing minimum trapping conditions in the upper mantle. This is because the fluid inclusions, in equilibrium with their host minerals, can be defined by the intersection of the geotherm with the density of the trapped fluid. Since a precise geobarometer for the spinel peridotites is not available yet, the minimum trapping pressure estimation from CO2 fluid inclusion densities provides the best method available for pressure estimation for these mantle rocks.
The Tihany orthopyroxene-rich spinel peridotite xenoliths (lherzolites and harzburgites) contain abundant CO2-rich inclusions. The inclusions are mostly hosted in orthopyroxenes showing two well-defined types: type-1 orthopyroxene-hosted negative crystal shaped inclusions with size up to 70
m containing one phase (liquid) at ambient conditions, and type-2: orthopyroxene- and olivine-hosted elongated or irregular shaped inclusions with a size varying between 5 and 20
m. Latter ones contain one (liquid) or two (liquid and vapor) phases at ambient conditions.
The CO2 fluid inclusions have been studied using heating and cooling stage and Raman microspectroscopy. The microthermometric data suggests that in most cases the fluid phase of the inclusions is pure CO2 (melting temperature /Tm/ = -56,6 - -56,9 °C). Furthermore, type-1 inclusions have higher densities (0.89-1.12 g/cm3) than type-2 ones (0.5-0.9 g/cm3) in all xenoliths. Results from microthermometry suggest presence of further fluid phase(s) in those xenoliths, which show values of Tm lower than pure CO2 (<-56,9 °C). Raman microspectroscopy confirms that these fluid inclusions also contain H2S besides CO2.
The geothermobarometric calculations and CO2-density values suggest that the studied peridotite xenoliths represent two, physically distinct, a shallower and a deeper domains in the lithosphere beneath the studied area. Minimum trapping pressure estimations of CO2 fluid inclusions support this idea and provide 12 °C/km temperature gradient for the upper mantle beneath Tihany 8 million years ago.
METASOMATIC PROCESSES IN THE MANTLE BENEATH THE VENETO VOLCANIC PROVINCE (NORTHERN ITALY): FLUID AND MELT INCLUSIONS EVIDENCE
BONELLI ROSSANA 1, FREZZOTTI MARIA LUCE 1
presenter's e-mail: bonelli5@unisi.it
1 - Dipartimento di Scienze della Terra, Università degli Studi di Siena, Siena, 53100 Italy
Keywords: peridotites, fluid inclusions, melt inclusions, metasomatism
Tertiary alkali-basalts of the Veneto Volcanic Province (VVP,SE Alps) contain abundant peridotite xenoliths, representing one of the few localities where to study the lithosphere mantle beneath Italy.
Previous studies showed that peridotites are depleted harzburgites and lherzolites, re-equilibrated under spinel-facies conditions. Beccaluva et al. (2001) described a pervasive metasomatism characterised by variable LREE-enrichment patterns, due to infiltration of alkali silicate basic melts related to the Tertiary volcanism, forming glass pockets and veins (pyrometamorphic textures). More recently, Gasperini et al. (2006) suggested two metasomatc events: 1) an older one induced by slab-derived material and characterised by crystallisation of amphibole and mica; and 2) a more recent alkaline metasomatic event consistent with the Tertiary volcanism of the area. We have investigated petrography, mineral chemistry, fluid and melt inclusions of 17 peridotites from VVP in order to constrain P-T conditions of mantle events, to determine the style of the metasomatic reactions, and the compositions of the metasomatic agents. Studied rocks show dominant protogranular and transitional (intermediate between protogranular and porphyroclastic) textures; only one sample shows pyrometamorphic features. They consist of a four–phase assemblage: clear olivine (43-70 % vol.), light brown orthopyroxene (20–42 % vol.), green clinopyroxene (5–15 % vol.) and brown spinel (2–8 % vol.). Clinopyroxenes in protogranular lherzolites show depleted LREE patterns, while those of transitional rocks are characterised by spoon-shaped REE patterns (La up to 60 times chondrite), and variable enrichments in LILE.
Two generations of fluid inclusions (1-30 µm) are recognised: 1) Type A (CO2-rich fluid), commonly present in orthopyroxene, while seldom observed in clinopyroxene and olivine; 2) Type B (CO2 and CO2-CO fluids) present only in orthopyroxene. Type B inclusions may contain very small amphibole (~10 µm) as trapped mineral, indicating a presence of an aqueous component in the fluid. Most of inclusions homogenise to the liquid, with ThL ranging between -44 and 24°C. The densest CO2 fluid inclusions (d = 1.13 g/cm3) indicate a trapping pressure of ~ 1 GPa at 900°C.
Melt/glass in studied xenoliths occur with different textures: 1) Type A clear glass occurs as microveins, melt inclusions (~50 µm), and pockets (e.g. in pyrometamorphic xenolith), and shows high SiO2 and alkali contents; 2) Type B melt is represented by colourless micro-channels of glass (~100 µm) around the porous rims of clinopyroxene (i.e. dendrites) and by melt inclusions (~50 µm), consisting of glass and a CO2 bubble. Type B glass shows lower SiO2 and alkali contents than Type A glass.
Mantle equilibration temperatures were calculated from mineral pairs in spinel peridotites, and range between 680 and 992°C for protogranular and transitional textured xenoliths, and between 941 and 1171°C for the pyrometamorphic sample.
We propose that the mantle beneath the VVP equilibrated at pressures >1 GPa, in a temperature range of 680-992°C. A major metasomatic event was induced by Type A melts with alkaline affinity, probably originating from the Tertiary alkali basalts of the VVP. Type A melts were responsible for the observed pyrometamorphic textures in the rocks, for a substantial enrichment in LREE in clinopyroxene, and for a 100-200°C temperature increase, similarly to what described by Beccaluva et al. (2001). Traces of an aqueous fluid (CO2 + brines) preserved as fluid inclusions in orthopyroxene suggest the existence of an older subduction related metasomatic event, confirming the hypothesis of a two stage metasomatism in the lithosphere beneath the SE Alps (Gasperini et al.,2006).
REFERENCES
Beccaluva L., Bonadiman C., Coltorti M., Salvini L. & Siena F. (2001) J. Petrology. 42: 173-187.
Gasperini D, Bosch D., Braga R., Bondi M., Macera P. & Morten L. (2006) J. Geochemical. 40: 377-404.
HYDROGEN CONTENT IN PYROXENES FROM MANTLE XENOLITHS IN THE TRIASSIC MAGMATIC COMPLEX OF PREDAZZO (DOLOMITES, NE ITALY)
CARRARO ANNA 1, RAEPSAET CAROLINE 2, BUREAU HÉLÈNE 2, VISONÀ DARIO 1, FUCHS YVES 3, KHODJA HICHAM 2
presenter's e-mail: a.carraro@unipd.it
1 - Dipartimento di Geoscienze, Università di Padova, Italy
2 - Laboratoire Pierre Süe, CEA Saclay, Gif-sur-Yvette, France
3 - Laboratoire G2I, Université Paris Est, Marne la Vallée, France
Keywords: Mantle xenoliths, Nominally Anhydrous Minerals, Hydrogen concentration, Dolomites, NE Italy
The amount of hydrogen dissolved in nominally anhydrous silicates from the upper mantle has been quantified using a nuclear microprobe. Studied samples are clinopyroxene (cpx) and orthopyroxene (opx) grains, which, together with olivine and spinel, form lherzolite xenoliths contained in Triassic dykes of the Predazzo Magmatic Complex (Dolomites, NE Italy). The xenoliths here considered have protogranular texture and no evidence of additional hydrous phases or melt was observed. Both clino- and orthopyroxene can be considered as host minerals for the largest concentrations of hydrogen among Nominally Anhydrous Minerals (Peslier et al., 2002); for this reason, their role in providing a repository of water in the upper mantle and in mantle water recycling processes is coming of crucial interest. An absolute analytical method, a combination of ERDA (Elastic Recoil Detection Analysis) and RBS (Rutherford Backscattering Spectroscopy), was used for H measurements. During the last years, the ERDA method was improved in the nuclear microprobe at the Laboratoire Pierre Süe (Saclay, France) and the detection limit of 130 wt ppm H2O has been now obtained for dehydrated San Carlos olivine (Bureau et al., 2007). Samples were prepared for analyses following a specific protocol, in order to decrease the effect of H absorbed at the surface of the mineral grains. The analytical conditions were: 4He+ energy = 3.0 MeV, beam spot size = 4x4 µm2, 4x16 µm2 for ERDA, scattering angle = 170°, recoil angle = 30°. The maps were processed using a specific software (Daudin et al., 2003): from RBS and ERDA maps, chemically homogeneous areas were selected and results are referred only to H depleted zones. Homogeneity of the selected zones was confirmed by electron microprobe analyses (CAMECA SX50, CNR – University of Padova).
H amounts in cpx vary from 716 to 1094 wt ppm H2O, whereas opx shows H concentrations from 378 to 456 wt ppm H2O. Total relative uncertainty is about 13 wt ppm H2O for all the samples. These values are slightly higher than those reported for pyroxenes (e.g., 450 wt ppm H2O for cpx) in spinel peridotite xenoliths from the Tertiary Veneto Volcanic Province (NE Italy; Carraro et al., 2007) and obtained with the same method. However, H concentrations in the Predazzo samples are in the ranges for mantle pyroxenes reported in the literature but higher than values usually found for diopside and enstatite forming spinel lherzolite xenoliths (Skogby, 2006). On one hand, an increasing trend in water concentration for cpx was observed when going from spinel lherzolite to garnet lherzolite stability field (Demouchy, 2004). In our case, the provenance of the xenoliths from higher pressure conditions is supported by clusters of opx-cpx-spinel (i.e., garnet peridotite precursor) observed in the texture of the Predazzo samples (Carraro & Visonà, 2003). On the other hand, the highest concentrations for opx may be attributed either to higher pressure environment or to metasomatic reactions at mantle level (Bell & Rossman, 1992).
REFERENCES
Bell D.R., Rossman G.R. (1992): Science 255, 1391-1397.
Bureau H., Raepsaet C., Khodja H., Carraro A., Aubaud C. (2007): Goldschmidt2007 (submitted).
Carraro A., Visonà D. (2003): Eur. J. Mineral., 15, 103-115.
