Journal of Conference Abstracts

Session E03:4A

E03 : 4A/09 : F1

New Constraints on U-Th-Pb Fractionation During OIB Generation: Basalts from Fogo, the Cape Verde Islands

Thomas F. Kokfelt (tkokfel@ugcvax.dnet.gwdg.de)¹,

Paul Martin Holm (paulmh@geo.geol.ku.dk)²,

Chris J. Hawkesworth (c.j.hawkesworth@open.ac.uk)³ &

David W. Peate (dwp@dlc.ku.dk)⁴

¹ Geochemisches Institut, Goldschmidt strasse 1, 37077 Göttingen, Germany

² Geological Institute, Oster Voldgade 10, 1350 Copenhagen K, Denmark

³ Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK

⁴ Danish Lithosphere Centre, Oster Voldgade 10, 1350 Copenhagen K, Denmark

The extent to which U-Th-Pb are fractionated during OIB generation remains central to the discussion on mantle sources, their ages and depths of origin. At issue is the so-called 'young HIMU' mantle (or OIB with negative  ^7/4Pb; c.f. Thirlwall (1997)), which shares many geochemical features of typical HIMU mantle, but without having extreme radiogenic Pb. Though such basalts may originate from sources that are younger than typical HIMU (~1.5-2 Ga), it is commonly inferred that their µ (²³⁸U/²⁰⁴Pb) and <omega> (²³²Th/²⁰⁴Pb) values greatly exceed those of their source regions, because Pb is thought to be much less incompatible than U and Th in the mantle during melting. However, even for small and yet realistic degrees of mantle melting (~1%), µ and <omega> are only fractionated about 10% for normal mantle mineralogy, and any extensive fractionation of µ and <omega> calls for residual sulphide in the source regions. In the Cape Verde islands the 'HIMU' source is found to be carbonatitic, partly on geochemical arguments (e.g. high Zr/Hf, low Nb/U) and partly because Fogo carbonatites lie on the extension of the lava arrays in Sr-Nd-Pb isotope-isotope plots. The Fogo lavas form striking arrays in diagrams of ²⁰⁶Pb/²⁰⁴Pb vs. µ and ²⁰⁸Pb/²⁰⁴Pb vs. <omega> which would yield concordant ages of ~150 Ma. A curved ²⁰⁸Pb/²⁰⁴Pb -²⁰⁶Pb/²⁰⁴Pb array is consistent with a semi-closed evolution of descrete source components with different ²³²Th/²³⁸U and furthermore, the curvature rules out any recent mixing event (including AFC processes which would produce straight lines). The array is consistent with a Cape Verde islands HIMU source age of ~200 Ma, based on measured µ and <omega> in the lavas. Older source ages (e.g. 600 Ma) would indicate a very significant (~100%) fractionation of µ and <omega>, which would not be due to residual sulphide, because there is a positive correlation between ppm Zn and µ. Also, in a diagram of µ vs. U ppm, the Fogo lavas do not define the same steep positive correlation as was found for MORB, and which White (1993) attributed to a significant U/Pb fractionation at shallow levels. Moreover, the young source age of ~200 Ma corresponds broadly to the opening of the Atlantic at this lattitude, and we favour a model of carbonatitic 'HIMU' generation through tectonically controlled mantle devolatilisation.

Thirlwall M, Chemical Geology, 139, 51-74, (1997).

White WM, Earth and Planetary Science Letters, 115, 211-226, (1993).

E03 : 4A/10 : F1

Melting beneath Sao Miguel, Azores; Inferences from U-Th-Pa Systematics

Govert Koetsier (koet@geo.vu.nl) &

Tim Elliott (ellt@geo.vu.nl)

Vrije Universiteit, Faculty of Earth Sciences, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands

Despite the potential power of U-series analyses for investigating melting processes, the interpretation of U-series disequilibria remains in some dispute. In order to improve our understanding of the variations in disequilibria we conducted an investigation on the scale of a single island, to place global variations in a better context. Previously interest has focussed on ²³⁸U-²³⁰Th disequilibrium, but simultaneous measurement and modelling of ²³⁵U-²³¹Pa disequilibrium should provide stronger constraints on the melting process. ²³¹Pa has a shorter half life (32 ka) relative to ²³⁰Th (75 ka), but both share a common parent, and the ²³⁵U-²³¹Pa pair should be more sensitive to changes in the dynamics of melting.We have analysed a suite of young (< 6500a) samples from Sao Miguel, Azores, for U-Th-Pa isotopes together with a range of other geochemical tracers. Basalts from Sao Miguel show some very intriguing geochemical features, notably an unusually large range in ⁸⁷Sr/⁸⁶Sr isotope ratios. Samples from this study have ⁸⁷Sr/⁸⁶Sr ratios of 0.7034 up to 0.7054, covering almost the whole range previously observed for the island (White et al., 1979, Hawkesworth et al., 1979, Widom et al., 1997). 'Enrichment' in ⁸⁷Sr/⁸⁶Sr, and associated incompatible element ratios, is attributed to mixing of different components of the Azores plume source (crustal or lithospheric contamination cannot account for such large variations in Sr isotope ratios these alkalic lavas with high Sr concentrations). ²³⁰Th excesses for the lava suite range from 16 to 18% but do not correlate with 'enrichment'. ²³¹Pa excesses range from 36-46% and likewise show no systematic variations with increasing ⁸⁷Sr/⁸⁶Sr. These results suggest that variations in source composition, resulting in rather dramatic differences in ⁸⁷Sr/⁸⁶Sr, have little effect on U-series disequilibria. This emphasises the power of U-series disequilibria to 'see through' some of the complications often associated with traditional geochemical tracers of melting. A further key feature of U-series tracers is that they enable constraints to be made on the dynamics of the melting process. Combined modelling of U-Th and U-Pa show melting rates of 1.26*10^-4kgm^-3a^-1 for the 'dynamic' melting model of M^cKenzie (1985) to 3.15*10 ^-4kgm^-3a^-1 for the equilibrium transport model of Spiegelman and Elliott (1993). The porosities are similar for both models and range from 0.6 to 0.8%. If we convert these melting rates to upwelling velocities we infer very high velocities of 10 to 100 cm/a. Such rapid estimates of upwelling would be yet greater (implausibly so) if melting rates are at their lowest at the onset of melting, as has been recently proposed (Asimow et al, 1996). Thus we feel it is unlikely that there are such 'melting tails' beneath Sao Miguel.

Asimow, PD, Hirschmann, MM, Stolper, EM, EOS, 77, 825, (1996).

Hawkesworth CJ, Norry MJ, Roddick JC, Vollmer R, Nature, 280, 28-31, (1979).

M^cKenzie, EPSL, 72, 149-157, (1985).

Spiegelman M, Elliott, T, EPSL, 118, 1-20, (1993).

White WM, Tapia MDM, Schilling JG, Con. Min. Pet, 69, 201-213, (1979).

Widom E, Carlson RW, Gill JB, Schmincke H-U, Chem. Geol, 140, 49-68, (1997).

E03 : 4A/11 : F1

Ocean Island Basalts and Garnet Pyroxenites

Olgeir Sigmarsson

(olgeir@opgc.univ-bpclermont.fr) &

Kevin W. Burton

(burton@opgc.univ-bpclermont.fr)

CNRS - Univ. Blaise Pascal, Clermont-Ferrand, France

Subducted oceanic crusts may be recycled into the mantle source of oceanic island basalts (OIB) as garnet pyroxenite veins in a dominantly lherzolite lithology. Preferential melting of garnet pyroxenite would produce melts characterized by high La/Yb and (²³⁰Th/ ²³⁸U). Mixing of such melts with those generated from spinel lherzolite would result in lower La/Yb and (²³⁰Th/ ²³⁸U) in OIB. Higher proportions of garnet pyroxenite melts might be expected from the periphery of mantle plumes compared to their hotter centres.

Holocene basalts from the off-rift volcanic zone on the Snæfellsnes peninsula in western Iceland offer the possibility of investigating over 100 km, lateral compositional variability in a plume head if the magmas ascended vertically from their source region. The alkalinity of these basalts, as well as the La/Yb, (²³⁰Th/ ²³⁸U) and ⁸⁷Sr/⁸⁶Sr, decreases along the volcanic zone, from west to east, towards the centre of Iceland. These geochemical variations, therefore, are also correlated with the SiO₂ contents of the basalts, suggesting a link between major and trace elements and isotope ratios in their mantle source. These compositional variations are taken to indicate that a significant component of the basalts, in the Snæfellsnes volcanic zone, is generated by melting of garnet pyroxenites. This component appears to decreases towards the east, and the composition of primitive basalts from the rift zones in the centre of Iceland may be dominated by melts of overwhelming lherzolitic source material, resulting from a larger extent of melting due to a higher temperature in the core of the mantle plume.

The alkali basalts furthest to the west in the Snæfellsnes volcanic zone, having the highest La/Yb and (²³⁰Th/ ²³⁸U), or garnet signature, also have the highest ⁸⁷Sr/⁸⁶Sr and ³He/⁴He lower than 8 (R/Ra). These isotope ratios are compatible with the hypothetical garnet pyroxenites in the periphery of the Iceland mantle plume being recycled oceanic crust.

E03 : 4A/12 : F1

Is the Hawaiian Basalt Progression a Consequence of Progressive Melt Extraction from the Upwelling Hawaiian Plume?

Jason Phipps Morgan (jpm@geomar.de)¹ &

W. Jason Morgan (wjmorgan@geo.princeton.edu)²

¹ Geodynamics Dept., GEOMAR, D-24148 Kiel, DE

² Geology Dept., Princeton Univ., Princeton NJ, USA

Arrays of basalts from the same hotspot usually plot within an elongate tube-like field in ⁸⁷Sr/⁸⁶Sr-¹⁴³Nd/¹⁴⁴Nd-²⁰⁶Pb/²⁰⁴Pb space [Hart et al., Science,1992]. Each hotspot array tube (HART) is commonly interpreted as the result of melting multiple basalt sources that are variably-proportioned mixtures of the hotspot source components. Alternatively, we have proposed instead that a HART is the isotopic trace of a melt-extraction trajectory which starts from an initial source mixture characteristic to that hotspot evolution [Phipps Morgan and Morgan, submitted to EPSL 1997; Phipps Morgan, submitted to Nature 1998]. Melt-extraction trajectories are produced when the sources of individual basalts differ in the amount of prior melt extraction they underwent at the hotspot. This melting physics also provides straightforward explanations for the ¹⁸⁷Os/¹⁸⁶Os contrasts between mid-ocean ridge basalts and their presumed abyssal peridotite source, and for the enigmatic trace element and isotopic patterns of pyroxenite veins and peridotite exposed within orogenic lherzolites.

Here we apply this idea to study the age-progression of volcanismafter passage over the Hawaiian hotspot. First we motivate the model by discussing our ongoing work with Marc Parmentier on 3-D numerical experiments of the upwelling and melting structure beneath the Hawaiian plume. These numerical experiments use finite-difference approximations to solve for variable viscosity flow and melting on a 64x64x64 computational grid. Our preferred model includes a temperature-dependent plume and lithosphere viscosity in addition to a viscosity increase due the dewatering associated with partial melt extraction from a peridotite [cf. Hirth and Kohlstedt, EPSL, 1996; Phipps Morgan et al., JGR, 1995; Phipps Morgan, EPSL, 1997]. A key feature of these experiments is that they delineate three distinct melting regimes for a plume upwelling beneath moving lithosphere: (1) A 'precursor' melting region along the colder leading rim of the plume (Loihi?); (2) A 'main-sequence' melting region above the hot core of the plume (Hawaii?). Here the melting rate is controlled by plume upwelling and lithosphere drag-removal of previously upwelled plume material; (3) The 'tail' melting region where melting is limited by the upwelling rate associated with the lateral spreading of the hotspot swell-root formed by the hot-but-depleted residues of 'main-sequence' melt extraction (Haleakala-Honolulu Volcanics?).

This physical model of plume upwelling and melt-extraction is compatible with the above geochemical scenario of a melt-extraction trajectory for Hawaiian basalt evolution. We present a sample melt-extraction trajectory that can fit the observed Hawaiian isotopic age-progression after passage over the Hawaiian hotspot.

