High-Pressure and High-Temperature Reactions between Lower Mantle Minerals and Metals in the Fe-Si-O System

Valérie Malavergne (malaverg@univ-mlv.fr)¹, Laurent Gautron (gautron@univ-mlv.fr)¹, Rossana Combes (combes@univ-mlv.fr)¹, François Guyot (guyot@lmcp.jussieu.fr)², Isabelle Martinez (martinez@ipgp.jussieu.fr)³ & Jean-Paul Poirier (poirier@ipgp.jussieu.fr)⁴

¹ Laboratoire des Géomatériaux, Université de Marne-la-Vallée, Cité Descartes, 5 boulevard Descartes, Champs-sur-Marne, 77454 Marne-la-Vallée Cedex 2, France

² Laboratoire de Minéralogie-Cristallographie et IPGP, Université Paris 7, Tour 16, case 115, 4 place Jussieu 75252 Paris Cedex 05, France

³ Laboratoire de Géochimie des isotopes stables, Institut de Physique du Globe de Paris, 4, Place Jussieu, 75252 Paris Cedex 05, France

⁴ Département des Géomatériaux, Institut de Physique du Globe de Paris, 4, place Jussieu, 75252 Paris Cedex 05, France

Metal-silicate at high pressure and high temperature are studied in order to understand the core-mantle boundary, and to elucidate the nature of the seismically anomalous D'' layer. Moreover, knowing high-pressure and high-temperature partitioning of elements between metal and silicates could constrain the nature of the light elements in the core and then contribute to the understanding of the segregation of the Earth's core.

Results from some previous studies (Knittle and Jeanloz, 1991; Ringwood and Hibberson, 1991; Ito et al., 1995; Goarant et al., 1992) suggest that silicon and oxygen could be light elements of the core. More recently, Gessmann and Rubie (1998) have observed that the solubility of silicon in liquid metal at 9 GPa seems to preclude this element as a major light component in the Earth's core.

In order to have a better understanding of these phenomena, we have performed new high-pressure and high-temperature experiments at (P,T) conditions of the lower mantle, in a laser-heated diamond anvil cell and in a multianvil apparatus, on mixtures of silicates and (Fe,Si) alloys. Quenched samples were observed by Scanning Electron Microscopy and Electron Microprobe.

Similarly to a preliminary study (Poirier et al., 1998), we observed chemical reactions between molten (Fe,Si) alloys and minerals leading to a dissolution of oxygen in the molten metallic phase. In some experiments, we also observed that silicates in contact with (Fe,Si) alloys are enriched in FeO at ambient or high pressure. We made some thermodynamical calculations to try to explain the reactions we observed, and we discussed the influence of pressure, temperature and oxygen fugacity on these reactions between silicates and metals.

Gessmann CK & Rubie DC, Geochimica Cosmochimica Acta, 62, 867-882, (1998).

Goarant F, Guyot F, Peyronneau J & Poirier J-P, Journal of Geophysical Research, 97, 4477-4487, (1992).

Ito E, Morooka K, Ujike O & Katsura T, Journal of Geophysical Research, 100, B4, 5901-5910, (1995).

Knittle E & Jeanloz R, Geophysical Research Letters, 16, 609-612, (1991).

Poirier J-P, Malavergne V & Le Mouël J-L, American Geophysical Union, The Core-Mantle Boundary Region, 131, (1998).

Ringwood AE & Hibberson WO, Earth & Planet Science Letters, 102, 235-251, (1991).

Al,Si Ordering and Cation Substitutions in Ferromagnesian Cordierites

Thomas Malcherek (thomas@crystal.unipv.it)¹, M. Chiara Domeneghetti¹, Vittorio Tazzoli¹, Luisa Ottolini¹, Catherine McCammon² & Michael A. Carpenter³

¹ CNR-CSCC, c/o Dipartimento di Scienze della Terra, Via Ferrata 1, 27100 Pavia, Italy

² Bayrisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany

³ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, England

A set of natural and heat treated cordierite crystals has been analyzed using single crystal X-ray diffraction, EMPA, SIMS and ⁵⁷ Mössbauer-spectroscopy.

Natural cordierites generally deviate considerably from their ideal stoichiometry, (Mg_x,Fe_2-x)[Al₄Si₅O₁₈], as is manifested by the presence of various stable and volatile channel constituents (Na⁺,H₂O).

The results of Moessbauer spectroscopy show that up to 11% of Fe²⁺ can be attributed to tetrahedral coordination in Mg-rich cordierite. As the radius of tetrahedrally coordinated Mg is similar to that of ^[4]Fe it is argued that minor amounts of Mg also can enter the tetrahedral framework on the T1 sites. This is supported by the observation of compositions with Mg>2 and by systematic changes in the size of the T1-tetrahedra. Charge balance for the substitution of divalent cations for the smaller Al³⁺ can be provided by introduction of Na⁺ into vacant Ch2(0,0,0) channel sites or by additional substitution of Si for Al (Schreyer 1985). Towards Fe-rich compositions the substitution scheme Na + Al = Si dominates. An Al/Si ratio of 41/49 is observed in samples from the Dolni Bory locality. The relatively large size of the T21 and T23 ring-tetrahedra confirms the presence of excess Al in these samples, indicating a reduced degree of Al/Si-order.

Based on these observations, tetrahedral site occupancies and corresponding Al,Si order parameters are calculated in a simple hard sphere approximation. Refined cation-oxygen mean bond lengths, the chemical composition and Fe-site occupancy refinements are used to constrain the site occupancies.

The Al,Si order parameter correlates with the orthorhombic shear strain (Putnis et al, 1987) if the effects of composition are taken into account (Selkregg and Bloss, 1980). Two models for the coupling between compositional variation and shear strain are discussed and the corresponding calibration functions for the order parameter as a function of composition and strain are derived.

Schreyer W, Bull. Minéral., 108, 273-291, (1985).

Putnis A, Salje E, Redfern S, Fyfe C & Strobl H, Phys. Chem. Minerals, 14, 446-454, (1987).

Selkregg KR & Bloss FD, Am. Mineralogist, 65, 522-533, (1980).