Carraro A., Bureau H., Visonà D., Raepsaet C., Fuchs Y, Khodja H. (2007): Goldschmidt2007 (submitted).
Daudin L., Khodja H., Gallien L.P. (2003): Nucl. Instr. and Meth. in Phys. Res. B, 210, 153-158.
Demouchy S. (2004): PhD Dissertation, Univ. Bayreuth, Germany.
Peslier A.H., Luhr J.F., Post J. (2002): Earth. Planet. Sci. Lett., 201, 69-86.
Skogby H. (2006): Rev. Mineral. Geochem., MSA, 62, 155-167.
INSIGHT INTO THE LITHOSPHERIC MANTLE BENEATH BALKAN PENINSULA: EVIDENCE FROM STUDY OF MANTLE XENOLITHS AND THEIR HOST BASALTS
CVETKOVIC VLADICA 1, DOWNES HILARY 2, PRELEVIC DEJAN 3, LAZAROV MARINA 4
presenter's e-mail: cvladica@rgf.bg.ac.yu
1 - UNIVERSITY OF BELGRADE, FACULTY OF MINING AND GEOLOGY, SERBIA
2 - BIRKBECK COLLEGE, LONDON, UK
3 - UNIVERSITY OF MAINZ, GERMANY
4 - UNIVERSITY OF FRANKFURT, GERMANY
Keywords: Geochemistry, Mineralogy, Alkaline basalts, Dinarides
Mantle xenoliths occurring in 40–60 Ma old basanites in East Serbia can be roughly distinguished as (i) depleted, (ii) orthopyroxene-rich, and (iii) fertile/metasomatized xenoliths. Their study provides important conclusions about the composition and geodynamics of mantle underneath East Serbia.
(i) The first sub-group predominates; it consists of harzburgite, clinopyroxene-poor lherzolite and rare dunite xenoliths. They contain mostly <5 vol% of modal clinopyroxene and are characterized by high Mg# in silicates (>91), high Cr# in spinel (mostly 0.5–0.7), and by distinctively low Al2O3 contents in orthopyroxene (1–2 wt.%). Non-modal batch and fractional melting models for REE contents in orthopyroxene revealed the extent of depletion of >>30% and 20-22% of melting, respectively.
(ii) Orthopyroxene-rich xenoliths are composed of tabular low-Al orthopyroxene (~70vol.%, Mg# 85–87) containing tiny inclusions of very Cr-rich spinels. Orthopyroxene shows a slightly U-shaped primitive mantle-normalized trace element pattern with strong peaks at U and Pb, similar to that of orthopyroxene from normal regional peridotitic mantle. These xenoliths are interpreted as having originally been hydrous olivine-bearing orthopyroxenites that crystallised from subduction-related SiO2-saturated, boninite-like magmas. (iii) Fertile/metsomatized lithologies include (a) Fe-rich dunite xenoliths and clinopyroxene megacrysts, (b) lherzolite xenoliths with metasomatic spongy-rimmed Ti–Al-rich clinopyroxene, and (c) tiny discrete metasomatic assemblages in veinlets and pockets in the highly depleted xenoliths. The fertile lithologies have low Mg#s in their silicates (ca. 88), low Cr#s in spinels (<0.4), and high Al2O3 in orthopyroxene (2–6 wt%). Trace element patterns of metasomatic clinopyroxene indicate that the inferred metasomatic melts are genetically related to the host basanites.
The most abundant East Serbian mantle xenoliths are more depleted than most non-cratonic sub-continental mantle xenolith suites, as well as orogenic peridotites and abyssal peridotites.
Geological and compositional evidence suggests that the xenoliths do not represent Archean mantle. The existence of Proterozoic mantle cannot be entirely excluded, although it is in disagreement with geological evidence. On the other hand, the studied xenoliths are compositionally very similar to peridotites of modern oceanic sub-arc settings. The existence of such a depleted lithospheric mantle segment is also inferred from the presence of rare orthopyroxene-rich xenoliths in the same suite. These are interpreted to have originated as lithospheric precipitates of high-Mg, SiO2-saturated magmas that require a highly depleted mantle source. Such source is typically required by boninitic-like magmas of intraoceanic suprasubduction settings. A proposed geodynamic model to explain these observations involves accretion or underplating of the lower parts of the Tethyan oceanic lithosphere during the Upper Jurassic closure of the eastern branch of the Vardar ocean.
Fertile lithology can be explained as addition of 5-20 wt% of a basanitic-like melt to a refractory mantle. The inversion modeling, performed on least contaminated and most isotopically uniform host basanites, implies a source which is enriched in highly and moderately incompatible elements (~35-40xchondrite for U-Th-Nb-Ta) suggesting that the primary magma of the host basanites was not derived by melting of a homogeneous asthenospheric source. D0 values of the calculated basanitic source are similar to D0 values of anhydrous metasomatized mantle with small additions of metasomatic clinopyroxene and carbonate (~5 %) and with traces of ilmenite (~1 %) and apatite (~0.05 %). A schematic two-phase model involves percolation of CO2-and H2O-rich fluids, precipitation of metasomatic hydrous minerals and their subsequent breakdown due to the further uplift of hot asthenospheric mantle.
METASOMATISM IN SPINEL DUNITE XENOLITHS FROM THE BEARPAW MOUNTAINS, MONTANA, USA
FACER JOHN 1, DOWNES HILARY 1, BEARD ANDY 1
presenter's e-mail: jrf@adidem.fsworld.co.uk
1 - SCHOOL OF EARTH SCIENCES, BIRKBECK, UNIVERSITY OF LONDON, MALET STREET, LONDON WC1E 7HX, UK
Keywords: ultramafic xenoliths, metasomatism, Montana
The Bearpaw Mountains are located on the northern margin of the Archean Wyoming craton and form part of an extensive Eocene high-K alkaline magmatic province. Ultramafic xenoliths hosted by this magmatism include highly depleted spinel peridotites (the tectonite suite) and chromite spinel dunites with iron-rich olivine (the Fe-rich dunites). Presented here are the initial results of a petrological and geochemical study of the unusual and previously unreported Fe-rich dunites.
The Fe-rich dunites have an olivine forsterite content in the range Fo83-87 which indicates that they are not the refractory end-members of a lherzolite-harzburgite-dunite mantle melt-depletion trend. Whether they are cumulates or the products of melt-rock reaction is at present unclear but their microstructure indicates high temperature grain boundary migration and dynamic recrystallization which are processes not usually associated with cumulates.
In the Fe-rich dunites, fluid films along olivine grain boundaries connect with a network of grain-penetrating veinlets and small (0.5 to 3.0 mm) "reaction pockets". These pockets typically contain grains of spinel and/or diopside (and, less commonly, enstatite or pargasite) undergoing breakdown reactions. This is evidenced in the spinel by embayments, striations and scalloped margins and, in the pyroxenes and amphibole, by fragmentation into numerous small patches. Some of the spinel show zoning with Mg, Al-rich cores and Al-poor, Fe, Ti and Cr-rich rims. The reaction pockets also commonly include fragments of olivine derived from the primary olivine surrounding the pocket as well as newly precipitated olivine grains and minute grains of spinel. The grains undergoing breakdown are surrounded by a groundmass of Mg-rich, Al-poor chlorite and/or an Mg and Mn-rich calcite. The interconnecting veinlets and films along grain boundaries appear to contain mainly chlorite with some patches of calcite. In a few xenoliths, the calcitic groundmass of reaction pockets appears to have been replaced by phlogopite.
Some of the Fe-rich dunite xenoliths also contain veins of Al-poor orthopyroxene. In one sample, the orthopyroxene forms fibrous to elongate grains: radiating clusters of fibrous grains form at intervals along the vein and elongated grains with length/width ratios as high as 140 grow out from the vein into the host olivine. Some of the radiating clusters have at their hub vesicles (now filled with chlorite) suggesting that nucleation may have been triggered by a release of volatiles. Small patches of calcite occur within the orthopyroxene veins and intergrowths of orthopyroxene and clinopyroxene (diopside or Mg-rich augite) also occur. Low-Al orthopyroxene veins also occur in some of the harzburgites of the tectonite suite and it is clear that several generations of such veins exist.
The modal metasomatism evidenced in the Fe-rich dunites by the reaction pockets and orthopyroxenite veins may both be explained by the migration of an H2O + CO2 fluid, rich in silica but poor in alumina, which reacted with the dunites. The origin of this hydrous fluid may be connected with the steepening subduction of the originally shallow-dipping Farallon plate under North America during the Eocene.
PETROLOGICAL INVESTIGATIONS ON DEEP-SEATED ROCKS (HARZBURGITE, DUNITE AND GABBROS) AROUND THE KAIREI HYDROTHERMAL FIELD IN THE CENTRAL INDIAN RIDGE
HARA KAORI 1, KUMAGAI HIDENORI 2, OKINO KYOKO 3, MORISHITA TOMOAKI 1, SAWAGUCHI TAKASHI 4, JOSHIMA MASATO 5, NAKAMURA KENTARO 6, NEO NATSUKI 7, SHIBUYA TAKAZO 8, SATO TAICHI 3
presenter's e-mail: kaori-h@earth.s.kanazawa-u.ac.jp
1 - KANAZAWA UNIVERSITY
2 - JAMSTEC
3 - TOKYO UNIVERSITY
4 - SHOUHOKU COLLEGE
5 - AIST
6 - JSPS, JAMSTEC
7 - NIIGATA UNIVERSITY
8 - TOKYO INSTITUTE OF TECHNOLOGY
Keywords: peridotite, Ocean core complex, Kairei hydrothermal field, Central Indian Ridge
We investigated petrological characteristics of deep-seated rocks, such as harzburgite, dunite and gabbros, collected from an oceanic core complex and two topographic highs (the so-called 25°S OCC and Uraniwa-hills, respectively) around the Kairei hydrothermal filed, which is characterized by a high concentration of H2 in the vent fluid (Takai et al., 2004), in the Central Indian Ridge.