E03 : 4A/13 : F1

S and O Isotopes in Cpx-Hosted Glass Inclusions from Miocene Submarine Hyaloclastites of Gran Canaria (Canary Islands): Constraints on Magma Degassing and Contamination

Andrey A. Gurenko (agurenko@geomar.de)¹,

Marc Chaussidon (chocho@crpg.cnrs-nancy.fr)² &

Hans-Ulrich Schmincke (hschmincke@geomar.de)¹

¹ GEOMAR, Wischhofstr. 1-3, D-24148 Kiel, Germany

² CRPG-CNRS, BP 20, Vandoeuvre-lès-Nancy, France

S and O isotopic compositions in Cpx-hosted glass inclusions from Miocene basaltic hyaloclastites drilled during ODP Leg 157 north and south of Gran Canaria were determined by SIMS. Most inclusions range in composition from tholeiite to alkali basalt and basanite, whereas more evolved inclusions resemble basaltic andesite. The inclusions show a large range in S concentrations (ca. 100 to 6000 ppm S) and S⁶⁺/S_total ratios (0.11 to 0.95). They are thus well suited to constrain crystallization, degassing and possible contamination of the Gran Canaria Miocene shield stage magmas based on S and O isotopes.

³⁴S and ¹⁸O values were measured with the Nancy IMS 1270 ion microprobe. Secondary negative ions of O and S were analyzed at a mass resolution of ~5000 under the sputtering of a primary negative Cs beam. At variance with ³⁴S/³²S ratios, a matrix effect on instrumental mass fractionation of ~4‰ was observed for ¹⁸O/¹⁶O ratios from the analysis of a set of standard basaltic glasses ranging in SiO₂ concentrations between 43 and 56 wt%. Counting statistics is of ±0.2-0.5‰ for ¹⁸O/¹⁶O ratios and of ±0.5-1.0‰ for ³⁴S/³²S ratios. Reproducibility on standard MORB glasses containing between 800 and 1200 ppm S is at the level of ±0.5‰ for ¹⁸O/¹⁶O ratio and of ±1‰ for 34S/32S ratios.

Analyses performed so far have shown that ³⁴S values range from -1.5 to +3.9‰ and ¹⁸O values from from +5.3 to +11.0. For most melt inclusions, ³⁴S correlate positively with (1) the concentrations of total S (1170 to 2080 ppm) and (2) the proportion of sulfate (S⁶⁺/S_total 0.4 to 0.95) dissolved in the melt. At first glance this relationship between ³⁴S values and S concentrations cannot only be the result of S degassing from the magma because for these S6+/Stotal ratios, S isotope fractionation factors between SO₂ and melt are ~+0.5‰ only or less. Another process, such as crystallization of a sulfate-rich phase (e.g., anhydrite) must be superimposed.

Two groups of inclusions can be recognized based on MgO-¹⁸O-³⁴S relationships. (1) Inclusions nearly corresponding to the upper mantle with respect to ¹⁸O (5.5±1.0‰) but having a range in ³⁴S (-1.0 to +3.9‰) increasing with the increase of ¹⁸O. They probably represent derivatives of mantle melts originated in the presence of a sulfate-rich phase. (2) Inclusions with relatively high ¹⁸O (+9.7±0.7‰) showing ³⁴S values in the higher range that could be samples of melts which are thought to have experienced assimilation or contamination by seawater.

E03 : 4A/14 : F1

Interactions of the Foundation Hotspot with the Pacific-Antarctic Ridge (South Pacific) Examined Using Sr, Nd and Pb Isotopes

Christophe Hemond

(chhemond@sdt.univ-brest.fr)¹ &

Colin W. Devey

(cwdevey@uni-bremen.de)²

¹ UMR 6538 Domaines océaniques, IUEM, Place Nicolas Copernic, F-29280 Plouzane, France

² Fachbereich 5 Geowissenschaften, Universität Bremen, Postfach 330440, D-28334 Bremen, Germany

The Foundation hotspot which activity resulted in a seamount chain (32°S/127°W-38°S/111°W) has interacted with the extinct Farallon spreading axis and interacts today with the Pacific-Antarctic Ridge at about 38°S. Trace element concentrations have shown that the chain can be divided in three parts: 1- the westernmost part made of transitional MORB; 2- the central part consisting of alkali basalts which are likely to be the chemical expression of the plume; 3- the easternmost part made of a series of elongated ridges connecting the last typical intraplate seamounts of the chain and the spreading axis. Their chemical compositions vary from enriched to depleted.

Sr, Nd and Pb isotopes have been determined on more than 60 samples. They agree with the trend defined by the trace elements. The westernmost part of the chain has unradiogenic Sr and ²⁰⁶Pb/²⁰⁴Pb ratio and rather radiogenic Nd compositions (0.51305) whereas the central section exhibits slightly more elevated Sr (0.7030), lower Nd compositions (0.51288) and much higher ²⁰⁶Pb/²⁰⁴Pb ratios (up to 20.2). The ²⁰⁶Pb/²⁰⁴Pb reaches such high value only in one seamount and is significant of the contribution of a HIMU component in the plume. Eastwards, isotope compositions essentially evolve gradually towards MORB like compositions. Nevertheless, the spreading axis samples remain intermediate isotopically. Modelling a mixing of two magmatic liquids which compositions are chosen to be the most extreme analyzed samples leads to assume a contribution of the plume to the spreading axis of about 40-60%.

It is also worth noting that the most unradiogenic Sr and Pb and radiogenic Nd compositions do not appear in samples from the spreading axis but in rocks dredged from the oblique ridges. This could be explained by a contribution of isotopically depleted melts coming from a depleted component of the plume. Alternatively, melts produced from the upper mantle underneath the spreading axis may have been deviated towards the oblique ridges which may be the locus of active interaction and mixing of melts from both origins.

Session E03:4B

E03 : 4B/25 : F1

Abyssal Peridotites and Associated Basalts Whole-Rock Trace Element Systematics: Evidence for Melt-Solid Interaction During Melt Ascent beneath Ocean Ridges

Laure Sevin (laure@earthsciences.uq.edu.au)¹,

Y. Niu (niu@earthsciences.uq.edu.au)¹,

R. L. Fisher²,

H. J. B. Dick³ &

R. Hékinian⁴

¹ Department of Earth Sciences, The University of Queensland, Brisbane Qld 4072, Australia

² Scripps Institution of Oceanography, La Jolla, CA 92093, USA

³ Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

⁴ Ifremer, DRO GM, 29287 Plouzané, France

Mid-ocean ridge basalts (MORB) and abyssal peridotites preserve complementarily the records of decompression melting and melt extraction processes beneath ocean ridges and the nature and the composition of the fertile mantle materials. Indeed, abyssal peridotites are thought to be residues of mantle melting. However, they have excess olivine relative to simple melting residues (Niu et al., 1997; Niu, 1997). Niu and Hékinian (Niu & Hékinian, 1997) showed that this olivine crystallization is accompanied by incompatible element enrichments, in a case study at the East Pacific Rise. We here extend this study to include abyssal peridotites dredged from the slow-spreading Southwest Indian Ridge (Fisher et al., 1987). Some of these samples have been previously studied in terms of modal mineralogy and mineral chemistry (Dick et al., 1984; Johnson and Dick, 1992). REE patterns of residual clinopyroxenes in these peridotites are consistent with varying extents of melt depletion (Johnson and Dick, 1992) whereas whole-rock peridotites show variable REE patterns, mostly having positive middle- to heavy REE slopes, and flat to enriched light REE (e.g., [La/Sm]_N=2.43±1.35; [Dy/Yb]_N=0.67±0.19). These whole-rock REE patterns cannot be explained by simple melting, but can be reasonably well modeled by the combination of near-fractional melting and subsequent refertilization of a depleted- to moderately enriched basaltic melt. We interpret this incompatible element refertilization, together with the creation of excess olivine, as the natural consequence of melt-solid interaction during melt ascent. Buoyant melts that ascend through previously depleted residues crystallize olivine at shallow levels as a result of cooling. Entrapment of these melts raises the abundance levels of incompatible elements in the whole-rock peridotites. The contrasted REE patterns of whole-rock abyssal peridotites from those of residual clinopyroxenes suggest that the shallow level melt-solid interaction during melt ascent is passive (vs. reactive), and does not affect the primary melting signals preserved in residual clinopyroxenes.

Niu et al, Earth planet. Sci. Lett, 152, 251-265, (1997).

Niu, J. Petrol, 38, 1047-1074, (1997).

Niu & Hékinian, Earth planet. Sci. Lett, 146, 243-258, (1997).

Fisher et al, Ofioliti, 11, 147-178, (1987).

Dick et al, Earth planet. Sci. Lett, 69, 88-106, (1984).

Johnson & Dick, J. Geophys. Res, 97, 9219-9241, (1992).

E03 : 4B/26 : F1

Non-Chondritic Platinum-Group Element Ratios in Abyssal Peridotites: Petrogenetic Signature of Melt Percolation ?

Mark Rehkamper

(markr@nwz.uni-muenster.de)¹,

A. N. Halliday,

J. Alt,

J. G. Fitton²,

J. Zipfel³ &

E. Takazawa⁴

¹ Dept. of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, USA

² Dept. of Geology and Geophysics, University of Edinburgh, Edinburgh EH9 3JW, UK

³ Max-Planck-Institut fuer Chemie, Postfach 3060, D-55020 Mainz, Germany

⁴ Dept. of Geology, Niigata University, Niigata 950-2102, Japan

Abyssal peridotites are commonly interpreted to be the residues of varying degrees of partial melting of the upper suboceanic mantle. Many studies, however, have emphasized that these rocks also display petrological and geochemical trends that reflect in-situ fractional crystallization from percolating melts and/or refertilization of the residual mineralogy by basaltic melts. We have investigated under what circumstances such processes may also be responsible for the variable platinum-group element (PGE) systematics of abyssal peridotites.

Whereas the Earth's mantle is generally considered to have nearly CI-chondritic relative PGE abundances, many abyssal peridotites are characterized by non-chondritic PGE systematics. We have analyzed the concentrations of the PGE Ir, Ru, Pt and Pd in 11 abyssal peridotites from ODP Sites 895 (Hess Deep, Pacific Ocean) and 920 (Mid-Atlantic Ridge, Kane Fracture Zone) and in 6 lherzolites and harzburgites from the Horoman Peridotite, Japan, which is generally considered to represent former suboceanic lithosphere. The majority of the ODP peridotites and all Horoman lherzolites exhibit suprachondritic Pd/Ir and Pt/Ir ratios, whereas Pd/Ir is subchondritic for only two samples. One ODP harzburgite, which shows textural evidence of melt impregnation, is characterized by a basalt-like PGE pattern with Pt/Ir and Pd/Ir at approximately 2 x CI-chondrite. It is notable, however, that the ODP dataset displays average Ru/Ir and Pt/Ir ratios that are within 15% of the CI-chondritic values, whereas Pd/Ir is significantly more elevated at 1.4 x CI-chondrite. The latter result is particularly surprising because oceanic tholeiites are characterized by highly suprachondritic Pd/Ir ratios, such that the residual mantle should be depleted in Pd, relative to the other PGE, following melt extraction.

We have performed detailed quantitative model calculations in order to investigate if petrogenetic processes alone could be responsible for the PGE systematics of abyssal peridotites. We find that bulk addition of basaltic melts to a depleted residue is unable to account for the variability of the observed PGE patterns. Fractional crystallization of silicate minerals from melts percolating through the residual mantle, however, is likely to be accompanied by the segregation of immiscible sulfide liquids. Addition of such sulfide liquids to a depleted mantle assemblage would result in variable PGE patterns and Pd/Ir ratios ranging from subchondritic to highly suprachondritic, in good agreement with our analytical results.

E03 : 4B/27 : F1

Ferropicrites: Geochemical Evidence for Fe-Rich Streaks in the Convecting Mantle?

Sally Gibson (sally@esc.cam.ac.uk)¹,

Robert Thompson² &

Alan Dickin³

¹ Dept of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, England

² Dept of Geological Sciences, University of Durham, Durham, England

³ Dept of Geology, McMaster University, Ontario, Canada

Adiabatic decompression melting of upwelling mantle plumes produces primary magmas that are typically more enriched in MgO and FeO* (total Fe) than those generated by mantle melting at plate boundaries or mid-ocean ridges. Additionally, anhydrous high-MgO magmas from both oceanic and continental intraplate settings exhibit wide variations in their bulk-rock contents of FeO* (9 to 16 wt% at 15 wt% MgO). The high-FeO* picrites (ferropicrites) are relatively rare at the Earth's surface and typically form isolated flows in thick lava piles (Madagascar, Siberia CFB, Paraná-Etendeka CFB), that are believed to be have been generated by partial melting processes associated with impacting mantle plumes. World-wide ferropicrites are characterised by high contents of compatible trace elements (e.g. Cr= 400 to 1650 ppm and Ni= 250 to 1050 ppm) and have relatively smooth, flat normalised multi-element patterns (e.g. [Ba/La]n= 0.5 to 1.0) and [La/Nb]n= 1.2 to 1.4); in this respect they are similar to other mantle plume-related high-MgO melts from localities such as Hawaii and West Greenland. The ferropicrites also have relatively low ⁸⁷Sr/⁸⁶Sr_i ratios (0.70366 to 0.70725) and high <epsilon>Nd values (-3 to 8) that are comparable to those of ocean-island basalts. They are distinguished from other picritic rocks, in addition to their high FeO* contents, by their relatively low abundances of Al₂O₃ and heavy rare earth elements (Lu= <10 x chondrite).