The 25°S OCC is a dome-like shape dipping shallowly toward the axis near the ridge. It is also characterized by continuous surfaces on which prominent striations parallel the spreading direction. Gabbros are recovered and are massive with variations both in composition (gabbro, gabbro norite, oxide-rich gabbro) and grain size at the scale of individual samples. Some gabbros are amphibolitized and are further locally altered under green schist facies conditions. Microstructures show magmatic textures, usually with no or very minor plastic deformation except for very localized mm-scale shear zones in altered gabbros. Two serpentinized peridotites are recovered along the ridge-facing slope. These peridotites have been statically serpentinized and show the typical serpentine mesh texture replacing olivine crystals, serpentine pyroxene pseudomorphs. Modal amount of clinopyroxene is low in the both samples. These peridotites were intruded by highly altered gabbroic veins now consisting of tremolitic amphibole, chlorite and saussurite with small amount of ilmenite (partly replaced by titanite), apatite and zircon. Plagioclase pseudomorph is frequently observed in a sample abundant in the gabbroic veins. On the other hand, we collected olivine-rich gabbroic rocks (troctolite including one plagioclase-bearing dunite) with small amount of olivine gabbros from the Uraniwa hills. These oceanic highs are interpreted to be exhumed footwall consisting of oceanic lower crust and upper mantle along low-angle detachment faults. Hydrogen can be produced by a reaction for the serpentinization of these olivine-dominate rocks due to sea water circulations related to the formation of the detachment fault.
Chemical compositions of chromian spinel without chemical modification due to formation of plagioclase (now saussuritized) and the gabbroic veins from recovered serpentinized samples in the 25°OCC shows Cr/(Cr+ Al) atomic ratio = 0.3-0.45. This combined with clinopyroxene-poor lithologies indicates that degree of partial melting of peridotite samples from the 25°S OCC is higher than typical peridotites recovered from slow-spreading ocean floor (Dick, 1989), although the 25°S Megamullion is located at a segment end of the ridge where generally smaller degrees of melting in the mantle are expected, the so-called transform fault effect (Ghose et al., 1996). Petrological and geochemical characteristics of serpentinites from the OCC show that the degree of partial melting is higher in the studied samples than in slow-spreading ridges. On the other hand, it is interesting to note that the dunite-troctolite-olivine gabbros in the Uraniwa hills were similar to those found from the first-spreading East Pacific Rise (e.g., Arai & Matsukage., 1996), where magmatic activity is expected to be high. We will also discuss the process of serpentinization in these samples.
SILICATE MELT INCLUSIONS IN AMPHIBOLE-BEARING SPINEL PERIDOTITE XENOLITHS FROM THE BAKONY-BALATON HIGHLAND VOLCANIC FIELD (WESTERN HUNGARY)
HIDAS KÁROLY 1, SZABÓ CSABA 1, GUZMICS TIBOR 1, BALI ENIKO 2, ZAJACZ ZOLTAN 3, KOVÁCS ISTVÁN 4
presenter's e-mail: karoly.hidas@gmail.com
1 - Lithosphere Fluid Research Lab, Eötvös University, Hungary
2 - Bayerisches Geoinstitut, Universität Bayreuth, Germany
3 - Department of Earth Sciences, Institute of Isotope Geochemistry and Mineral Resources, ETH, Switzerland
4 - Research School of Earth Sciences, The Australian National University, Canberra, Australia
Keywords: spinel peridotite xenoliths, silicate melt inclusions, CO2-bearing fluid inclusions, Pannonian Basin, Hungary
In this work we present a detailed textural and geochemical study of two equigranular textured amphibole-bearing spinel lherzolite xenoliths of the subcontinental lithospheric mantle (Szg07 and Szg08) from Szigliget, Bakony-Balaton Highland Volcanic Field (Pannonian Basin) containing abundant primary silicate melt inclusions (SMI) in clinopyroxene rims and secondary SMI in orthopyroxene (and rarely spinel) along healed fractures. The SMI are dominantly composed of silicate glass and CO2-bearing fluid phase at room temperature. Only the primary SMI show evidence for significant post-entrapment modification as a result of crystallization on the wall of the host clinopyroxene.
Microprobe study reveals that clinopyroxene and orthopyroxene are zoned in both studied xenoliths, especially in Fe, Mg, Na and Al contents. Cores of clinopyroxenes in both xenoliths show trace element distribution close to primitive mantle. Rims of clinopyroxenes are enriched in Th, U, LREE and MREE. Amphiboles, particularly in Szg08 xenolith, exhibit elevated Rb, Ba, Nb, Ta, LREE and MREE content.
The composition of silicate glass in the SMI covers a wide compositional range from the basaltic trachyandesite and andesite to phonolitic composition through the most common trachyandesite. The glasses are rich in P2O5 (up to 2.88 wt%). There are no notable differences in glass compositions neither in the different host minerals nor in the different xenoliths, except the Cl and TiO2 content. The SMI, either primary or secondary ones, are strongly enriched in incompatible elements (particularly in U, Th, La, Zr), displaying slight negative Hf anomaly. The SMI in Szg07 show approximately one order higher enrichment in LILE and LREE compared to those of SMI in xenolith Szg08.
Based on microthermometry of the fluid phase of SMI, melting temperatures show values below -56.6 °C, which indicate the presence of other fluid components beside CO2. Furthermore, homogenization of fluids into the liquid phase at low T (in the range of -40 and -53 °C) suggests extremely high density for the CO2–rich fluids. Furthermore, numerous opx- and cpx-hosted SMI reveal various glass-fluid ratios.
During high-temperature experiments of cpx-hosted SMI, the silicate glass started to melt at 850-900 °C. At 1150 °C the glass was completely melted, however the fluid phase was not homogenized into the melted glass. The SMI-cpx boundary at 1150 °C reveals that some cpx from the host dissolved into the glass in all runs. At 1200 °C the cpx-hosted SMI decrepitated without complete homogenization. In contrast, the opx-hosted SMI, indicating the same melting features, were not decrepitated at same temperature.
The development of zoned pyroxenes, as well as the trapping of primary SMI in the clinopyroxene rims happened after the partial melting and subsequent crystallization of clinopyroxenes, most probably due to an interaction between hot volatile-bearing mafic melt and mantle wall-rock. This interaction formed evolved silicate melts, which filled microfractures in orthopyroxenes (and rarely spinels), resulting in secondary SMI ranging from the basaltic trachyandesite and andesite to phonolitic compositions. Their trace element distributions and high volatile contents also suggest that the studied melt, which were trapped as SMI, could be the product of fractionation crystallization of a mafic magma at mantle T-P conditions. The relatively high fluid content permits to assume the migration of these evolved melts along veins or grain boundaries in the mantle, causing significant metasomatism on reactive mantle minerals. The different fluid-glass ratios in the studied SMI indicate a pre-entrapment phase separation. Besides this, the trace element composition modeling suggest that melt similar to the host alkali basaltic magma, can be responsible for the presence of zoned mantle minerals together with SMI, which, therefore, could act as a metasomatic agent in the upper mantle.
MIDDLE MIOCENE VOLCANISM IN THE VICINITY OF THE MIDDLE HUNGARIAN ZONE: EVIDENCE FOR AN INHERITED ENRICHED MANTLE SOURCE
KOVÁCS ISTVÁN 1, SZABÓ CSABA 2
presenter's e-mail: Istvan.kovacs@anu.edu.au
1 - AUSTRALIAN NATIONAL UNIVERSITY AND EÖTVÖS UNIVERSITY, (LRG)
2 - EÖTVÖS UNIVERSITY, (LRG)
Keywords: geodynamics, xenoliths, subduction, volcanism, Carpathian-Pannonian region
Middle Miocene igneous rocks in the vicinity of the Middle Hungarian zone (MHZ) show subduction-related geochemical characteristics of which strength faded away with time. In contrast to previous models, which suggest that southward-dipping subduction of the European lithosphere beneath the Alcapa microplate along the Western Carpathians was responsible for the source enrichment of middle Miocene volcanics, we propose that source enrichment happened along the subduction of either the Budva-Pindos or Vardar Oceans. Recent seismic studies have revealed that the proposed southward-dipping subduction had not developed beneath the entire Western Carpathians or even if it had, it was overprinted after the collision between the European plate and the Alcapa unit at 16 Ma. This subduction is thought to have started 30 Ma ago, therefore the time between the onset of subduction and collision cannot account for extensive source enrichment in the overlying mantle wedge. Furthermore, the middle Miocene volcanics along the MHZ in their reconstructed position are not parallel to the supposed suture, as would be expected for subduction-related arc volcanoes.
We propose, alternatively, that the source enrichment was related to the subduction of either the Budva-Pindos or Vardar Ocean during the Mesozoic-Paleogene. The Alcapa microplate was transferred from this distant tectonic setting to its present tectonic position via extrusion and rotations. Geophysical modeling and mantle xenoliths provide evidence that this process happened at the scale of the lithospheric mantle, therefore the subduction-modified lithospheric mantle was not decoupled from the crust. Melting in the lithospheric mantle of the Alcapa unit was triggered by the extension during the formation of the Pannonian Basin. The preserved subduction-related geochemical character of volcanics in intra-plate settings that are otherwise directly unaffected by subduction can be accounted for by tectonic transport of metasomatized mantle from a previous subduction-affected setting. In general this model provides an alternative approach to understand the geochemical complexity of intra-plate calc-alkaline volcanics without the involvement of plumes.
INSIGHTS INTO REFERTILIZATION PROCESSES IN LITHOSPHERIC MANTLE FROM INTEGRATED ISOTOPIC STUDIES IN THE LHERZ MASSIF
LE ROUX VÉRONIQUE 1, BODINIER JEAN-LOUIS 2, ALARD OLIVIER 1, O'REILLY SUZANNE Y. 3
presenter's e-mail: le-roux@gm.univ-montp2.fr
1 - GÉOSCIENCES MONTPELLIER - GEMOC
2 - GÉOSCIENCES MONTPELLIER
3 - GEMOC
Keywords: Lherz, refertilization, erosion of lthosperic mantle
Differentiation of the Earth's mantle occurs principally through partial melting and extraction of basaltic melt. Among the mantle rocks occurring at the Earth's surface, harzburgites are widely considered as refractory mantle residues left after extraction of a basaltic component. In contrast, fertile lherzolites are generally regarded as pristine mantle, only weakly affected by partial melting. However in the Lherz Massif (France), structural mapping shows that the lherzolites are secondary rocks formed at the expense of the harzburgites. Variations of major, minor and trace elements across the harzburgite-lherzolite contacts indicate that the lherzolites were formed through a refertilization process involving interaction of refractory, lithospheric mantle with upwelling asthenospheric partial melts. Rare-earth elements (REE) in clinopyroxenes display variable chondrite-normalized REE patterns. Massive harzburgite bodies show U-shaped REE patterns whereas lherzolites have classic N-MORB REE patterns as observed in orogenic lherzolites worldwide. However at the contact, both lherzolite and harzburgite show more LREE-enriched clinopyroxenes than their distal counterparts. These REE enrichments also cannot be explained by partial melting model and provide further evidence for refertilization.
In order to further constrain the mechanisms involved in the refertilisation process, we investigated Sr, Nd and Hf isotopic compositions of over 15 samples accross a harburgite-lherzolite contact, as well as "distal" samples. Sr isotopes were measured in whole-rocks and clinopyroxene separates (TIMS and MC-ICP-MS), combined with the Sr in-situ method on clinopyroxenes (LA-MC-ICPMS). Al2O3 is negatively correlated with 87Sr/86Sr and positively correlated with 143Nd/144Nd. These correlations are consistent with REE patterns. Distal harzbugites have 87Sr/86Sr = 0.703638(6), while distal lherzolites have 87Sr/86Sr between 0.7020 and 0.7025. Towards the contact, the lherzolites show a steady increase in 87Sr/86Sr up to 0.7032. The harzburgites within the contact radiogenic compositions up to 0.7055. The highly radiogenic composition of the contact zone is not compatible with melting models. Further investigations are in progress to characterise the scale and range of isotopic variations across the transition from harzburgite and lherzolite.
ORIGIN OF XENOLITHS FROM WINNA GÓRA BASALT (SW POLAND)
MATUSIAK MAGDALENA 1, PUZIEWICZ JACEK 1
presenter's e-mail: MAMATU@ING.UNI.WROC.PL
1 - WROCŁAW UNIVERSITY
Keywords: Basalt, Xenoliths, Cumulates, SW Poland
The basalt from Winna Góra near Jawor in SW Poland (K–Ar age ~ 22Ma) belongs to the Tertiary Central European Volcanic Province. Small ( 5 cm) peridotite (dunite, lherzolite, harzburgite), websterite, clinopyroxenite and gabbro xenoliths occur in the rock.
The xenoliths consist of olivine clinopyroxene orthopyroxene plagioclase. Small (200 m) pools consisting of younger generation of olivine, clinopyroxene, spinel and plagioclase occur between primary phases in peridotite and websterite. Clinopyroxene forming gabbro has spongy structure due to voids (up to 100 m) filled with plagioclase and olivine.
The main constituent of dunite is olivine (Fo 89–82%, Ca300 – 1450 ppm, NiO 0.11–0.43 wt.%). In olivine occurring (together with spinel, clino- and orthopyroxene) in pools the Fo content varies from 82 to 85% (Ca 750–1800 ppm, NiO 0.19–0.35 wt.%), #mg of clinopyroxene (CaO 20-23 wt.%) is 88-82 , in orthopyroxene (Al2O3 0.5–0.6 wt.%) is 86-87. The #cr of spinel is 74–78.
Fo content in olivine forming lherzolite varies from 75 to 80 % (Ca< 300–1300 ppm, NiO 0.22–0.37 wt.%), #mg of clinopyroxene is 0.86 (CaO 23 wt.%) and in orthopyroxene is 0.83 (Al2O3 0.6–0.74 wt.%). Spinel #cr is 0.73-0.76.
Olivine forming harzburgite contains 82–88% of Fo (Ca 300–1700, NiO 0.2–0.5 wt%), the #mg of orthopyroxene is 85–87 (Al2O3 0.9–1.6 wt.%), #cr of spinel is variable from 0.5 to 0.7. Younger olivine (Fo 80–85, Ca 600–1500 ppm, NiO 0.09-0.3 wt.%) occurs in pools together with clinopyroxene (#mg 0.80-0.91, CaO 18–23 wt.%) and spinel (#cr 0.8–0.86).
Websterite consists of orthopyroxene (#mg 0.74-0.84, Al2O3 1.2-3.9 wt.%) and clinopyroxene (#mg=0.86, CaO 21–23 wt.%). Clinopyroxene occurring in pools together with olivine (Fo 71–78, Ca 1400–2300 ppm, NiO 0.01–0.06) contains from 19 to 21 wt.% of CaO (#mg=0.86).
Clinopyroxene forming clinopyroxenite contains 23–24.5% of CaO, #mg is variable from 0.71–0.87.
Gabbro consists of clinopyroxene (#mg=68–74, CaO 19–22 wt%) and plagioclase (An 40– 56%).
An content in plagioclase occurring with olivine (Fo 64–70, CaO 1600–2200 ppm, NiO 0.02–0.09 wt%) in voids in clinopyroxene varies from 0 to 24%.
Temperatures of primary clino- and orthopyroxene equilibration (Brey and Köhler algorithm) are 850-880°C (lherzolite, dunite) and 960-990°C (websterite).
Spinel and olivine occurring in xenoliths from Winna Góra do not fall in Olivine-Spinel Mantle Array of Arai (1994) but are rather similar to those from volcanic rocks or cumulates. Volcanic chemical composition of spinel is also suggested by plot of data on Al2O3/TiO2 and Al2O3/(Fe2+/Fe3+) diagrams by Kamenetsky et al., 2001.
The presented data indicate that the xenoliths from Winna Góra are not of mantle origin: olivine is too rich in Fe, spinel has compositions which differ from those of OSMA. The xenoliths may be products of cumulative processes from host basaltic magma or may come from older, ultramafic intrusion located at the crust-mantle boundary. Size of the xenoliths suggest rather slow ascent of magma, which resulted in sinking of larger xenoliths. Small sizes maybe also due to disintegration of primarily larger xenoliths.
Due to small size of the xenoliths, which precludes determination of whole-rock geochemistry, mineral chemistry is the only possible source of information about their origin. Mineral-chemical data supplied from minerals are unequivocal, but suggest that gabbro and clinopyroxenite are cumulates of host basaltic magma. Some features (e.g. NiO and Ca contents in olivine) of phases forming peridotites and websterite suggest their mantle origin, whereas the other (e.g. Fo contents in olivine) are typical for crystallization as shallow as mantle/crust boundary. The variation in mineral chemistry is similar to that of Beaunit layered intrusion (Féménias et al, 2003). Younger generation of minerals is probably a result of decompressional melting of xenoliths during magma ascent.
A SUBCONTINENTAL MANTLE-LOWER CRUSTAL ASSOCIATION FROM CENTRAL CORSICA (FRANCE): PRELIMINARY DATA ON THE ST. LUCIA NAPPE MANTLE SEQUENCE
MONTANINI ALESSANDRA 1, TRIBUZIO RICCARDO 2
presenter's e-mail: alessandra.montanini@unipr.it
1 - DIPARTIMENTO DI SCIENZE DELLA TERRA UNIV. DI PARMA
2 - DIPARTIMENTO DI SCIENZE DELLA TERRA UNIV. DI PAVIA
Keywords: Corsica, mantle, lower crust, peridotite, pyroxenite
The Corsica island was linked to the Pyrénées–Provence domain in southern France until the Early Miocene drifting from the European mainland following the collision between Europe and Adria. Corsica is divided into two main geological domains. The northeastern area ("Alpine Corsica") is formed by a complex stack of nappes derived from the Ligurian Tethys oceanic litosphere and older continental crust, both commonly displaying HP-LT metamorphism developed in Late Cretaceous to Eocene times.
The S. Lucia nappe belongs to the Alpine Corsica and consists of a pre-Alpine crystalline basement overlain by a Middle Cretaceous detritic cover. The S. Lucia basement includes a High Grade Mafic complex and a Granitoid complex (Libourel, 1985; Zibra, 2006), which partly escaped the Alpine tectono-metamorphic event. In particular, the High Grade Mafic complex consists of a layered gabbroic sequence of Early Permian age (Paquette et al., 2003) which contains lenses of felsic granulites and displays a pervasive recrystallisation under granulite facies conditions (P ca. 0.9 GPa, T = 800-900°C, Caby & Jacob, 2000). As a whole, the High Grade complex shows many similarities to the Ivrea-Verbano lower crustal sequence from the Western Alps (Libourel, 1985). The base of the mafic sequence is associated with up to 50-m thick mantle slices. The mantle rocks consist of spinel-bearing lherzolites (clinopyroxene = 10-15 % vol) with minor, up to cm-thick pyroxenite layers. The peridotites show a subvertical mylonitic to ultramylonitic foliation roughly concordant with that of the nearby metagabbroic rocks. The pyroxenite layers are oriented subparallel to the peridotite foliation.
The mylonite microstructure in the peridotite is characterised by aligned porphyroclasts of pyroxene (and rare spinel) in a fine-grained polyphase matrix composed of olivine (Fo88-89) + pyroxenes + spinel occurring as mm-size bands, lenses and porphyroclast tails. Orthopyroxene porphyroclasts are stretched along the mylonitic foliation with high aspect ratio. Both pyroxenes commonly show evidence for intracrystalline deformation. Relics of an older low-strain spinel tectonite predating the mylonite deformation are locally preserved. The composition of clinopyroxene (Al2O3 = 4.5-5.1 wt%, Na2O= 0.70-0.90 wt%) and spinel porphyroclast cores (Cr# 0.14) reveal a rather fertile character. The pyroxenites are Opx-poor spinel websterites containing small amounts of Ti-rich amphibole and plagioclase. They have fine-grained granoblastic texture and may show elonged porphyroclasts of green spinel (with altered plagioclase rims) and Al-rich clinopyroxene.
Temperature estimates based on clinopyroxene-orthopyroxene equilibria for both peridotites and websterites (Wells, 1977; Brey & Kohler, 1990) are in the range 830-890°C, similar to the temperature conditions reported for the granulite-facies recrystallization of the associated lower crustal rocks. There is no significant decrease from the porphyroclastic to the neoblastic assemblage equilibration temperatures of the peridotites. However, the orthopyroxene porphyroclasts record up to ca. 1000°C values, employing the Ca-in-Opx geothermometer (Brey & Kohler, 1990). This suggests a former higher-T stage in the spinel stability field, in agreement with the occurrence of mutual pyroxene exsolution lamellae and tiny Cr-spinel exsolutions in the pyroxene porphyroclasts.