The high FeO* contents of world-wide ferropicrites, relative to 'normal' picrites (e.g. Hawaii, Deccan and West Greenland) cannot simply be attributed to variations in degrees of partial melting and/or depth of melt segregation of an anhydrous lherzolite mantle source. The contributing parental melts of the ferropicrites appear to have been derived by adiabatic decompression melting of a mantle source that was more Fe-rich than experimental melts of Fe-rich lherzolites (HK66). They appear to represent large-degree, anhydrous, partial melts that segregated at either greater depths and/or from a more garnetiferous mantle source than 'normal' picrites. The relatively low volume of the ferropicrites and their association with igneous rocks of 'normal' FeO* contents in mantle plume-related igneous provinces suggests that the former may be derived from Fe-rich streaks in the convecting mantle. The ferropicrites provide direct evidence that the convecting mantle is chemically heterogeneous on a small scale, and this has important implications for models concerned with the degree and rates of partial melting and melt segregation.

E03 : 4B/28 : F1

²³⁸U-²³⁰Th-²²⁶Ra Disequilibrium and Melt Generation Processes in the Lau Basin

Chris J. Hawkesworth (c.j.hawkesworth@open.ac.uk)¹,

David W. Peate (dwp@dlc.ku.dk)^1,2,

Thomas K. Kokfelt (tkokfel@ugcvax.dnet.gwdg.de)^1,3,

Peter W. van Calsteren (p.v.calsteren@open.ac.uk)¹,

Janet M. Hergt (j.hergt@earthsci.unimelb.edu.au)⁴ &

Julian A. Pearce (j.a.pearce@durham.ac.uk)⁵

¹ Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, U.K.

² Danish Lithosphere Centre, Øster Voldgade 10, Copenhagen, Denmark

³ Geochemisches Institut, Goldschmidtstr 1, 37077 Göttingen, Germany

⁴ School of Earth Sciences, University of Melbourne, Parkville, VIC 3052, Australia

⁵ Department of Geological Sciences, University of Durham, Durham DH1 3LE, U.K.

The Lau Basin offers an excellent setting in which to investigate the role of fluid induced and decompression melting, because the active spreading centres are at varying distances from the arc front and the trench. Samples were selected from the Central Lau Basin and the Valu Fa Ridge (VFR), which is only ~40 km from Ata volcano. Most samples have significant ²²⁶Ra-²³⁰Th disequilibria, confirming their young eruptive ages, and (²³⁴U/²³⁸U) values within 1% of secular equilibrium. There is a clear contrast between the samples from the Valu Fa and those from the Central Lau Basin to the north, in that the former all have relatively high U/Th and 'arc-like' (²³⁰Th/²³⁸U) <1, whereas the Central Lau Basin samples all have lower U/Th and 'MORB-like' (²³⁰Th/²³⁸U)~1.

The Valu Fa samples have a relatively limited variation in both (²³⁸U/²³²Th) (1.34-1.37) and (²³⁰Th/²³²Th) (1.15-1.21), with the exception of the northernmost sample 48/3-55 GC which has a lower (²³⁸U/²³²Th) of 1.28 and (²³⁰Th/²³²Th) of 1.13. All samples show significant excess (²³⁸U) with (²³⁰Th/²³⁸U) of 0.85-0.90. In contrast, samples from the central Lau Basin have (²³⁸U/²³²Th) between 0.98-1.15, and all samples show either ²³⁰Th-²³⁸U equilibrium or ²³⁰Th excesses of up to 11%. The Valu Fa rocks may provide the best indication of the isotope composition of the mantle wedge prior to subduction, and hence indicate that the timescale of fluid transfer from the subducted slab for the Tonga arc is ~40 ka. Many of the Central Lau Basin rocks are indistinguishable from N-MORB, and if the data from the back arc island of Niuafo'ou are included (Turner et al., 1997), there is a crude overall negative relation between (²³⁰Th-²³⁸U) and Na at 8% MgO in the back arc rocks. This is explored in melting models with variable fluid contributions, different Indian and Pacific MORB reservoirs in the mantle wedge, and mantle upwelling rates.

E03 : 4B/29 : F1

Melting History of an Island Arc Mantle Wedge: Izu Arc, Japan

James Gill (jgill@es.ucsc.edu) &

Hochstaedter Alfred

Earth Sciences Dept., University of California, Santa Cruz CA, USA

Over 100 submarine sites have been sampled and studied from throughout the 100 km wide Izu volcanic arc, Japan, which is in the rifting stage of backarc basin formation. Fluid-immobile elements indicate that the subarc mantle is more enriched beneath the rear of the arc than beneath the volcanic front, and is more depleted at the front than beneath mid-ocean ridges. This is similar to the difference between off-axis versus on-axis at mid-ocean ridges. Differences can be modeled quantitatively as sequential melts of mantle initially more fertile than DMM which convects trenchward. Some of the enrichment in the rear arc appears to be ancient as in off-axis MORB sources, but there is ambiguity how much of this enrichment comes from recently melted subducted sediment. The Izu arc consists of an active volcanic front, an active extensional zone, and a series of 3-9 Ma southwest-trending across-arc seamount chains. The volcanic front (VF) contains one of the most depleted suites of any volcanic arc, with basalt containing 0.2-0.7 ppm Nb, 25-50 ppm Zr, Nb/Zr<0.015, and Zr/Y<2.5. Basalts from the western seamount chains (WS) have higher concentrations of these incompatible elements: 1-8 ppm Nb, 50-130 ppm Zr, Nb/Zr=0.02-0.1, and Zr/Y=2-7. Basalts from the intervening extensional zone are intermediate in concentrations and ratios. Trace element modeling shows that VF and WS compositions cannot be produced by different degrees of melting of a homogeneous source. Extraction of fractional melts from the WS source can produce a residual, depleted source capable of generating VF magma. Both the VF and WS suites have retained similar compositions over the last 15 million years, implying that largely steady state processes of melt generation have been maintained throughout the Neogene.

E03 : 4B/30 : F1

Trace Element Partitioning in Amphiboles: Implications for Magma Genesis and Magma Composition

Richard C. Brumm (rbrumm@gwdg.de),

Stephen F. Foley (sfoley@gwdg.de)¹,

Massimo Tiepolo (tiepolo@crystal.unipv.it)² &

Riccardo Vannucci (vannucci@crystal.unipv.it)

¹ Mineralogisch-Petrologisches Institut, Universität Göttingen, Goldschmidtstraße 1, Germany

² Dipartimento di Scienze della Terra, Università di Pavia, I-27100 Pavia, Italy

Due to its crystal chemical flexibility allowing the incorporation of many geochemically relevant trace elements, amphibole may control the geochemical characteristics of partial melts, either generated within a metasomatised mantle beneath stable cratons or at convergent plate boundaries, producing the island arc basalts. Furthermore, it has been suggested that amphiboles (as well as phlogopites) are the major hosts for Nb, Ta, thus playing an important role for the fractionation of HFSE in the mantle. Based on the results of high-pressure partitioning experiments between a variety of amphiboles (Kaersutite, Pargasite, K-Richterite) and silicate melts, showing the effect of different bulk compositions and amphibole types on partition coefficients (Tiepolo et al., 1999), we investigate the influence of amphibole-bearing mantle assemblages on the composition of partial melts. Using the measured partition coefficients for 29 trace elements, we concentrate on processes during the generation of melts at convergent plate boundaries, but also focus on the role of K-Richterite, which is a common constituent of mantle xenoliths from kimberlites.

In contrast to pargasites and kaersutites, most trace elements (REE and HFSE) are incompatible for K-Richterites (D<0.4) and D-values greater than 1 are observed only for Sr, F, V and Cr. Therefore, the influence of K-Richterites for the generation of ultraalkaline melts is very small, leading only to negative Sr, F anomalies and high Rb/Sr ratios, whereas REE and HFSE are not greatly affected. Thus, in assemblages containing clinopyroxene and K-Richterite, Cpx will exert the main control on trace element abundances in coexisting melts.

Due to the higher D-values especially for Ba (0.1-2), HFSE (0.05-2) and SEE (0.5-4) pargasites and kaersutites can lead to a significant depletion of the calculated melt composition in these components. Furthermore, decoupling of Zr, Hf and Nb, Ta is observed, because these elements are incorporated by different substitution mechanisms and occupy different sites within the amphibole structure: Zr, Hf, and also Ba, are mainly controlled by the ^[4]Al content (^[6](Zr⁴⁺, Hf⁴⁺)+^[4]Al ^[6](Fe,Mg)²⁺_-1+^[4]Si⁴⁺_-1) and D-values greater than 1 occur for high-Al pargasites. In contrast, the incorporation of Nb, Ta depends on the dehydrogenation of the amphiboles (Ti⁴⁺+2O^2- 2OH_-1+Mg²⁺_-1) and the availability of Ti during the formation of amphibole. In Ti-depleted systems, high D-values are observed for Nb, Ta (1-2) and Ti (>4), resulting in significant depletions of Nb and Ta in the calculated melt compositions and also resulting in trace element patterns very similar to those of natural island arc basalts.

Tiepolo M, Vannucci, R, J. Conf. Abs, 4, (1999).

E03 : 4B/33 : F1

Magma Pulses in the Mantle. Evidence from Glass in Peridotitic Xenoliths

Birte Hoejsteen (Bih@sfi.dk)¹ &

Paul Martin Holm (PAULMH@geo.geol.ku.dk)²

¹ Tvaervej 79, DK 2830 Virum, Denmark

² Geological Institute University of Copenhagen, Oester Voldgade 10, DK 1350 Copenhagen K, Denmark

Glass occurs in cracks in olivine and as matrix in fine grained domains of metamorphic mantle minerals and minerals crystallized from an intruding melt in peridotitic xenoliths from Torre Alfina, Italy. Micro probe analysis of the glass show compositions which gathers in two overlapping clusters. One glass occurs most frequently as matrix. It is rich in Si, Al and volatiles. The original melt partly crystallized as phlogopite and it reacted with chromite to form sanidin and very chrome-rich spinel. The other glass occurs most frequently in cracks. It is poorer in Si, Al, Na and volatiles and richer in all other elements. This glass reacted with chromite to form sanidin and very chrome rich chromite. Estimations on mass balance shows that: 1) Neither glass are intruded host lava. 2) The variation between the two glasses can not develop from one melt by in situ crystallization and reactions with primary minerals. 3) It is obvious from the occurrence of phlogopite and sanidin and from an enriched LILE and LREE pattern, that if the glass did origin in decompressionel melting, then a former intrusion had taken place. The variation in the glass composition however can not be explained by decompressionel melting from minerals now present. A steady melt or fluid flow through would not leave a deposit with compositions in two clusters. The implications are, that three melts intruded the mantle rock. 1) The first melt intruded fine grained domains with crushed minerals. 2) The second intruded (new) cracks in olivine and only mixed with the first melt to a limited extend. 3) The third melt extruded and brought pieces of the mantle to the surface

E03 : 4B/34 : F1

Intergranular Glassy Layers Along Mineral Interfaces in Mantle Derived Xenoliths: the Early Stage of Melting

Richard Wirth (wirth@gfz-potsdam.de)¹ &

Leander Franz (lfranz@mineral.tu-freiberg.de)²

¹ GeoForschungsZentrum Potsdam, D-4.1 Telegrafenberg, Germany

² TU-Berg Akad. Freiberg, Brunnhausgasse 14, 09569 Freiberg, Germany

The presence of intergranular glassy layers and pockets along mineral interfaces, on microfractures and as inclusions in minerals in mantle peridotite xenoliths from different locations is revealed by optical microscopy and transmission electron microscopy (TEM). That all represent former melts, is confirmed by electron diffraction as well as their typical geochemical signatures. Often very thin glass layers are present on grain boundaries and show characteristic chemical compositions that strongly depend on the adjacent minerals. The composition of these layers differs distinctly from the bulk melt composition of partial melt experiments or from the compositions of wider meltpools of glasses observed in other xenoliths. Furthermore, a relation of these glasses to the adjacent host basalt can be excluded by the distinctly different geochemistry of the melts. The chemical composition of the melt changes with increasing thickness of the glass layers, which is due to mixing processes of the different types of glasses in the xenoliths. Wider melt films (>1000 nm) are more similar to glasses observed in large melt pools and veins given in the literature as well as partial melting experiments. Thus, the chemical composition varies from that of the very first melt at different interfaces to the bulk composition of partial melts created by experiments depending on the melt film thickness. Melts are probably formed by grain boundary melting due to lattice mismatch and impurity segregation in the xenolith triggered by decompression processes during the uplift of the xenolith. This is testified by corrosion textures and the absence of chemical equilibrium between melt and adjacent olivine crystals. Chemical equilibrium is only found for very few melt films along olivine boundaries and melt inclusions in olivine neoblasts. These early melts were generated during thermal overprint and dynamic recrystallisation of the xenolith in the mantle. The occurrence of melt on grain boundaries has important geological and petrological implications. Intergranular layers give an insight into the very first melting processes and the development of melt composition with time and degree of partial melting. Furthermore, melt films on interfaces have an important significance for the rheology of the mantle by distinctly increasing the creep rate of the rock. Finally, diffusion processes are distinctly enhanced by the presence of melt and may give way for a very fast reequilibration of the mineral chemistry.