Brey and Kohler, 1990. J. Petrol., 31, 1353-1378
Caby and Jacob, 2000. Geol. de la France 1, 21-34.
Libourel, 1995. These doct., Univ. Toulouse, 405 pp.
Paquette et al., 2003. Chem Geol. 148, 1-120
Wells, 1977. Contrib. Mineral. Petrol. 62, 129-139
Zibra, 2006. PhD Thesis, Univ. Pisa, 204 pp.
CRYSTALLOGRAPHIC ORIENTATION AMONG SYMPLECTITE MINERALS IN THE HOROMAN PERIDOTITE COMPLEX, JAPAN
NAGASHIMA RYOKO 1, ODAJIMA NORIHIRO 2, MORISHITA TOMOAKI 1, OZAWA KAZUHITO 2
presenter's e-mail: galikson@earth.s.kanazawa-u.ac.jp
1 - Kanazawa University
2 - Tokyo University
Keywords: Two-pyroxene spinel symplectite, crystallograghic orientation, Horoman peridotites complex
Symplectites consisting of orthopyroxene, clinopyroxene and spinel in the Horoman Peridotite Complex, Japan, are thought to be the subsolidus reaction product after garnet with addition of olivine component during decompression of the complex from the garnet-lherzolite stability conditions. Automated electron backscattered diffraction (EBSD) analysis using a field-emission gun SEM was applied for determination of crystallographic orientations of symplectite minerals. Minerals have similar crystallographic orientations in each symplectite, although they occur as isolated grains in a two-dimensional plane. Systematic crystallographic orientations among them are the same among the studied samples collected from the upper part to the lower part of the complex and are summarized as follows: ① the (100) and (010) planes and [001] axis of orthopyroxene are parallel to the (100) and (010) planes and [001] axis of clinopyroxene, respectively, and ② the (100) and (010) planes are parallel to the {111} and {101} planes of spinel, respectively. ①, the crystallographic relationships between orthopyroxene and clinopyroxene in the Horoman symplectites are the same as those of Bushveld-type exsolution in pyroxenes. A qualitative model for the formation of symplectitic mineral intergrowth was proposed. Garnet in peridotites become a thermodynamically unstable high-Cr, Al orthopyroxene as pressure-temperature conditions were changed from garnet-lherzolite stability conditions to spinel-lherzolite stability conditions. Then high-Cr, Al clinopyroxenes were exsolved from the unstable orthopyroxene following the crystallogaraphic rules of the Bushveld-type exsolution. The reaction volumes from the precursor garnet to the orthopyroxene-clinopyroxene intergrowth might be large enough for minerals to be grown in three-dimensional directions the reaction was caused by decompression. Finally, spinel was selectively nucleated at grain boundaries between orthopyroxene and clinopyroxene by the reaction with olivine components that were introduced by diffusion from the surroundings.
EBSD data also suggest that orientation contrast of symplectite spinel in each symplectite tends to occur around one of <111>axis of spinel. Furthermore the <111>axis of spinel, the rotation axis, are orientated the direction parallel to the foliation and perpendicular to the lineation. Further investigations on relationships between deformation and crystallographic orientations of symplectite minerals are needed because rotation of crystallographic orientations of symplectite minerals might be caused by deformation of the complex after the formation of symplectite textures.
THE ORIGIN OF THE ALKALINE AND HIGH-K CALC-ALKALINE MAGMAS ALONG THE SE MARGIN OF THE STYRIAN BASIN, AUSTRIA
NTAFLOS THEODOROS 1, KRUMPEL GEORG 1, HARANGI SZABOLCS 2, TSCHEGG CORNELIUS 1
presenter's e-mail: theodoros.ntaflos@univie.ac.at
1 - DEPARTMENT OF LITHOSPHERIC SCIENCES, UNIVERSITY OF VIENNA, AUSTRIA
2 - EÖTVÖS UNIVERSITY, DEPARTMENT OF PETROLOGY AND GEOCHEMISTRY, BUDAPEST, HUNGARY
Keywords: Calc-alkaline rocks, Alkaline rocks, Pre-enriched mantle, Styrian Basin
The potassic volcanism occurred in the Pannonian-Carpathian area could provide valuable information about the nature of the lithosphere and the melting/metasomatic processes that took place beneath this region.
The Styrian basin, the westernmost sub-basin of the Pannonian basin system, is located as well at the eastern margin of the Eastern Alps. The majority of the studied samples were revealed by drilling cores in the south-eastern part of the Styrian Basin. The rest of the samples have been collected from quarries.
The rocks are high-K calc-alkaline (andesites and dacites) and alkaline (latites and shoshonites). K/Ar-ages range from 16.3 to 13 Ma with high-K calc-alkaline rocks representing the oldest suite. A striking petrographicdifference between both groups is that the calc-alkaline rocks are amphibole bearing and lack clinopyroxene. Alkaline rocks include zoned clinopyroxenes that have magmatic corroded cores overgrown by more basic rims suggesting basic magma reflux.
In the spiderdiagrams all the samples show remarkable negative Nb and Ta troughs and high contents of LILE. Additionally the calc-alkaline rocks have negative P anomaly. However, the calc-alkaline rocks are characterized by low contents of LILE and HFSE when compared to the alkaline rocks.
The calc-alkaline rocks from Styria are similar to those from the Carpathian-Pannonian area that have long been recognized as being typical for subduction related-magmas (Seghedi et. al. 2004). They can be modelled by an AFC-process controlling the composition of an IAB-magma with F=0.6 and r=0.4. The upper continental crust is proposed as contaminant.
While high-K calc alkaline rocks can be explained by an AFC-process the composition of the alkaline rocks (latites and shoshonites) seems to be a consequence of batch melting processes of a pre-enriched mantle, which can be derived by mixing of primitive lithospheric mantle with material from the continental crust. Harangi et al. 1995 proposed a similar explanation for the Pannonian alkaline volcanism. Model calculations reveal that the fraction of continental material in the mixture was 4% and the batch melting process took place in the stability field of spinel-lherzolite at 5%.
From the geodynamical point of view, the calc-alkaline lavas can be seen as a consequence of the oceanic subduction, which exhibit the basis of the Rhenodanubian-flysch or of the West-Carpathian-flysch. This subduction was related to the N-S convergence of Africa and Europe in the Early Miocene. In contrast the alkaline magmatism was the result of decompressional melting of a metasomatic pre-enriched mantle. The process of decompressional melting may be related to the extension that characterizes the western margin of the Panonnian Basin during the Middle Miocene.
Literature
Harangi Sz., Wilson M. & Tomarini S. (1995) Petrogenesis of Neogene potassic volcanic rocks in the Pannonian Basin. Acta Vulcanologica, 7, 125-134.
Seghedi I. Downes H., Szakacs A., Mason P.R.D, Thirwall M.F., Rosu E., Pecskay Z., Marton E. & Panaiotu C. (2004) Neogene-Quaternary magmatism and geodynamics in the Carpathian-Pannonian region: a synthesis. Lithos, 72, 117-146.
CRYSTAL CHEMISTRY OF CLINOPYROXENES ENCLOSED IN UPPER MANTLE XENOLITHS FROM THE CARPATHIAN-PANNONIAN REGION (HUNGARY): CONTRIBUTIONS TO PETROGENESIS AND PRESSURE ESTIMATION
NÉDLI ZSUZSANNA 1, PRINCIVALLE FRANCESCO 2, DOBOSI GÁBOR 3, EMBEY-ISZTIN ANTAL 4, HIDAS KÁROLY 1, BERKESI MÁRTA 5, SZABÓ CSABA 1
presenter's e-mail: nedlizs@yahoo.com
1 - LITHOSPHERE FLUID RESEARCH LAB, DEPARTMENT OF PETROLOGY AND GEOCHEMISTRY, EOTVOS UNIVERSITY BUDAPEST
2 - DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF TRIESTE
3 - INSTITUTE OF GEOCHEMISTRY, HUNGARIAN ACADEMY OF SCIENCES, BUDAPEST
4 - NATURAL HISTORY MUSEUM, BUDAPEST
5 - LITHOSPHERE FLUID RESEARCH LAB,
Keywords: upper mantle xenoliths, clinopyroxene, crystal chemistry, pressure estimation, Carpathian-Pannonian Region
Crystal structural characteristics provide significant information about the petrogenesis and equilibrium pressure conditions of the mantle xenoliths. Most of the crystal structural analysis have been carried out on xenoliths showing undeformed texture until now. We selected a texturally heterogeneous upper mantle xenolith series from different part of the Carpathian-Pannonian Basin 1) to reveal crystal structural and chemical variation in xenolith series with textural and deformation variety, 2) to estimate equilibrium pressure conditions of different textured mantle xenoliths, and 3) to contribute to understand the petrogenesis of the special group of poikilitic and mosaic textured xenoliths.
For analysis we selected a well described, texturally heterogeneous xenolith series hosted in Plio-Pleistocene alkaline basalts from the Bakony-Balaton Highland Volcanic Field (central part of the Carpathian-Pannonian Region), in Late-Cretaceous lamprophyres from Villàny Mts (S Hungary) and in Late-Cretaceous- Paleogene alkali basalts from Poiana Rusca (Romania, peripherial part of the region). Clinopyroxenes from xenoliths were studied by means of single crystal X-ray diffraction and electron microprobe analysis.
Xenoliths studied show wide textural variety: the dominant textural types (protogranular, porphyroclastic, equigranular) compose the main series, whereas the rarer poikilitic and mosaic textured samples form a special group. The main series samples represent common textural evolution of the upper mantle, governed by polimetamorphic process and increasing deformation in the progressive protogranular - porphyroclastic - equigranular series. For mosaic and poikilitic textured xenoliths several authors proposed formation in relation with mantle derived melts.
Previous crystal chemistry studies on mantle xenolith clinopyroxenes have shown that crystal structure depends notably on the equilibrium physical conditions, and relations between cell and site (M1, M2) volumes are indicative of different equilibrium pressures. These observations allowe us to use structural parameters as a useful and chemically independent estimation of equilibrium pressure of mantle xenoliths even for spinel peridotite facies.
Our results suggest that protogranular, porphyroclastic and equigranular xenoliths define the main trend of chemical, structural and textural changes in the suites which is characterized by the progressive increase of deformation in proportion with the decrease of equilibrium pressure. Protogranular xenoliths record pressure near to garnet stability field, whereas equigranular xenoliths were equilibrated near to plagioclase stability field. This inverse proportion of pressure and deformation geodynamically can be explained by the mantle diapir beneath the Carpathian-Pannonian Region, which could cause significant deformation and lithosphere thinning in the centre of the basin, in accordance with the fact that xenoliths from the peripherial parts of the basin record higher pressures.
Poikilitic and mosaic xenoliths are detached in crystal chemistry from the main trend, suggesting that their origin and evolution may not be explained by the continuous deformation/depletion. Despite of their similar texture, poikilitic xenoliths from different parts of the basin show very different structural parameters, suggesting that thay likely were formed at different pressure conditions. Xenoliths deriving from the centre show pressure conditions near to plagioclase stability field, whereas that ones from the peripherial areas of the Basin indicate remarkably higher pressure. These observations are in accordance with works which link them to percolating melts in the lithospheric mantle indicating that they can be formed by interaction of this melts at different depths of the subcontinental mantle.
S-WAVE VELOCITY MODELS OF THE LITHOSPHERE-ASTHENOSPHERE BENEATH THE TYRRHENIAN SEA: IMPLICATION FOR MAGMATISM, GEODYNAMICS AND MANTLE DEGASSING
PECCERILLO ANGELO 1, PANZA GIULIANO 2, FREZZOTTI MARIA LUCE 3, AOUDIA ABDULKARIM 2, DOGLIONI CARLO 4
presenter's e-mail: pecceang@unipg.it
1 - DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITA' DI PERUGIA
2 - DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITA' DI TRIESTE
3 - DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITA' DI SIENA
4 - DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITA’ DI ROMA "LA SAPIENZA"
Keywords: S-wave tomography, Magmatism, Tyrrhenian Sea, Mantle degassing, Mantle metasomatism
Shear-wave velocity models, inverted from Rayleigh-wave dispersion tomography, for the lithosphere-asthenosphere system along four transects across the Western-Central Mediterranean area, reveal the presence of a thick layer, at about 70 - 120 km of depth, with low S-waves velocity (VS ~ 4.0-4.2 km/sec), that can identify low-rigidity, fluid-bearing mantle material. This layer extends from the Balearic Sea to the central and southern Tyrrhenian margins of the Italian peninsula (Panza et al., 2006), where it thins and raises to a depth of about 30 km, below the presently active Campanian and Aeolian volcanoes. Low S-waves velocity layers are absent along the Sicily channel, where thin shallow lenses with low S-wave velocities (Vs ~ 4.0-4.2 km/sec) are restricted to volcanic active areas of Ferdinandea-Graham Island and Etna.
We suggest that the low S-wave velocity layer across the Balearic Sea and the central and southern Tyrrhenian Sea is the effect of mantle contamination by subduction-related C-O-H fluids (e.g. H2O, CH4, CO2) released by the eastward retreating Adriatic-Ionian subduction plates from Oligo-Miocene to present. The upraise of this layer to shallow depths beneath the active volcanic areas of southern Italy is attributed to eastward mantle flow above the retreating slab and to the release of aqueous fluids from the Ionian slab, that generates mantle melting and active volcanism. The detected low-velocity layer lies at pressures corresponding to a minimum in the mantle+H2O+CO2 solidus. The presence of fluids in the mantle across the Balearic and Tyrrhenian basins is attributed to the decarbonation-dehydration of the subducting slab during its retreat from the Provence area to its present position off the Calabria coast. Decarbonation-dehydration of marly sediments, which have been demonstrated to have been involved in mantle contamination beneath Italy, is suggested to represent a main source of mantle fluids. The persistence of this layer along the track of the migrating slab for long time after the end of active subduction and volcanism could testify the preservation of small amounts of melts that were unable to separate from parent rocks. Degassing of this CO2-rich layer may occur along deep lithospheric faults, like the so-called 41° Parallel alignment,, and the Tindari-Letojanni-Malta Escarpment fault. The diffused CO2 release in central-southern Italy may be well related to this process.
Recently, Panza et al. (2007) have shown that outside the western Mediterranean backarc setting, specifically beneath northern Africa, a very pronounced low-velocity layer occurs at the top of the asthenosphere, between 130-200 km. To the north, in the western Mediterranean backarc basin, this layer is less evident, possibly diluted and apparently connected to the shallower aforementioned low-velocity layer. This would suggest that, besides the fluids contamination, an eastward upraise of the upper part of the fluid-rich, ultra-low viscosity asthenosphere at shallower layers might be responsible for the low-velocity layer at shallow depth in the Tyrrhenian Sea.
The ensemble of geophysical and geochemical data rules out the presence of a mantle plume beneath the Tyrrhenian sea, and excludes that the Plio-Quaternary intraplate volcanism of Sardinia and along the northern border of the African foreland is related to deep mantle process.
References
Panza, G.F., Peccerillo A., Aoudia K., Farina B., 2006. Geophysical and petrological modelling of the structure and composition of the crust and upper mantle in complex geodynamic settings: the Tyrrhenian sea and surroundings, Earth Sci. Rev., 80, 1-46.
Panza G.F., Raykova, R.B., Carminati E., Doglioni C., 2007. Upper mantle flow in the western Mediterranean. Earth Planet. Sci. Lett., 257, 200-214.
EXPERIMENTAL INVESTIGATION ON PERIDOTITE/ALKALINE-MELT REACTIONS: IMPLICATIONS FOR METASOMATISM OF NORTHERN VICTORIA LAND (ANTARCTICA) UPPER MANTLE
PERINELLI CRISTINA 1, ORLANDO ANDREA 2, CONTE AIDA MARIA 3, ARMIENTI PIETRO 1, BORRINI DANIELE 4, FACCINI BARBARA 5, MISITI VALERIA 6
presenter's e-mail: cperinelli@dst.unipi.it
1 - Dipartimento di Scienze della Terra, Università di Pisa
2 - C.N.R.-I.G.G. U.O. di Firenze
3 - C.N.R.-I.G.G. U.O. di Roma
4 - Dipartimento di Scienze della Terra, Università di Firenze
5 - Dipartimento di Scienze della Terra, Università di Ferrara
6 - I.N.G.V. Sezione di Sismologia e Tettonofisica
Keywords: Mantle metasomatism, peridotites, melt-rock reaction experiments, Antarctica
Cenozoic alkaline basic lavas of the Mt. Melbourne Volcanic Province (northern Victoria Land – NVL, Antarctica) host spinel peridotite and pyroxenite xenoliths that testify the compositional heterogeneity of the NVL upper mantle due to the combined effects of partial melting and metasomatic processes (Coltorti et al., 2004; Perinelli et al., 2006). Metasomatism is revealed by the occurrence of amphibole (modal metasomatism) and by incompatible element enrichments in bulk rock and clinopyroxenes (cryptic metasomatism). Basing on the mineral chemistry of the xenoliths, Coltorti et al. (2004) and Perinelli et al. (2006) inferred that the composition of the metasomatic agent was possibly similar to that of a mela-nephelinitic sample outcropping at Greene Point (NVL).
To investigate the effects of melt /mantle rocks interaction, high pressure (P) - high temperature (T) experiments were performed by placing in close contact a layer of nephelinite with a layer of lherzolite or wehrlite.
The experiments were carried out in a piston-cylinder apparatus using the graphite-Pt double capsule technique at P=1.0 GPa, T=1050-1250°C for wehrlite and 1.5-2.0 GPa and T= 975-1300°C for lherzolite. Experimental results show that metasomatic reactions took place producing clinopyroxene, olivine and spinel with different compositions respect to the original phases. In particular, clinopyroxene exhibits the largest chemical changes ranging from primary diopside to high Mg-Cr-(Na) augite and omphacite in lherzolite and to low Mg and high Ti-Al-Fe-Na augites in wehrlite; olivine in wehrlite shows enrichments in Fe while spinel in lherzolite displays higher Cr/(Cr+Al) ratios. The lack of amphibole crystallization in our experiments could be explained by the fixed temperature imposed by the run setting which does not allow to meet the stability field for this phase that, instead, can crystallize in the natural system where hot metasomatising melts infiltrate a colder matrix.
The observed systematic variations in mineral compositions are tied to modifications of invading melt chemistry. This is particularly evident in the experiments involving wehrlite paragenesis in which a network of melt veins and/or interstitial glass pools is observed in all runs. In these experiments the variation of glass composition is related to olivine+clinopyroxene crystallization coupled with primary clinopyroxene dissolution at the contact between the metasomatising melt and the solid matrix. The composition of these glasses approaches that of melt patches associated to both amphibole-free and amphibole-bearing natural samples.
This mechanism can be identified with a moving reaction front that, though implying a relative low amount of melts, may induce widespread metasomatic effects in a wide mantle region.
References
Coltorti, M., Beccaluva, L., Bonadiman, C., Faccini, B., Ntaflos, T. & Siena, F. 2004. Lithos, 75, 115-139.
Perinelli, C., Armienti, P. & Dallai, L. 2006. Contrib. Mineral. Petrol., 151, 245-266.