E03 : 4B/35 : F1

Caught in the Act: Carbonatite in Progress?

Jeff M. Rosenbaum (j.rosenbaum@rdg.ac.uk)¹ &

Marjorie Wilson (marge@earth.leeds.ac.uk)²

¹ P.R.I.S; Reading University, Whiteknights, Reading RG6 6AB, United Kingdom

² Dept. of Earth Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom

Carbonatites are carbonate-rich, magmatic rocks often containing economically important quantities of rare earth and other trace elements. Despite many years of study, the origin of these rocks remains elusive. In a spinel peridotite xenolith from the Pannonian-Carpathian Region, we found intimate mixtures of carbonate and silicate glasses in veins which may have direct bearing on the problem of carbonatite petrogenesis. Major element compositions of the primary silicates suggest the xenolith last equilibrated in the upper mantle at about 1.2-1.5 GPa and 920°C. The textures of the two glasses probably reflect either fluid unmixing or fluid immiscibility, indicating that the two phases currently observed were originally a single phase. The silicate phase always wets the host silicates. The carbonate phase forms rounded 'bubbles' within the silicate phase, always surrounded by a thin reaction zone. Contacts between glasses and host minerals are usually sharp and show little sign of reaction. The glasses are not in equilibrium with the host xenolith, however, as vein pyroxenes and those in the host matrix have different major element compositions. We have determined the major element compositions of the glass phases and host silicates by electron microprobe and the trace element compositions by ion microprobe (Univ. of Edinburgh). The silicate glass is rich in Na, K, Al, and Si, but depleted in Mg and Fe relative to volcanic rocks of similar silica content (~63% SiO₂). The carbonate glass is extremely Ca-rich (>93% CaCO₃) with variable Mg contents and virtually no Fe. The trace element patterns of the two glasses are similar to each other with absolute concentrations being approximately an order of magnitude higher in the silicate phase. Both glasses have trace element patterns broadly similar to those of carbonatites. The chondrite-normalized, rare earth patterns of the silicate glasses show a weak negative Eu anomaly which is absent from the carbonate glass patterns. Evidence of interaction between the glasses of the host xenolith can be seen in the trace element data of the host clinopyroxenes which show both light rare earth enriched and depleted chondrite-normalized patterns. Our data suggest the carbonate and silicate glasses may be representative of the magmas which give rise to carbonatites in the mantle.

E03 : 4B/36 : F1

Genesis of Silica-Saturated and Calcio-Carbonatitic Melts by Immiscibility and Wall Rock Reactions in the Upper Mantle: A Natural Case Study in Mantle Xenoliths (Persani Mts, Romania)

Françoise Chalot-Prat

(chalot@crpg.cnrs-nancy.fr)

CRPG - CNRS, BP 20, 54501-Vandoeuvre Cedex, France

This presentation concerns the textural, mineralogical and chemical study of veinlets crosscutting peridotite xenoliths from the lithospheric mantle (Chalot-Prat and Boullier, 1997; Chalot-Prat and Arnold, in press). The veinlets line grain boundaries or crosscut any mineral, and always form an interconnected network. They are filled with carbonate patches included in a silicate matrix. Both products are holocrystalline. Carbonate products have alkali-poor calciocarbonatitic to sövitic compositions, while the silicate matrix composition ranges from monzodioritic to monzonitic and alkali feldspar syenitic, depending on the host-sample, i.e. within a rather alkaline silica-saturated series. The mineral phases present in the silicate matrix (F-apatite, armalcolite, chromite, diopside-enstatite series, plagioclase-sanidine series) are usually present in the carbonate zones, where forsterite is also found. Some minerals crosscut the interface between both types of zones. Only the matrix is different, feldspathic (oligoclase to sanidine) in the former and pure calcite in the latter. Thus, mineralogical and textural relationships between both products are consistent with an origin with equilibrium liquid immiscibility. Mantle minerals crosscut by veinlets are sometimes resorbed at grain boundaries. Subhedral diopside/augite formed at the expense of mantle enstatite or olivine at the contact of the most alkaline silicate or carbonate melts. The compositional variations recorded by vein minerals vary along a continuous trend which superpose to those observed from lherzolites to harzburgites, or to that observed between rims and cores of mantle minerals crosscut by veinlets. The Ca-rich pyroxenes of veinlets, whether magmatic or metasomatic, are Al-poor and Mg-rich, slightly Ca-richer in the carbonate zones than in the silicate matrix; spinels are Al- and Mg-poor but rather Cr- and Fe-rich. Existence of armalcolite alone and various pairs of pyroxenes suggest crystallization at 1100-1200°C between 10-15 kb. Feldspar compositions in silicate materials, which vary continuously from labradorite to sanidine, are consistent with hypersolvus and dry crystallization conditions. The cooling rate until total crystallization was very slow and limited. All of these results provide evidence that immiscibility occurred at mantle depth as the liquid was forcibly injected during hydraulic fracturing of the mantle. The compositions of conjugate melts suggest a very large miscibility gap, as expected at high pressure in a dry environment from the experiments of Kjarsgaard and Hamilton (1988, 1989). The parental melt was carbonate, silica-undersaturated and rich in F, Cl and CO₂. Wall rock reactions leading to the formation of Ca-rich pyroxene occurred only at the contact with alkali-rich carbonatitic or silicate melts. Calcite, always anhedral, is a differentiation product by magmatic crystallization or wall rock reaction, or in some cases the product of the crystallization of a pure sövite immiscible melt.

Chalot-Prat F, Boullier AM, Contr. Miner. Petrol, 129, 284-307, (1997).

Chalot-Prat F, Arnold M, Lithos, (in press).

Kjarsgaard BA, Hamilton DL, Mineral. Mag, 52, 43-55, (1988).

Kjarsgaard BA, Hamilton DL, Carbonatites: Genesis and Evolution. Unwyn Hyman,London, 388-404, (1989).

E03 : 4B/37 : F1

Kinetics of Carbonatite Percolation in the Mantle

T. Hammouda (taharh@opgc.univ-bpclermont.fr) &

D. Laporte (laporte@opgc.univ-bpclermont.fr)

Université Blaise Pascal, département des Sciences de la Terre and CNRS-UMR 6524, 5, rue Kessler, 63038 Clermont-Ferrand cedex, France

Carbonatitic melts are often invoked as possible metasomatic agents in the mantle because of their low viscosity and good wetting properties. One possible mechanism for melt migration in a crystalline matrix is interfacial energy-driven infiltration along grain edges (Watson, 1982). In order to investigate the kinetics of such a process we have performed high pressure infiltration experiments on couples consisting of carbonate melts and dry synthetic dunites. The melt composition was Na-carbonate saturated in forsterite at run conditions. The dunites were prepared by sintering synthetic forsterite sieved to 20 micrometers. The couples were run in a piston-cylinder with the melt on top and the dunite at the bottom. Infiltration run conditions were 1300°C-10 kbars with durations ranging from 0 (zero-time experiment) to 5 days. Experimental charges were characterized by optical and electron microscopies and the distances of penetration of the carbonatitic melt in the crystalline aggregate (initially dry and non-porous) were measured.

The results are that carbonatite percolation is a very fast process. For example we found a penetration distance of 800 µm after 10 minutes and 2.5 mm after 1 hour at run conditions. Those rates are much faster (more than 2 orders of magnitude) than what we previously determined for basalt infiltration using the same experimental procedure. Observation of the experimental assemblages shows that euhedral forsterite crystals are present in the carbonatite reservoir just above the dunite interface. We take this as evidence that material is dissolved from grain edges in the dunite and subsequently transported along melt channels before being precipitated back into the melt reservoir. Therefore carbonatite percolation in the dunite can be described as a dissolution-reprecipitation process. A plot of the melt travel distance as a function of the square root of time yields a linear relationship suggesting that diffusional transport in the melt along forsterite grain edges could be the governing process.

The implications of the present results are that carbonatitic magmas are highly mobile in the mantle. Therefore they can chemically interact with the surrounding silicates and alter their geochemical characteristics. On the other hand the experiments suggest that carbonatitic melts would rather disperse (by infiltration in the neighbouring mantle rocks) than collect and segregate which might explain their scarcity as eruptive products.

Watson EB, Geology, 10, 236, (1982).

E03 : 4B/38 : F1

Melt Extraction Under MORs: Transition from Channel Flow to the 'Magmafracture'

Alexei N. B. Poliakov

(aljosha@dstu.univ-montp2.fr) &

Adolphe Nicolas

Laboratoire de Geophysique et Tectonique, CNRS, case 060, Universite Montpellier II, 34095 Montpellier Cedex 5, France

The objective of this work is to investigate the physics of melt segregation and ascent under mid-ocean ridges. The data from Oman Ophiolite suggest that extraction of the basaltic melt probably took place by magma-fracturing which can be traced by numerious dikes found in the field (Nicolas and Jackson, 1982). However, the physical mechanism of this process is still controversial: is magma transported by diffused porous flow, channeled flow or by dikes? The difficulty arises in the explanation of the transition from the porous flow to the localized flow and then to magma-fracturing of hot and low-viscous mantle rocks. The recent model of magma extraction from visco-elasto-plastic matrix suggests that the strong waves of overpressure can be generated in the low permeability zones (Connoly and Podladchikov, 1998). This process can lead to the fracturing and rapid episodic extraction of the melt. In this work we discuss the physics of dike formation and estimate the periodicity of melt surges, dimensions of the dikes and volumes of extracted magma. We also discuss the origin of dunites bordering observed dikes in Oman.

Connoly JAD and YuYu Podladchikov, Geodinamica Acta (Paris), 11, 2-3, 55-84, (1998).

Nicolas A and M Jackson, J. Petrol, 23, 568-582, (1982).

Session E03:4P

E03 : 4P/01 : PO

Melilite-Bearing Rock Series within Alkaline-Ultrabasic Complexes: Derivatives from SiO₂-Poor, Ca-Rich Mantle Magmas?

Irene Rass (rass@igem.msk.su)

IGEM RAS, 35 Staromonetny, Moscow, Russia

Two series - melilite-free and melilite-bearing rocks, different in Ca-content - compose alkaline-ultrabasic complexes (Kravchenko and Rass, 1985). The geological setting of melilite-bearing associations relatively to melilite-free rocks is different in different massifs and provinces. Autonomous displays of melilitic rocks do also exist, e.g., in Central Europe

Rock-forming mineral compositions demonstrate their essential distinctions that depend on their affinity to rocks of the two series. Clinopyroxenes, olivines and nephelines from the series with melilite are enriched in Ca-content as compared with the same minerals from the series without melilite. Thus, CaO in olivines is 0,42-2,08 wt.% contre 0.28-0,46, and in nephelines 0,32-2,93 contre 0,04-0,61 wt.%. According to experimental melting data, the Ca-content in olivine and nepheline depends on the Ca-content in the parental melt.

Clinopyroxenes from melilite-bearing rocks in any massif are enriched by REE, as compared with clinopyroxenes from melilite-free rocks in the same massif. When taken into account that REE concentrations in melilites themselves are of the similar magnitude, it may inferred the parental magmas for melilite-bearing rocks are enriched by REE.