GEOCHEMICAL FEATURES OF SPINEL PERIDOTITES IN THE UPPER MIOCENE VALLE GUFFARI DIATREME (HYBLEAN PLATEAU, SICILY): IMPLICATIONS ON EVOLUTION OF SOUTH-EASTERN SICILY LITHOSPHERE
PERINELLI CRISTINA 1, SAPIENZA GIOVANNA TIZIANA 2, ARMIENTI PIETRO 1, MORTEN LAURO* 3
presenter's e-mail: cperinelli@dst.unipi.it
1 - Dipartimento di Scienze della Terra, Università di Pisa, Italy
2 - Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Italy
3 - Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Italy; * deceased, Nov 18 2006
Keywords: Spinel peridotite xenoliths, pyroxenes, trace elements, metasomatism, Hyblean Plateau
Upper Miocene Valle Guffari diatreme (Hyblean Plateau, Sicily, southern Italy) contains large number of upper mantle xenoliths among which spinel-facies peridotites with coarse-grained texture are the dominant type. Rarely the xenoliths host fresh glass vein poorly crystallized. The whole rock and mineral chemistry record distinct events of partial melting and metasomatism. Partial melting led to variable removing of basaltic components and produced depleted lherzolites with Fo89-91, En88-91, Cr-Diopside: En48-49 Fs4-6 Wo45-48 and Cr-rich spinel with cr# = 25-39. The signature of metasomatic event(s) is revealed by bulk rock incompatible element enrichments and by the REE clinopyroxenes patterns that show three different profiles: a) LREE-enriched (Lan/Ybn = 7-17); a) spoon-shaped (Lan/Ybn = 18-20; Lan/Smn = 21-34; Smn/Ybn < 1); c) nearly flat (Lan/Ybn ~3). These patterns can be associated to more or less complete equilibration with at least two distinct metasomatic melts: an alkaline silicate melt resembling the host basalt (patterns a and b) and an hawaiitic melt (in case of a peridotite containing a fresh hawaiitic glass veinlet, pattern c). Trace element distribution shows also that the alkaline silicate melt influenced the HFSE content, and in particular caused the increase of Nb/Ta and Zr/Hf ratios.
fO2 calculation gives a redox state above FMQ (up to +1.7 Δlogunits) related to melt-driven metasomatism.
P-T estimates on these rocks yield 0.9-1.2 GPa and 870-1050°C, suggesting that different metasomatising melts percolated the spinel-peridotite matrix near the Crust-Mantle boundary or just below it. Moreover the P-T data are in agreement with paleogeotherm reported by Nimis (1998) that is consistent with a high geothermal gradient. However, the inferred mantle potential temperatures (two hundred degrees lower than values typical for a mantle plume), indicates that the assessed thermal regime does not fit with the occurrence of an active mantle plume beneath the Hyblean area.
References
Nimis, P. 1998. European Journal of Mineralogy, 10, 521-534.
PETROGENESIS OF POST-VARISCAN OLIVINE-BEARING CUMULATES AND ASSOCIATED BASALT DYKES FROM BOCCA DI TENDA (NORTHERN CORSICA): IMPLICATIONS FOR MANTLE SOURCES
TRIBUZIO RICCARDO 1, RENNA MARIA ROSARIA 1, DALLAI LUIGI 2, BRAGA ROBERTO 3
presenter's e-mail: renna@crystal.unipv.it
1 - Dipartimento di Scienze della Terra, Università di Pavia
2 - C.N.R. - Istituto di Geoscienze e Georisorse, Unità di Pisa
3 - Dipartimento Scienze della Terra e Geologico Ambientali, Università di Bologna
Keywords: Post-Variscan basic magmatism, Sardinia-Corsica batholith, mafic cumulates, mineral trace element compositions, crustal contamination
The western European Variscides record a widespread intrusion of gabbroic complexes at 300-275 Ma, associated with post-collisional lithosphere thinning. In western-central Alps and northern Apennine, these post-Variscan gabbroic complexes show the emplacement of mantle-derived melts at different lithosphere levels, from the crust-mantle boundary (Hermann et al. 2001) to the middle crust (Tribuzio et al. 1999). The intrusion of the basic melts was commonly associated with a significant process of crustal contamination (Voshage et al. 1990; Tribuzio et al. 1999), thus hampering to constrain accurately the nature of their mantle sources. Nevertheless, on the basis of trace element and isotope studies, some of these complexes provide evidence for the involvement of MORB-type melts, derived from depleted mantle sources (e.g. Voshage et al. 1990; Hermann et al. 2001; Montanini & Tribuzio 2001).
In the Sardinia-Corsica batholith, there are several gabbroic sequences, dated at ~285 Ma, which were emplaced at rather shallow crustal levels (Cocherie et al. 2005).
There is still some uncertainty about the origin of the basic melts that gave rise to these gabbroic sequences (cf. Tommasini et al. 1995; Cocherie et al. 2005), as a comprehensive study determining the possible chemical effects induced by the continental crust is lacking. We have thus examined the gabbroic complex of Bocca di Tenda, cropping out in northern Corsica. This complex intrudes a plutonic sequence made of hornblende-bearing granitoids and is crosscut by basalt dykes with chilled margins. Whole-rock chemical compositions indicate that the most primitive Bocca di Tenda intrusives (i.e. the olivine-gabbronorites) are of cumulus origin. They are mainly composed of olivine (Fo78-72) and plagioclase (An68-60), and poikilitic clinopyroxene and orthopyroxene. Olivine-gabbronorites show nearly homogeneous Nd isotope (initial εNd +2.5 to +1.4) and mineral trace element compositions. In particular, clinopyroxene compositions indicate that the parental melts of olivine-gabbronorites were LREE- and LILE-enriched relative to N-MORB.
The somewhat elevated δ18O values (+6.3‰ to +6.7‰) of pyroxene indicate that a substantial crustal contribution was involved in the petrogenesis of these cumulates. The basalt dykes have initial εNd values and clinopyroxene trace element compositions overlapping those of olivine-gabbronorites, thus indicating that these two rock types formed by mantle-derived magmas with similar compositions.
A comparison with the gabbroic complexes of Porto (western Corsica, Renna et al. 2007) and Sondalo (central Alps, Tribuzio et al. 1999) was carried out. Although the most primitive rocks of Bocca di Tenda, Porto and Sondalo complexes record a process of crustal contamination, their geochemical features indicate the intrusion of chemically similar basic melts in different sectors of the original Europa-Adria lithosphere, at shallow to intermediate crustal levels.
We propose that the primary melts of these complexes were derived from a slightly enriched mantle source, involving a significant contribution from a plume-type or lithospheric component, thus implying that the post-Variscan basic magmatism did not involve only typical depleted mantle sources.
Cocherie A, Rossi P, Fanning CM, Guerrot C (2005) Lithos 82: 185-219
Hermann J, Müntener O, Günther D (2001) J Petrol 42: 189-206
Montanini A, Tribuzio R (2001) J Petrol 42: 2259-2277
Renna MR, Tribuzio R, Tiepolo M (2007) Contrib Mineral Petrol DOI: 10.1007/s00410-007-0205-9
Tommasini S, Poli G, Halliday AN (1995) J Petrol 36: 1305-1332
Tribuzio R, Thirlwall M F, Messiga B (1999). Contrib Mineral Petrol 136: 48-62
Voshage H, Hofmann AW, Mazzucchelli M, Rivalenti G, Sinigoi S, Raczek I, Demarchi G (1990) Nature 347: 731-736
PETROLOGY AND GEOCHEMISTRY OF ULTRAMAFIC XENOLITHS FROM SARDINIA.
ROCCO IVANA 1, LUSTRINO MICHELE 2, MELLUSO LEONE 1, MORRA VINCENZO 1
presenter's e-mail: ivana.rocco@unina.it
1 - Dipartimento di Scienze della Terra, Università degli studi di Napoli "Federico II"
2 - Dipartimento di Scienze della Terra, Università degli studi di Roma "La Sapienza"
Keywords: Mantle xenoliths, Sardinia, Clinopyroxenes
Mafic alkaline late Miocene-Quaternary volcanic rocks from Sardinia are often associated with mantle-derived ultramafic xenoliths. They are lherzolites, harzburgites and websterites with protogranular, porphyroclastic, equigranular-mosaic and pyrometamorphic textures. Olivine content ranges from ~50 to ~91%, orthopyroxene from ~6 ~45%, clinopyroxene from 0 to ~17% and spinel from 0 to ~8%. Pyrometamorphic glass enclosing secondary (subhedral to euhedral) olivine, clinopyroxene, spinel and plagioclase microlites has been found in a harzburgite and in a websterite-harzburgite composite xenolith.
Secondary olivine (in glass) shows slightly higher Fo (Fo91-93) compared to primary olivine (Fo89-90). Enstatite (Wo0.7-4.3En72.5-90.8Fs7-24.4) shows Mg# and Cr# ranging from 75 to 93 and from 0.86 to 22, respectively, and sometimes is characterized by clinopyroxene exolution lamellae. Clinopyroxene is diopside (Wo38.7-51.3En41.4-52Fs4-10) and often occurs as small grains between larger forsterite and enstatite, or as spongy-textured crystals (indicating incipient partial melting) while the secondary clinopyroxene occurs as subhedral crystals in pyrometamorphic glass; spinel occurs both as holly-leaf shaped (sometimes with a darker rim indicating incipient partial melting) and as symplectitic crystals; spinel Mg# and Cr# range from 64 to 84 and from 6 to 61, respectively. Orthopyroxene shows a large compositional range in the websterite-harzburgite composite xenolith from Gerrei: FeO ranging from 6.50 to 7.38 wt.% in harzburgite, while clustering around 14 wt.% in websterite. MgO content ranges from ~24 wt.% in websterite to ~32 wt.% in harzburgite. Clinopyroxene shows two main patterns in primitive mantle-normalized diagrams: the first (found in large clinopyroxenes in a lherzolite from Logudoro) is incompatible elements-depleted (IED; LaN <0.25 times PM), the second type is incompatible elements-enriched (IEE; LaN >3.3 times PM). This latter group partially overlaps the composition of the mantle clinopyroxenes of Sardinia described by Beccaluva et al. (2001). All lherzolite clinopyroxenes shows a positive anomaly at Eu. The (La/Yb)N ratio ranges from 0.008 to 0.08 for lherzolites IED clinopyroxenes, from 0.74 to 5.5 for lherzolites IEE clinopyroxenes, from 0.15 to 0.3 for secondary clinopyroxenes in glass of harzburgites, and from 3.1 to 6.5 in websterite clinopyroxenes.
Pyrometamorphic glasses in harzburgites show a basaltic andesite composition, while glasses of websterites show a basaltic composition. The glass in harzburgite and in websterite have a similar composition to “LKT” and “HKT” glasses described by Lustrino et al. (1999) in mantle xenoliths from the same area (Gerrei), respectively. Primitive mantle-normalized REE patterns of pyrometamorphic glasses show a LREE enrichment (LaN=5-20 times PM in harzburgites and 60-130 times PM in websterites) Both glasses in harzburgites and in websterites show a positive peak at Sr and Ti. Glasses in websterites show many geochemical similarities with host rock composition: this is consistent with an origin of the glass as consequence of infiltration of host magma in the xenolith.
Primitive mantle-normalized diagrams of host basalt and liquids in equilibrium with primary clinopyroxenes (calculated on the basis of literature KDs) show that melts in equilibrium with IEE-clinopyroxenes (excluding the samples with a positive peak at Zr) match the geochemical composition of the host basalt, while the melts in equilibrium with IED-clinopyroxenes match the composition of N-MORB melts.
The temperatures of these nodules range from 900 (eastern Sardinia) to 1200 °C (western Sardinia) and the pressure estimates cluster around 10-11 kbar, corresponding to 35-40 km in depth, just below the Moho discontinuity which is found 30 km depth in Sardinia.
AGE OF THE LITHOSPHERIC SOURCE OF THE VENDIAN FLOOD BASALTS OF THE VOLYN REGION, SOUTH-WESTERN EAST-EUROPEAN CRATON
SHUMLYANSKYY LEONID 1, NOSOVA ANNA 2, PER-GUNNAR ANDREASSON 3
presenter's e-mail: lshumlyanskyy@yahoo.com
1 - Institute of geochemistry, mineralogy and ore formation of the National Academy of Sciences of Ukraine, Kyiv, Ukraine
2 - Institute of geology of ore deposits, petrography, mineralogy and geochemistry of the Russian Academy of Sciences, Moscow, Russia
3 - Lund University, Lund, Sweden
Keywords: Continental flood basalts, Volyn region, Rodinia break-up, zircon, Nd isotopic composition
A large volume of data regarding the age of flood basalts of the Volyn region (north-western part of Ukraine, south-western Belarus and eastern Poland) now exists, including results from SIMS analysis of zircons isolated from basalts and tuffs, as well as whole-rock Rb-Sr-isochron results. The bulk of the data suggests an age of formation of the volcanogenic sequence at ca. 550 Ma.
Nd isotopic compositions were recorded from a number of samples that represent a wide range of rocks from basalts to rhyolite and dacite. Volynian basalts are rather evolved (#Mg = 0,35 – 0,54) and evidently do not represent primary mantle-plume related melt. Instead, they carry geochemical evidences of substantial lithospheric, predominantly lower crustal, contamination. With respect to Nd and Sr isotopic compositions, the evidence includes predominantly negative values of Nd550 and high positive values of Sr550. Nd model ages according to depleted mantle (DM) model vary from 2050 to 1090 Ma. There are two prominent spikes on the histogram of DM model age distribution at 2000 and 1500-1700 Ma. Zircons separated from Volynian basalts revealed a wide spectra of 206Pb/207Pb ages that can be subdivided into four groups: (1) ca. 2000 Ma, corresponding to the age of the Osnitsk-Mikashevichi igneous belt and a very common orogenic event in the Ukrainian shield; (2) ca. 1820 Ma, coinciding with age of the Belarus-Baltic granulite belt; (3) ca. 1470 Ma. Rocks of this age are unknown so far in this region; however, rocks of similar age are rather widely distributed in the western part of the East-European platform (EEP); (4) “Volynian” 549 ± 29 Ma old zircons that date the flood basalt event. The first three groups of zircons are apparently inherited from various crust/lithospheric sources by contamination with the basaltic melt.
There is a good correspondence between whole-rock Nd DM model ages and 206Pb/207Pb zircon ages. Both embrace the same time interval while main Nd model age spikes are 100-150 Ma older than main zircon ages. We believe this is due to the different characters of the whole rock Nd model ages and zircon ages – while the former correspond to the time of separation of material from the mantle, the latter date crust-forming processes. Meanwhile there is a good correspondence between zircon ages and ages of main crust-forming events in the south-western part of the EEP (i.e. the formation of the Osnitsk-Mikashevichi Igneous and Belarus-Baltic granulite belts and widespread magmatism in the western EEP).
Such correspondence suggests significant addition of lower-crustal and probably upper mantle (lithospheric) material to parental basalt melts. The plume source was relatively suppressed probably by the large thickness and low permeability of the lithosphere. However, the role of the plume component increased during the latest stages of the Volynian flood basalt event when high-Ti basalts formed. Their trace element patterns and Nd isotopic compositions approach values typical for OIB that may be related to break-up of the lithospheric plate that occurred already during the outpouring of high-Ti basalts. The ca. 1200-1400 Ma component of the lithospheric source of the Volynian flood basalts is so far unknown in the basement underlying the Volynian province. However, such material is rather abundant to the north-west of the province and may testify to the location of the plume’s centre not immediately below the Volynian flood basalt province, but further to the north-west. From there, melts spread south-eastward along the Trans-European Suture Zone that defines the western boundary of the EEP. The zone itself represented one of the shoulders of the classical rift system that developed in western (in modern coordinates) Rodinia and led to its break-up into three continents – Baltica, Laurentia and Amazonia. The triple point was located nearby present-day Scotland while the Trans-European Suture Zone experienced gradual opening in a south-easterly direction.
QUANTITATIVE TEXTURE ASSESSMENT OF MANTLE XENOLITHS
TABOR BRYAN ERNEST 1, TABOR FELICITY A. 1, DOWNES HILARY 1
presenter's e-mail: b_e_tabor@brise39.demon.co.uk
1 - SCHOOL OF EARTH SCIENCES, BIRKBECK, UNIVERSITY OF LONDON, MALET STREET, LONDON WC1E 7HX, UK
Keywords: Xenolith, Texture, Quantitative
A method for the quantitative characterization of grain size, as observed in petrological thin-sections, has been established on the basis of a set of spinel lherzolite xenoliths from the French Massif Central. The method makes use of whole, or large area, optical scanning of the thin-section, skeletonization of the mineral identity outlines and computerised measurement of the individual grain section areas.
The method has now been extended to xenolith materials from other locations and the results obtained are consistent with the linear plot of arithmetic mean against additive standard deviation obtained with the original study. This all suggests that xenoliths of different overall textures tend to form a continuous series from coarse "protogranular" to fine grained "equigranular", rather than discrete qualitatively assessed groups that have some times been implied on the basis of the original Mercier and Nicolas designations (1975).
The statistical significance and character of grain-size distributions observed has been explored both in relation to their description and possible mechanistic origin. The variance of the measured samples has been examined in relation to size and number of grain areas examined. The variation within samples has been explored using multiple and orthogonal sections cut from several xenoliths, which were of sufficient size to make this possible. These results are consistent with the observed linear relationship although some xenoliths show differences in grain size distribution between orthogonal sections.
By direct measurement and comparison of cumulative number and area distribution curves, it can be seen that the often quoted qualitatively assessed "typical" grain sizes tend to be influenced by a small number of the larger grain areas. Our method therefore provides an objective standard for the classification of xenolith textures.
Although the use of the additive (arithmetic) mean and standard deviation provide a convenient means for correlation, in practice the distributions are best described by lognormal functions with multiplicative (geometric) parameters, for which there are mechanistic theoretical derivations. Linear log-probability curves again support the idea of continuous series rather than bi-modal distributions that have sometimes been implied for some "porphyroclastic" xenoliths.
GEOCHEMICAL FEATURES OF SPINEL PERIDOTITES IN THE UPPER MIOCENE VALLE GUFFARI DIATREME (HYBLEAN PLATEAU, SICILY): IMPLICATIONS ON EVOLUTION OF SOUTH-EASTERN SICILY LITHOSPHERE
PERINELLI CRISTINA 1, SAPIENZA GIOVANNA TIZIANA 2, ARMIENTI PIETRO 1, MORTEN LAURO* 3, GRIFFIN WILLIAM L.4, O’REILLY SUZANNE Y.4
presenter's e-mail: cperinelli@dst.unipi.it
1 - Dipartimento di Scienze della Terra, Università di Pisa, Italy
2 - Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Italy
3 - Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Italy; * deceased, Nov 18 2006
4 – GEMOC, Macquarie University, Australia
Keywords: Spinel peridotite xenoliths, pyroxenes, trace elements, metasomatism, Hyblean Plateau
Abstract
Upper Miocene Valle Guffari diatreme (Hyblean Plateau, Sicily, southern Italy) contains large number of upper mantle xenoliths among which spinel-facies peridotites with coarse-grained texture are the dominant type. Rarely the xenoliths host fresh glass vein poorly crystallized.
The whole rock and mineral chemistry record distinct events of partial melting and metasomatism. Partial melting led to variable removing of basaltic components and produced depleted lherzolites with Fo89-91, En88-91, Cr-Diopside: En48-49 Fs4-6 Wo45-48 and Cr-rich spinel with cr# = 25-39. The signature of metasomatic event(s) is revealed by bulk rock incompatible element enrichments and by the REE clinopyroxenes patterns that show three different profiles: a) LREE-enriched (Lan/Ybn = 7-17); a) spoon-shaped (Lan/Ybn = 18-20; Lan/Smn = 21-34; Smn/Ybn < 1); c) nearly flat (Lan/Ybn ~3). These patterns can be associated to more or less complete equilibration with at least two distinct metasomatic melts: an alkaline silicate melt resembling the host basalt (patterns a and b) and an hawaiitic melt (in case of a peridotite containing a fresh hawaiitic glass veinlet, pattern c). Trace element distribution shows also that the alkaline silicate melt influenced the HFSE content, and in particular caused the increase of Nb/Ta and Zr/Hf ratios.
fO2 calculation gives a redox state above FMQ (up to +1.7 Δlogunits) related to melt-driven metasomatism.
P-T estimates on these rocks yield 0.9-1.2 GPa and 870-1050°C, suggesting that different metasomatising melts percolated the spinel-peridotite matrix near the Crust-Mantle boundary or just below it. Moreover the P-T data are in agreement with paleogeotherm reported by Nimis (1998) that is consistent with a high geothermal gradient. However, the inferred mantle potential temperatures (two hundred degrees lower than values typical for a mantle plume), indicates that the assessed thermal regime does not fit with the occurrence of an active mantle plume beneath the Hyblean area.
References
Nimis, P. 1998. European Journal of Mineralogy, 10, 521-534.