The zoning of clinopyroxenes, titaniferrous garnets, and especially apatites and perovskites from the two series are characterized by persistently different trends of component microfractionation.

Compositions of alkaline-ultrabasic igneous rocks, when traced on almost each petrochemical diagram, especially those using Ca-content as a coordinate, demonstrate two different trends related to melilite-bearing and melilite-free rocks, with more and less Ca, respectively. The normative tetrahedron Ln-Fo-Q-Ne (Yoder, 1979) with traced rock compositions shows that two rock sequences could not be derivatives from one parental magma, and, as compared with the parental magma for melilite-free rocks, a possible parental magma for melilite-bearing rocks must be enriched in Ca, and be compositionally close or similar to Siberian kimberlites with approximatly 14 wt.% CaO, and to the IB kimberlite type (Smith et al., 1985).

Recent melting experiments at a pressure 7 kbar (Rass et al., 1996), the high-pressure experiments on Ca-rich kimberlite from Wesselton (Edgar and Charbonneau, 1993), and isotope evidence from Eurropian melilitites (Wilson et al., 1995) support the idea, that melilite-bearing rocks may be derivatives from deep, Ca-rich, mantle magma. One of the necessary conditions for the differentiation of Ca-rich kimberlite-like or kimberlite magma to reveal melilite is the existence of an intermediate magma chamber under pressures at least as low as those within the melilite stability field.

The author thanks RFFI for financial support, N98-05-65017

Edgar AD & Charbonneau H.E., Amer. Mineral., 78, 132-142, (1993).

Kravchenko SM & Rass IT, Trans. (Doclady) USSR Acad. Sci., 283, 111-116, (1985).

Rass IT, Girnis AV & Laputina IP, Experiment in Geosciences, 6, 41-42, (1996).

Smith CB, Gurney JJ, Skinner EMW, Clement CR & Ebharim N, Trans. Geol. Soc. S. Afr, 88, 267-280, (1985).

Wilson M, Rosenbaum JM & Dunworth EA, Contrib. Mineral. Petrol, 119, 181-196, (1995).

Yoder HS, jr, Yoder HS, jr (Ed) The evolution of the igneous rocks. Princeton, N. J, 391-412, (1979).

E03 : 4P/02 : PO

Geochemistry and Petrology of Oman Ophiolite Peridotites

Laure Gerbert-Gaillard

(gerbert@dstu.univ-montp2.fr),

Marguerite Godard

(margot@dstu.univ-montp2.fr) &

Françoise Boudier (fran@dstu.univ-montp2.fr)

Laboratoire de Tectonophysique, Université Montpellier II, place E. Bataillon, 34095 Montpellier cedex 5

We present here a chemical study of peridotites sampled along 3 cross-sections (from bottom to the top of the exposed mantle) in the Oman ophiolite, a paleo-spreading center. Our purpose is to characterize the chemical heterogeneity in mantle composition as a fonction of depth and distance to an ocean propagator. The chemical study includes major and trace elements (REE, Zr, Hf, Rb, Ba, U, Th, Nb, Ta, and Y, analysed by ICP-MS at Montpellier university) on whole rocks and separated minerals.Petrological and major elements study show that samples deep in mantle sections have higher cpx contents (cpx/opx>0.2), than uppermost samples (cpx/opx<0.2), while their Mg-number (=(100*Mg²⁺)/(Mg²⁺+Fe²⁺)) remain constant along the section. This points to a secondary origin for cpx in cpx-rich harzburgites (type II) by opposition to the uppermost harzburgites (type I). Oman harzburgites are extremely depleted in all incompatible elements. Nevertheless, a drastic change in REE patterns is observed at the type I/type II harzburgites limit. Type I harzburgites have 'flat' REE patterns characterized by (Nd_N/Gd_N>0.2; N=normalized to Primitive Mantle), while type II harzburgites show 'convex' patterns with (NdN/GdN<0.2).Other incompatible elements contents are more or less constant along the mantle section. Cs, Rb, Ba, Nb, Ta and U are enriched relative to Th and light REE. Later degrees of alteration could be invoked for mobile elements, such as Ba, but the trace elements low contents and inter-element fractionations are consistent with mantle processes.The extremely low contents in incompatible elements may be explained by extraction processes in the mantle, but the relative enrichment in incompatible elements, the changes in REE patterns and the cpx contents for constant Mg-number, point to later magmatic processes toward spreading ridge, superimposed on melting. The type I/type II harzburgites limit doesn't correspond to the mechanical LT-HT deformation boundary. Drastic change in cpx contents suggest that type I/type II harzburgites limit is a thermal boundary resulting from later cpx precipitation. In this case, it would trace the upper limit of magmatic infiltrations in the oceanic lithosphere. The incompatible elements enrichment may result from the low fraction liquid circulation created by this cpx precipitation, or, by later magmatic events associated with the beginning of the compressive tectonic.

E03 : 4P/03 : PO

Cr-Diopside Veins in Vitim Mantle. Origin and Evolution

Igor V. Ashchepkov (garnet@uiggm.nsc.ru) &

Luc Andre. (lucandre@africamuseum.be)

The suites of the Cr-Diopside pyroxenite (CDP) veins from the volcanics in Vitim plateau have been studied. At the initial "picrite basalt" stage they mainly are saturated at the percolating unit in lherzolites within 950-1050°C. At the HT° part CDP are more ferriferous and demonstrate the spikes of the HFSE due to the remelting of the metasomatic front assemblages. They equilibrate with the lherzolites when rising. In CDP the Cr content decrease together with the calculated temperatures what suppose the differentiation at the interconnected vein system during the uplift. 'Magmatic' type CDP Cr-diopside veins demonstrate sharp contacts and lower LILE, 'anatexic' CDP reveal them upper (Fig.1). The TRE in lherzolites are similar to CDP but they have only Zr dips while CDP show the joint Zr-Hf-Ti depressions due to minor Ilm precipitation. At the Middle Miocene stage of the lava plateau creations the CDP are more HT° and Fe-rich due to mixing with the basalts. At the Pliocene stage they are located at the several horizons of the stratificated mantle. Newly formed HT° CDP mark two horizons of percolation and the LT° of subsolidus type with the signs of the garnet decomposition are located within the LT° part representing the older vein system from the Miocene stage. The calculated shift of subsolidus pyroxenites from the garnet facies is about 3-7 kbar (Fig.2). The Pleistocene stage mainly with the microvein Gar-bearing CDP locates at the Ga-Sp transition and rarely at the spinel facies. The levels of the Cr-Di vein location coincide with the percolating units at the mantle column and probably with the locations of the intermediate magmatic chambers. The CDP parental melts have the inflected in Gd REE paterns very similar to the most of the Vitim flood basalts and were likely participated at the basaltic genesis as the result of remelting and assimilation.

Trace elements in Cr- diopside veins and lherzolites.

The geotherms and the possible diapiric displacements of pyroxenetes in mantle beneath Vitim.

E03 : 4P/04 : PO

Across-Arc Variations for Mantle Xenoliths Compositions in Kamchatka (Russia)

Tatiana Churikova (tanya@africamuseum.be)¹,

Luc Andre¹ &

Alexander Koloskov (ivgg@svyaz.kamchatka.su)²

¹ Royal Museum of Central Africa, Geological Department, Leuvensesteenweg. 13, 3080 Tervuren, Belgium

² Institute of Volcanic Geology and Geochemistry FED RAN, Peep avenue 9, 683006 Petropavlowsk-Kamchatsky, Russia

Active arc volcanism on Kamchatka comprises from E to W three major zones: (1) the Eastern volcanic front (EVF), (2) the Central Kamchatka zone and graben depression (CKD), and (3) the western volcanic chain of the Sredinnyrange (SR). We have sampled the ultramafic plagioclase-free xenoliths as well as the host volcanic rocks from these zones from Avachinsky and Bakening volcano at EVF through the Kluchevskaya Group of CKD (Kluchevskoy, Kharchinsky and Shiveluch volcanoes) into the back arc of the Ichinskystrato volcano at SR.

Two main groups of xenoliths are generally observed: dunite-harzburgite and pyroxenite-wehrlite associations (except for SR, where dunite-harzburgites are absent). Despite large difference in the host-rock composition (from magnesian basalts to andesites and from tholeitiic to alkaline rocks) the xenoliths of the first group have many common peculiarities in terms of the whole rock and olivine compositions (Koloskov & Khotin, 1978). Within the xenoliths of the second group, good correlations are observed between whole rock and mineral chemistry (Ol, Cpx) from the xenoliths and host-rocks. Amphibole-rich volcanics of Avachinsky and Shiveluch volcanoes bring more metasomatized and recristallyzed amphibole-bearing xenoliths whereas amphibole-free volcanic rocks of Bakening, Kluchevskoy, Kharchinsky and Ichinsky volcanoes do not show any amphibole-bearing ultramafic inclusions. Most high-Mg olivines (Fo_93-94) were found in harzburgites of Kluchevskoy volcano. All minerals are characterized by very low Al₂O₃, TiO₂ concentrations and high CaO content. Moreover Cpx and Opx are characterized by low Na, Fe³⁺, and Cr. This suggests that sub-arc mantle is strongly depleted below Kamchatka. Different steps of mantle metasomatism were observed in various locations. Most fluid-metasomatized rocks with veins and metasomatic phases (amphibole, phlogopite) occur in CKD. This correlates with the highest fluid signatures in volcanic rocks (Churikovaet al, 1997; Dorendorf et al, 1997).

The trace element distribution in olivines and pyroxenes are determined by LA-ISP-MS. The preliminary results will be discussed.

Koloskov AV & Khotin MYu, Inclusions in the volcanic rocks of Kurile-Kamchatka island arc, Moscow, Nauka, 36-66, (1978).

Churikova T, Dorendorf F, Woerner G, Eisenhauer A & Heuser A, EOS Transactions, AGU Fall Meeting Abs, 78 (46), F804, (1997).

Dorendorf F, Wiechert U, Koloskov A, Volynets O, Hoefs J & Woerner G, Terra Nova Abs, 9, 473, (1997).

E03 : 4P/05 : PO

Geochemistry and Petrology of Megacrysts from Plio-Pleistocene Basanites of Vitim Volcanic Field (Baikal Rift System, Russia)

Konstantin D. Litasov (kostik@uiggm.nsc.ru)

Institute of Geology SB RAS, Koptyuga ave., 3, Novosibirsk, RUSSIA

Megacryst assemblage in Plio-Pleistocene basanites of the Vitim volcanic field includes clinopyroxene, garnet, ilmenite, Ti-magnetite, biotite, and K-Na feldspars. Also, coarse intergrowths of clinopyroxene with garnet, ilmenite, and phlogopite, ilmenite-clinopyroxene symplectites, glimmerites chemically belong to the megacryst assemblage.

The megacryst clinopyroxenes reveal certain trends of compositional variations, such as an increasing in TiO₂, Al₂O₃, Na₂O, CaO and decreasing in Cr₂O₃, while Mg-number (Mg# = 100 x Mg / (Mg+Fe)) decreases from 87 to 61. This feature evidences for the model of polybaric melt fractionation during channeling and in local magma chambers. The melt differentiation occurs during the melt ascent due to a precipitation of certain phases on walls of cracks and magma chambers (e.g. Ashchepkov et al., 1995).

Biotite megacrysts has Mg#=54-64 and contain 9-11.5% TiO₂. Ilmenite is Mg-rich (5-6.5% MgO in bulk nodules) and often contain Ti-magnetite lamellas. Feldspar megacrysts has composition An_3.8-5.0, Ab_75-58, Or_21-37.

Trace element patterns of clinopyroxene was obtained by ion microprobe for nodules from the different part of the Mg#-variation trend. (La/Yb)_n ratio varies from 1.0 to 2.7. Highest values correspond to the last stage ilmenite-phlogopite-clinopyroxene nodules. Zr/Sm and Ti/Eu ratios varies from 0.3 to 0.7 and from 0.7 to 0.9 correspondingly. This suggest about significant Zr and Ti accumulation in the melt during clinopyroxene fractionation.

The crystallization temperatures of the clinopyroxene megacrysts decrease in accordance with Mg#-decreasing from 1350 to 1000^oC. Indirect pressure estimations (projection to the geotherm, derived from garnet-bearing nodules from Miocene picrite basalts, Ashchepkov et al., 1995) are 25-14 kbar. Each volcano forms its local vein system propagating to the depths 60-80 km, despite resembling melt sources, because the megacrysts of different volcanoes form separate clasters within different parts of a single trend. The order of phases crystallization during the melt evolution was established from chemical composition and coexisting in intergrowths as following: 1) clinopyroxene, 2) clinopyroxene±garnet, 2) clinopyroxene, 3) clinopyroxene+garnet, 4) clinopyroxene+ ilmenite+biotite, ) K-Na feldspar.

Ashchepkov IV, Andre L, Litasov KD, & Mal'kovets VG, Ext Abst 6th Int Kimb Conf, Novosibirsk, 17-19, (1995).

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Ti-Rich Lherzolite from Pliocene Basanite of the Vitim Volcanic Field: Evidence of Unique Type of Mantle Metasomatism

Konstantin D. Litasov (kostik@uiggm.nsc.ru)¹,

Yury D. Litasov (yuryl@uiggm.nsc.ru)¹,

Vladimir G. Mal'kovets (vomal@uiggm.nsc.ru)² &

Alexey S. Mekhonoshin³

¹ Institute of Geology SB RAS, Koptyuga ave., 3, Novosibirsk, RUSSIA

² Institute of Mineralogy and, Petrography SB RAS, Koptyuga ave., 3 Novosibirsk, RUSSIA

³ Institute of Geochemistry, SB RAS Favorsky st., 1a, Irkutsk, RUSSIA

The abundant mantle xenoliths were found in Pliocene basanites of the Dzhilinda River (Vitim volcanic field). Two main and one detached peridotite groups were recognized: (1) spinel and garnet-spinel lherzolites with coarse-grained protogranular texture (two pyroxene temperature T=1060-1210^oC, Brey & Kohler, 1990); (2) spinel peridotites with various textures from protogranular to tabular equigranular and secondary recrystallized (T=790-910^oC); (3) mosaic equigranular lherzolite with modal composition ol_42-48, opx_25-32, cpx_18-23, sp_3-6. (T=710-750^oC).

Minerals of group 3 peridotite are characterized by unusual chemistry: NiO contents in olivine are up to 0.7%; clinopyroxene contains 1.8-2.2% TiO₂; spinel has up to 0.8% NiO and high ZnO contents (0.8-1.0%). Bulk rock Zn is 135-170 ppm and TiO₂ is 0.42-0.55% against 36-56 ppm Zn and 0.01-0.15% TiO₂ in group 1 and 2 peridotites.

Trace element data were obtained by ion microprobe. The group 3 clinopyroxene shows depleted patterns (La_n/Yb_n=0.01-0.08) with positive Nb and Ti anomalies and light negative Zr anomaly. Melt coexisting with cpx has REE pattern similar to N-MORB, although it has significant Nb, Zr, Ti positive peaks. Numerical modeling of partial melting and melt percolation showed that the most suitable metasomatizing agent is a basaltic melt derived from ilmenite-phlogopite-bearing spinel lherzolite. The presence of ilmenite in the source is supported by high contents of V, Ni, Zn, Ti in the peridotites.

High contents of compartible elements in the group 3 lherzolite and high Mg# of composing minerals are explained by significant primary depletion - more than 15% of primitive mantle melting. Trace element variations suggest that primary assemblage was dunite or harzburgite with high amount of spinel (>3%). Pyroxenes have secondary origin and were crystallized from percolating basaltic melt. Thus, unique Ti-rich lherzolite of Dzhilinda River characterizes rare type of shallow level mantle-melt interaction and it is not described somewhere yet.

Brey GP & Kohler T, J Petrol, 31, 1353-1358, (1990).

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Experimental Evidence for the Generation of High-Mg Andesite from Pyroxenitic Mantle

Hassan El Hmidi,

Antonio Castro Dorado (dorado@uhu.es) &

Mohammed El Biad (biad@uhu.es)

Dpto de Geología, University of Huelva, campus de La Rábida, Huelva, Spain

Melting phase relations of a high-Mg norites from the Hercynian Aracena Metamorphic Belt (AMB), Southern Spain, have been studied under anhydrous conditions. The high-Mg norites are characterised by high Mg number (Mg#=75) and considered to represent primary or nearly primary magma derived from upper mantle. The experiments were carried out in solid-medium piston-cylinder apparati at the University of Huelva. 5 mg of sample was contained in graphite capsule and welded in Pt tube to eliminate the problem of iron loss to the Pt capsule. Cell assemblies consisted of graphite furnace tubes, crushable MgO inner parts for sample plug and thermocouple bushing, and outer sleeves of pyrex and NaCl. Temperature were measured with Pt-Pt₈₇Rh₁₃ thermocouple and controlled with an Eurotherm 808 digital controller.The results of the experimental study indicate that under anhydrous conditions at the pressure from 8 to 14 kbar and temperatures from 1120 to 1190°C, clinopyroxene is the liquidus phase followed by orthopyroxene and then plagioclase. The mineral-in curves for these phases converge at the liquidus at the pressure about 8 kbar. The absence of olivine at near liquidus conditions suggests that the composition under investigation cannot be derived from a more mafic parent by olivine fractionation at any pressure in the investigated range, and supports the interpretation that it is primary. This study indicates that high-Mg andesite may be generated under anhydrous conditions by partial melting of a shallow pyroxenite mantle consisting predominantly of clinopyroxene, orthopyroxene and plagioclase. The resulting melt have been contaminated with crustal components as it is deduced from their isotopic ratio characteristics (Sm/Nd and Rb/Sr). These contamination may have had only negligible effects on the major element composition and then on the phase relations.

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Experimental Study on a Near-Primary Basanite from the McMurdo Volcanic Group, Antarctica: Inference on its Genesis and Source

Andrea Orlando (aorlando@steno.geo.unifi.it)¹,

Sandro Conticelli (sandro@cesit1.unifi.it)²,

Pietro Armienti (armienti@dst.unipi.it)³ &

Daniele Borrini (dborrini@steno.geo.unifi.it)¹

¹ Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira, 4 - I-50121 Firenze, Italy

² Centro di Geodinamica, Università degli Studi della Basilicata, Via Anzio, Potenza, Italy

³ Dipartimento di Scienze della Terra, Università degli Studi di Pisa, Via S. Maria, 53 - I-56100 Pisa, Italy

Cenozoic and Quaternary magmatism that extensively affected the Ross Sea area, Antarctica, is characterized by a wide spectrum of Na-alkaline to mildly alkaline igneous rocks comprehensively known as "McMurdo Volcanic Group". Magmatism is related to the continental break-up responsible for the Ross Sea rifting. Volcanic rocks are the most widespread products of this activity, and their composition ranges from basanite to phonolite, and from alkaline basalt to trachite and riolite. Though less evolved rocks seldom have primitive compositions, a basanite from the Malta Plateau, North Victoria Land, possibly represents a near-primary composition being characterised by high amounts of Ni (357 ppm), Cr (735 ppm), a high Mg-V (71) and by a sub-aphyric texture with olivine (Fo_88-92) and subordinate clinopyroxene phenocrysts.

The knowledge of phase relations of a near-primary melt allows to investigate the composition of its source and to estimate P-T conditions of melt generation. Thus a set of experiments was performed on this basanite in order to reconstruct its phase relations in the range 1.0-3.0 GPa and 1175-1550°C. No water addition was provided to the starting material. Liquidus curve is found to occur between 1370°C (at P=1.0 GPa) and above 1525°C (at P=3.0 GPa). The liquidus phase is olivine up to P=1.5 GPa and changes to clinopyroxene at higher pressures. Spinel is stable up to P=2.5 GPa while garnet approaches the liquidus at P=3.0 GPa. Olivine and garnet never coexist near the liquidus. These data are compatible with basanite generation either from a spinel lherzolite (at P=1.5-2.0 GPa; T=1390-1490°C) or a garnet pyroxenite (at P>3.0 GPa; T>1550°C) source. Several petrological and geochemical lines of evidence support the latter hypothesis.

Some experiments were also performed adding 5 wt.% H₂O to the starting material. In these runs primary mica crystals are found among the products, suggesting that mica can represent a residual phase in the source if it is assumed that magma lost some water during its ascent to the surface. This hypothesis is also supported by the presence of phlogopite and amphibole in the mantle xenoliths hosted in the most primitive volcanic rocks of the McMurdo Volcanic Group. This evidence, and the pyroxenitic nature of the magma source, suggest that mantle underwent a metasomatic event before partial melting episodes linked to the Cenozoic phase of rifting.

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The Lamprophyres of the Spessart and Ruhla Crystalline Complexes: Differences in Hercynian Mantle Metasomatism and Partial Melting

Volker von Seckendorff,

Peter Wrobel &

Martin Okrusch

Mineralogisches Institut, Univaersitaet Wuerzburg, Am Hubland 97074 Wuerzburg, Germany

Late Hercynian spessartites and kersantites intruded into metamorphic and igneous rocks of the Mid-German Crystalline Rise in the Spessart and Ruhla crystalline complexes. Within the lamprophyre dikes of the Spessart, a transition is observed from spessartitic centres to kersantitic margins. Petrographic details are described in the companion abstract of Wrobel et al. (this volume). From the various dike rocks of the Ruhla crystalline complex, only spessartites were considered in this study. They contain phenocrysts of hastingsite-pargasite (with edenite rim), of magnesiohornblende-actinolite, of olivine (replaced by tremolite-actinolite and chlorite), and of rare plagioclase (An_30-70) set in an actinolite-plagioclase (An_30-63) matrix. Pseudomorphs of green hastingsite after pyroxene and xenocrysts of quartz are always present.

The Ruhla spessartites show chondrite-normalized LREE around 100, HREE around 10 (La/Yb = 12) with slight negative Eu anomalies. MORB normalized spidergrams display enrichment in incompatible elements (e.g. Rb 90, Ba 20, Th 60) and slight negative Nb, P and Ti anomalies. The Cr contents scatter around 500 ppm with MgO values between 8 and 9 wt.%. The chondrite-normalized patterns of the Spessart lamprophyres are similar to shoshonitic patterns and have higher LREE of 150-300, HREE around 7 (La/Yb 21-57) with slight negative Ce and Eu anomalies. The MORB-normalized spidergrams show a higher enrichment of the incompatible elements as compared to the Ruhla spessartites (e.g. Rb 100, Ba 120, Th 120) with similar negative Nb, P and Ti anomalies. The MgO-richest sample (7.5 wt.%) also displays the highest Cr-contents of 320 ppm, both values are smaller than those of the Ruhla spessartites.

Trace element patterns indicate that fluids rising from subducted oceanic crust and overlying sediments led to mantle metasomatism prior to melting. The observed patterns are consistent with experimental data showing that Rb, Ba, Sr, Pb are preferably transported in a Cl-containing fluid, to a lesser degree also U and Th, whereas Nb, Ta, Zr and Ti remain in the source rock. The REE patterns in combination with the contents of MgO and Cr indicate partial melting of a metasomatized garnet-lherzolite. We discuss two models to explain the differences in trace element contents between the Ruhla and Spessart lamprophyres: (i) a different extent of mantle metasomatism generating different amounts of amphibole or/and phlogopite, (ii) a lherzolite with smaller garnet contents and higher degree of partial melting for the Ruhla spessartites. The major and trace element data indicate the presence of a deep-seated magma chamber allowing the melts to fractionate. Multiple intrusions result from subsequent pulses of melt released from the reservoir and triggered by tectonical processes. Large-scale contamination by crustal material is not indicated.

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Multiple Stage Evolution of the Spessart Lamprophyres

Peter L. Wrobel

(peter.wrobel@mail.uni-wuerzburg.de),

Volker von Seckendorff &

Martin Okrusch

(okrusch@rzbox.uni-wuerzburg.de)

Mineralogisches Institut, Univaersitaet Wuerzburg, Am Hubland, Germany

Lamprophyre dikes transitional from kersantite (margin) to spessartite (center) intruded into dioritic rocks of the southern Spessart during the latest stage of the Hercynian Orogeny (younger than 320 Ma). Thick dikes (up to 8 m) show multiple intrusions with more pronounced transitions between kersantite and spessartite. Spessartite and kersantite contain phenocrysts of zoned biotite (cores up to 5.8, rims up to 2.0 wt.% TiO₂ ), chromian augite (cores 1.0 wt.% Cr₂ O₃, 1,5 wt.% NiO, rims 0.5 wt.% Cr₂O₃, <0.1 wt.% NiO), and olivine replaced by talc, tremolite and chlorite and mantled by Mg- rich biotite. The matrix consists of amphibole, biotite and plagioclase (An_10-50). A striking feature of one kersantite dike is the presence of strongly zoned phlogopite phenocrysts (mg 80) rimmed by biotite (mg 60) together with biotite phenocrysts (mg 50-60). The pseudomorphs after olivine are always mantled by phlogopite, but never by biotite. Small amounts of magnesiohornblende are restricted to the matrix. In one transitional kersantite-spessartite dike, strongly zoned magnesiohastingsite to pargasite phenocrysts consist of cores with resorption features, overgrown by amphibole rims of slightly different composition.

The chondrite-normalized REE-pattern of the investigated lamprophyres have a shoshonitic character with strong LREE enrichment (150-300) and low HREE (ca. 7; La/Yb 21-57) with slight negative Ce and Eu anomalies. The MORB-normalized spidergrams show high enrichments of incompatible elements (e.g. Rb 100, Ba 120, Th 120), the negative Nb, P and Ti anomalies are also present. The highest Cr-contents of 320 ppm are present in the MgO-richest samples (7.7 wt.%).

The trace element patterns suggest mantle metasomatism prior to melting caused by a Cl-rich fluid derived from a dehydrating oceanic crust and its sedimentary cover. As experimental data show, in a Cl-containing fluid Rb, Ba, U, Sr, Pb are preferably transported, to a lesser degree also U and Th, whereas Nb, Ta, Zr and Ti remain in the residuum. Intrusion mechanism and melt differentiation are coupled. As indicated by the trace element zonation within a single dike, in addition to mineral zonation, post-metasomatic batch-melting occurred with successive melt accumulation in a deep-seated reservoir. SiO₂ and incompatible elements were enriched by fractionation at the top of the magma chamber leading to the stability of phlogopite, whereas amphibole was restricted to deeper parts of the reservoir. The resorbed and zoned amphibole, the zoned phlogopite and the kersantitic character of the dike margins are the result of a first intrusion event of the more differentiated melt. The rocks with spessartitic composition and their higher concentrations of Mg, Cr, Ni point to the influx of more primitive or less fractionated melt. Xenocrysts (orthoclase, quartz and glomerophyric plagioclase (An₃₀) point towards interaction of a lamprophyric magma with a granodioritic magma, which was not completely solidified.

Chatterjee ND, Nachr. Akad. Wiss. GoettII. Math-Phys, H 1, 24 pp, (1959).

Foley SF, Proc. Indian Acad. Sci, 99, 57-80, (1990).

Keppler H, Nature, 380, 237-240, (1996).

Rock NMS, Lamprophyres Blackie & son ltd. Glasgow, 285 pp, (1991).

Righter K, Carmichael ISE, Contrib. Mineral Petrol, 123, 1-21, (1996).

Wagner C, Velde D, Eur. J. Mineral, 5, 85-96, (1993).

E03 : 4P/11 : PO

Mantle Melting Systematics for Tertiary Sill Complexes, East Greenland Volcanic Rifted Margin

Per G. Gisselø (pgg@geo.aau.dk)¹,

Christian Tegner (christian.tegner@geo.aau.dk)¹,

Sverre Planke (sverre.planke@geologi.uio.no)² &

J. Richard Wilson (jrw@geo.aau.dk)¹

¹ Dep. of Earth Sciences, Aarhus university, Denmark

² Dep. of Geology, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway

Sorgenfri Gletscher Sill Complex (SGSC) and Traill Ø Sill Complex (TØSC) have tholeiitic compositions and are part of the Tertiary North Atlantic Basalt Province. SGSC was emplaced into Mesozoic-Palaeocene sediments in the Kangerlussuaq region (~68^oN) at ~56 Ma. TØSC is located approximately 500 km further North on Traill Ø (~73^oN) and was emplaced in Mesozoic sediments at ~54 Ma.

Rare earth element compositions of uncontaminated SGSC and TØSC magmas imply differences in mantle melting processes and/or source compositions in these two regions. SGSC shows markedly higher Dy/Yb_N (~1.5-1.7) and lower Lu/Hf_N (~0.3-0.4), than TØSC that has Dy/Yb_N ratios of ~1.3-1.5 and Lu/Hf_N ratios of ~0.4-0.6. Both of these ratios depend on the proportion of garnet in the melting source since Yb and Lu are retained in residual garnet relative to Dy and Hf respectively. It thus appears that the proportion of garnet- to spinel-lherzolite at the melting source was larger at Kangerlussuaq than at Traill Ø.

SGSC also shows lower La/Sm_N (~1.2-1.3) and higher Zr/Nb_N (~0.9-1.0) than TØSC which has La/Sm_N ratios of ~1.3-1.4 and Zr/Nb_N ratios of ~0.7-0.8. Both of these ratios, which are insensitive to fractional crystallisation, imply that the degree of melting was larger for SGSC than for TØSC assuming a common mantle source. This is supported by the TiO₂ content which is slightly lower in TØSC than in SGSC for a given Mg#.

Rare earth element compositions of the regional flood basalts suggest concurrent melting beneath thick, partially extended crust (continental rift system with high Dy/Yb_N and high TiO₂) and in a nascent oceanic rift system (low-Ti MORB-like lavas with low Dy/Yb_N ) (Tegner et al., 1998). The La/Sm_N and Dy/Yb_N ratios of SGSC show similar values (or slightly higher for Dy/Yb_N ) to the base of the flood basalts, suggesting that SGSC formed during the early phases of continental type magmatism. The Dy/Yb_N and La/Sm_N ratios of TØSC are comparable to the upper portion of the flood basalts, suggesting that the TØSC magmas were formed beneath more extensively extended crust than the SGSC magmas. We thus speculate that the sill complexes and the continental type flood basalts formed from the same melting and plumbing system, and were emplaced in and fed through the pre-volcanic sedimentary rift basins.

Tegner C, Lesher CE, Larsen, LM & Watt, WS, Nature, 395, 591-594, (1998).

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Platinum-Group Element (PGE) Geochemistry of Tertiary Flood Basalts and Intrusions, East Greenland Volcanic Rifted Margin

Peter Momme (momme@geo.aau.dk)¹,

C. Kent Brooks (kentb@geo.geol.ku.dk)²,

Reid R. Keays (rkeays@nickel.laurentian.ca)³ &

Christian Tegner (christian.tegner@geo.aau.dk)¹

¹ Geologisk Institut, Aarhus Universitet, C.F. Møllers Alle bygn 110, Denmark

² Geologisk Institut, Københavns Universitet, Øster Voldgade 10, Denmark

³ Departement of Earth Sciences, Laurentian University, Sudbury, Ont., Canada

The recognition of Au-PGE-enriched horizons in the Tertiary Kap Edvard Holm and Skaergaard intrusions of East Greenland (Bird et al., 1991; Arnason & Bird, 1994) has triggered a study of Ni-PGE-Au-Cu geochemistry of mafic and ultramafic magma types formed along the volcanic rifted margin during continental breakup above the ancestral Iceland plume ~55 Ma ago. We present 40 new PGE (Ir, Ru, Rh, Pt, Pd) and Au determinations, obtained using Ni-sulphide fire assay pre-concentration with ICP-MS finish.The samples analysed include the main magma types known in the >7 km thick flood basalt succession (high-Ti picrite-ankaramites of the lower lava series and low-, high-, and very high-Ti basalts of the more voluminous middle lava series) together with a chilled microgabbro (GGU366912) and related dykes of the Skaergaard intrusion. All samples (except one) are enriched in Pd, Au and Cu, and depleted in Ni, Ir, Ru, Rh relative to primitive mantle and thus show patterns that increase from left to right in a mantle-normalised diagram arranged with decreasing compatibility during fractionation of chromite and olivine. These patterns arise because in S-undersaturated magmas undergoing fractionation, Ni is removed in early silicate phases such as olivine, Ir, Ru and Rh are co-precipitated in both early silicate (e.g., olivine) and oxide (e.g., chromite) phases while Pd, Au and Cu accumulate along with S in the residual melt (Keays, 1995). In whole-rock data, this is reflected in high Ni concentrations and a positive Ni_N anomaly (N denotes mantle normalisation).The effect of olivine addition/removal is clearly indicated in metal ratio diagrams (e.g. Ni/Pd vs. Cu/Ir and Pd/Ir vs. Ni/Cu). The high- and very-high Ti lavas show characteristic precious- and base metal ratios of flood basalts, whereas the low-Ti group shows ratios comparable to high MgO- and mid ocean ridge basalts (MORB). Ni/Pd and Cu/Ir values of the high-Ti series are comparable to MORB while the low- and very high-Ti series are comparable to high-MgO- and flood basalts respectively. On a Pd vs Cu discriminant diagram (cf. Vogel and Keays, 1997), all of the samples plot well within the field of S-undersaturated magmas, and well away from the field of S-saturated magmas, in which MORB plot. Sulphur-undersaturated magmas are one of the critical requirements necessary to form major Ni-Cu-PGE sulphide deposits (Keays, 1995). This played a crucial role in the formation of the Platinova Reef in Skaergaard (Andersen et al., 1998) where S-saturation occurred at a late stage (~upper MZ), and PGE were pre-concentrated by efficient fractional crystallisation. The Skaergaard magma is not unique in this respect and the region as a whole must be concluded to possess high potential for PGE mineralisation, as reported from Skaergaard, Kap Edvard Holm and Kruuse Fjord intrusions (also noted by Brooks et al., 1999).

Andersen JC, Rasmussen H, Nielsen TFD & Rønsbo JG, Econ. Geol, 93, 488-509, (1998).

Arnason JG & Bird DK, Trans. Am. Geophys. Un (EOS), 75 (supplement), 710, (1994).

Bird DK, Brooks CK, Gannicott RA & Turner PA, Econ. Geol, 86, 1083-1092, (1991).

Brooks CK, Keays RR, Lambert D, Frick L & Nielsen TFD, Lithos, in press

Keays RR, Lithos, 34, 1-18, (1995).

Vogel DC & Keays RR, Chem. Geol, 136, 181-204, (1997).

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Carbonates in Magmatic Processes

Vsevolod N. Anfilogov (iminchf@ilmeny.ac.ru),

Victor K. Purtov (purtov@ilmeny.ac.ru) &

Alexey Yu. Volkov (alexw@ilmeny.ac.ru)

Institute of Mineralogy, Miass, Cheliabinsk obl, 456301, Russia

The modern tectonic model, based on the plate tectonic allow the plunging of sedimented carbonates into mantle.

The interacting of mantle substance with carbonates may be in mantle plumes and other hot points of mantle. Carbonates are able to be either the main components of magmatic processes in this situation or components which determinate the acid-base parametrs of magmatic melts and fluids. It is possible to determinate the acid base characteristic of CO₂ and other acid oxides concerning to SiO₂ by the calculation of the free energy of formation of the corresponding compounds from oxides: Na₂O+SiO₂ = Na₂SiO₃ (G_sil^oxide)Na₂O+CO₂ = Na₂CO₃ (G_car^oxide) Na₂O+H₂O = 2NaOH (G _hydrox^oxide)

The following row of the accidity of oxides is obtained: H₂O, TiO₂, SiO₂, CO₂, P₂O₅. There are only exchange between cations in 'dry' silicate-carbonate systems: 2NaAlSi₃O₈ + CaCO₃ = CaAl₂Si₂O₈ + Na₂CO₃ + 4SiO₂

The strong acid CO₂ takes out the strong base, Na₂O from the silicate to the carbonate in this reaction. The hydrolisis of carbonate and reaction of the products of hydrolisis with silicates are in the silicate-carbonate systems when H₂O is the component of these systems: CaCO₃ + H₂O = Ca(OH)₂ + CO₂

The hydrolisis of carbonates gives the alkaline mineral forming enviroment which produces the alkaline mineral associations 2NaAlSi₃O₈ + 5Ca(OH)₂ = CaAl₂Si₂O₈ + 4CaSiO₃ + 5H₂O2NaAlSi₃O₈ + 4NaOH = NaAlSiO₄ + 2Na₂Si₂O₅ + 2H₂O

Carbonatic melts are able to transport the silicate components and form kimberlites. CO₂ formed in reactions of dissociation and hydrolisis of carbonates is the main component of the fludizated streams which transport silicate substances and form diatrems.

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Numerical Solutions of the Boundary Problems of the Compaction for Investigation Geodynamical and Fluid Dynamical Processes in the Crust and Mantle

Sergey A. Karakin (karakin@glas.apc.org)

OIFZ RAN (Institute of Earth's Physics at Russian Academy of Science), Bolshaya Gruzinskaya 10, 123810, Moscow, Russia, Russia

Introduction. Results of a numerical investigation of the 1D compaction boundary value problem in the finite domains with moving boundaries are presented. The compaction equations describe a melt migration in a partially molten rock and dynamics of the water saturated soil, mud and silt. They were proposed in the works (Karakin A., 1974; M^cKenzie, 1984). The compaction model in a finite domain with the moving boundaries was proposed (Karakin A., 1990).

The compaction model. The governing equations of the 1D compaction at small porosity is given in the cited papers (using the notation from (Khodakovskii (1995), S=w-W)). There are trailing and leading fronts functions z₊(t) and z_-(t) in the finite domain of definition {z_- < z < z₊}. Solutions satisfy the boundary and initial conditions given by: f=f₀, S=0 at z=z_- and z=z₊, f(z,0)-f₀=<phi>(z) at t=0, z_- < z < z₊. On the leading front the additional condition is complemented:

(-<eta>(f) S/z - <sigma>_*) = a dz₊/dt

The numerical calculations helped to investigate the stability problem of the solitary waves at the various parameter values. Another task was to investigate a great solution in an «infinite» domain. The interval with size of 50 compaction lengths imitated an «infinite» domain. M^cKenzie (1984) obtained numerically a nonstationary solution of the compaction boundary value problem in an infinite domain. The initial disturbance had a bell-type form. Here the analogous problem was solved with the same material functions and initial porosity. But against the M^cKenzie's solution in this work there were the boundary conditions on the leading and tailing fronts of the finite initial domain.

The proposed model explains some geological phenomena more successful than usual known compaction model (McKenzie, 1984): igneous dykes, boiling sands (volcanoes) arose during earthquakes, mud volcanoes and other traces of unstable compaction movements.

Karakin AV, "Fizika lda i ledotechnika (Physics of ice and ice technique)". Yakutsk: Yakutsk branch of Siberian Department of Science Academy of USSR, 87-97, (1974).

Karakin AV, Earth Physics, 2, 3-15, (1990).

McKenzie D, J. Petrol, 25 (3), 713-765, (1984).

Khodakovskii G, Rabinovich M, Ceuleneer G, Trubitsyn VP, Earth and Planet. Sci. Lett, 134, 267-281, (1995).

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Major Element Solubilities in Aqueous Fluids in the System MgO-SiO₂-H₂O

Roland Stalder (stalder@erdw.ethz.ch),

Peter Ulmer,

Alan Bruce Thompson &

Detlef Günther

Institut für Mineralogie und Petrographie, ETH Zentrum, Sonneggstr.5, 8092 Zürich, Switzerland

Considerable amounts of silicates dissolve in fluid phases at high pressure; conversely water-saturated silicate melts become increasingly hydrous with increased pressure. In some systems these trends result in a total convergence of these two phases at a distinct pressure and temperature. This singular point in the given p-T-x-space is called the "second critical end point". At pressures exceeding this point, a volatile bearing silicate system has no solidus (sensu strictu). Although for some simple systems (e.g., albite-water, silica-water) this behaviour is understood, for more complex systems results are still controversially debated.

Here we report results from high pressure experiments in the MSH-system (MgO-SiO₂-H₂O). Experiments were carried out at 6 to 9 GPa and 900 to 1200°C in a Walker-type multi-anvil press. Starting materials were synthetic mixtures with variable Mg/Si containing 10-15 wt.% water (added as hydrous phases). Fluids or melts were trapped in a diamond layer which was added to the experimental charge as a layer separate from the silicate phases. Traps were analysed by laser ablation - ICP - MS.

Results at 6 and 9 GPa for serpentine starting-composition indicate that with increasing temperature the amount of dissolved silicate in the fluid increases more or less continuously up to 25 wt% at 1100°C (at 6 GPa) or 1050°C (at 9 GPa). A sharp increase (to 65 wt%) is observed between 1100 and 1150°C (at 6 GPa) and between 1050 and 1100°C (at 9 GPa). These steps are interpreted to reflect the solidus (melting) temperature at the given pressure - in accord with Irifune et al. (1998). The MgO/SiO₂ weight-ratios in the subsolidus fluid are around 1 at 6 GPa and around 2 at 9 GPa. The high Mg/Si in the fluid suggest, that at solidus temperatures the fluid-melt two-phase-region crosses the join serpentine-water - if at all - at still higher pressures and therefore the subsystem Fo-En-Water remains subcritical.

Irifune T, Kubo N, Isshiki M, Yamasaki Y, Geoph. Res. Lett, 25, 203-206, (1998).

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The Stages of the Melt Percolation in the Mantle beneath Vitim (TransBaikal)

I. V. Ashchepkov (garnet@uiggm.nsc.ru),

L. Andre (lucandre@africamuseum.be),

V. G. Malkovets (vomal@uiggm.nsc.ru),

Yu. D Litasov (yuril@uiggm.nsc.ru) &

A. V. Travin (travin@uiggm.nsc.ru)

Five episodes of volcanic activity and melt percolation in mantle were found in Vitim plateau according to K-Ar ages and deep seated xenoliths study.

1st stage (18-14 Ma picrite basalts tuffs explosion and associated K-silicious basalts, filling the depression). Xenoliths compile from the depth : a) intrusion zone within the submelted peridotite; b) metasomatic front and HT° zonal veins; c) percolation unit with abundant Cr-diopside veins and intercalating Sp and Gar lherzolites, d) deformed and LT° Ga-peridotites (upper diapiric unit); e) Sp peridotites fertile and depleted. 2nd stage (14-7 Ma, Mg-basanites, creation lava plateau) LT° Na-Al percolating unit with HT° harzburgites ± melt conductors, Cr-hybridic pyroxenites. 3rd stage (5±2 Ma basanites-hawaiites-alkali basalts, top of plateau) the a) LT° unit with Al-Na pyroxenes, b) MT° filtration uinit with Fe-enriched peridotites. 4th stage (hawaiites, valley flows 3-1.5 Ma) a) HT° intrusion unit, b) two layers of melt percolation and diapiric ascend with the relics of garnet, c) reduced peridotites with subsolidus veins transported from garnet facies, d) the LT° percolating unit Na-Ti enriched and the accumulation layers of the hybridic (basaltic) melts. 5th stage (0.8 Ma, limburgitic basalts, scoria cones and flows): a) HT° FeTi-lherzolites- the thransfers of basaltic melts, b) garnet peridotite percolating unit decreasing the Fe# to the Ga-Sp transition, c) MT° spinel lherzolite with the garnet relics d) LT coarse lherzolites. The layered mantle was created due coupling of the ascending diapirs formed due to he influx of volatiles and melts from the coming plum basalts. (Fig.1) The lower parts of such diapirs are the active percolation units while upper are transported mechanically.

Distribution of different varieties of mantle rocks at the temperature axes for mantle section beneath Vitim.

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Phlogopite-Bearing Dunitic Ejecta from Mount Vesuvius: Evidence for a Deep Magma Chamber in the Upper Mantle-Lower Crust Region

Corrado Cigolini (cigolini@dsmp.unito.it)

Dipartimento Scienze Mineralogiche e Petrologiche, Universita' di Torino, Via Valperga Caluso 35, TORINO, Italy

Dunitic enclaves found in pyroclastic ejecta of the 1944 eruption of Mount Vesuvius show a orthocumulate-heterogranular texture consisting of euhedral to subeuhedral olivine (Fo95-86), locally surrounded or enclosed in poikilitic greenish pyroxene of diopsidic composition, and interstitial glass. Some samples may contain abundant Mg-spinel and interstitial phlogopite, the latter coexisting with a yellowish glass. A late, acicular, seldom radiating brown-greenish clinopyroxene of salitic composition is locally growing around olivine cystals.Olivine may locally exhibit deformation bands that reflect a deep environment of formation. Moreover, olivine may show mircoinclusions of phlogopite, Cr-spinel (Cr-hercynite and chromite) and deep brownish to clear glass.Microinclusions of phlogopite are coexisting with glasses of phono-tephritic and basaltic trachy-andesitic composition (Mg# ranging 29-65). In contrast, more primitive liquids included in olivine do not coexist with mica and they have a trachybasaltic and basanitic composition (Mg# up to 68-75). Interstitial glasses are essentially phono-tephritic straddling the trachybalsalt field (Mg# 48-62).Trace element data for cumulitic dunites indicate moderate to high contents in incompatible elements (Rb/Sr range 0.19-0.24), with enrichments up to about two order of magnitude above primordial mantle values. In particular spider diagrams indicate strong positive anomalies for Ta and P, whereas some sample may exhibit strong negative anomalies in Hf. REE patterns are relatively regular with moderate enrichments in LREE, and HREE contents comparable or slightly above average chondrites: (La/Yb)_N varies between 6 and 16. Thermobarometric estimates indicate that these dunitic enclaves have crystallised within a magma chamber at temperatures ranging 1290 ± 40°C and pressures of about 12-14 (± 2.6) kbar. These pressure estimates are somehow consistent with the geophysical data that indicate the crust-mantle transition, below Mount Vesuvius, is at about 30-36 km. Analysed interstitial phlogopite coexisting with glass (including microinclusions of phlogopite+melt in cumulitic olivine) indicate crystallisation of Mg-mica at lower pressures and temperatures (1000-1150°C). It is inferred that this process, associated with decompressiom, is nearly coeval with the nucleation of a late-stage acicular pyroxene around olivine, at temperatures of about 1000-1120°C and pressures not higher than 4-5.6 kbar. It is thus suggested that cumulitic dunite may derive by in situ crystallisation of basaltic-basanitic melts within a magma reservoir located in proximity of the lower crust-upper mantle boundary layer. Following this process, cumulitic materials are eventually mobilised at depth and are transported at upper crustal levels by uprising basanitic-basaltic magmas.

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Mantle-Melt Interactions at Low Pressure Producing High Mg-Al Liquids beneath the MARK Zone (Atlantic Ocean)

Hubert Whitechurch¹ &

Fabienne Chatin²

¹ Ecole et Observatoire des Sciences de la Terre; 5, rue René Descartes; 67084- Strasbourg Cedex, France.

² Earth Sciences Department, University of Leeds, Leeds LS29JT, UK.

Evidences of magma percolation through the mantle peridotite drilled during ODP-Leg153 and subsequent chemical interactions have been clearly shown. Impregnation figures are illustrated by the presence of clinopyroxene or secondary spinel. The peridotites are regularly cut by gabbroic veinlets and their mineral composition, close to the veins, have been largely modified. The question is to know if such interactions generate chemical heterogeneities observed in the overlying mafic rocks.

The overlying cumulate bodies exhibit, beside common N-MORB gabbros, Mg-rich olivine and Mg-Cr-Al-rich (depleted in REE) clinopyroxenes bearing gabbroic sills. The mineral composition of these last plutonic rocks is close to the mineral composition of the impregnated peridotites and intrusive patches of clinopyroxenite. Calculations, using parental liquid compositions, show that these two types of gabbos cannot derive from each other by simple fractionnal or in-situ crystallization. The plot of these high Al-Mg liquids, in the Walker diagram, fall on a line which can be interpreted as the result of an olivine and diopside enrichment of the magmatic liquid. We relate these liquids to the presence of high Al-Mg basalts sampled close to the peridotite hill.

We propose to interpret their composition as the result of mantle-melt interactions leading to a second stage partial melting in the plagioclase facies. The calculation of the Al-partition coefficient between the liquid and pyroxenes, using Walter and Presnal's data (1994), show clearly a higher value in the plagioclase facies compared to the value obtained for the spinel facies. So, second stage melting during magma percolation at low pressure in the uppermost mantle would increase consistently Al and Mg contents in the liquids. These results are supported by the observation of depletion in Al in the residual pyroxenes of the peridotite without significant enrichment in iron.

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Evidence for the Presence of a Carbonated Mantle and Carbonatite Complex beneath Saghro (Anti-Atlas, Morocco)

Ibhi Abderrahmane,

Abia El Hassan,

Nachit Hassane &

Berrahma Mhamed

Geology department, Ibn Zohr University, B.P. 28/S, Agadir Morocco, Morocco

The calcite carbonatite inclusion has been found in a pyroxene nephelinite from Saghro (Anti-Atlas, Morocco). Major phases are SrO-rich (6 wt.%) calcite, SrO-rich (1,78 wt.%) fluorapatite (3,98 wt.% F), ZrO-rich (24,10 wt.%) pyrochlore (19,40 wt.% Nb₂O₃), barian titanian biotite (21wt.% BaO and 14 wt.% TiO₂), magnetite (ilmenite exsolution) and Salite. Some spinel lherzolitic xenoliths scavenged by this nephelinites, display mineralogical evidence of interaction between lithospheric mantle and carbonated fluids. The Saghro nephelinites contain numerous carbonatite inclusion and spinel lherzolitic xenoliths which could indicate the presence of an underlying carbonatite complex.