J. Matas  Y. Ricard  L. Lemelle  F. Guyot An improved thermodynamic model of metal-olivine-pyroxene stability domains Received: 17 November 1999 /Accepted: 14 May 2000 Abstract Interactions between several silicate and metallic phases are studied by applying a self consistent thermodynamic approach and using recent thermody- namic data. We compute proportions and compositions of oxidized silicates and of reduced metallic phase in equilibrium at various temperatures and oxygen fugaci- ties. The empirically observed activity-composition rela- tionships for ternary metallic alloys are used and their applicationstoageneralthermodynamicexpressionfora non-regular ternary system is explicitly discussed. We show that the stability limits of olivines and pyroxenes with respect to precipitation of metallic phases under re- ducing conditions are directly related to the presence of nickel impurities. We precisely evaluate the modi®cations of the stability limits as a function of nickel content. For typical mantle olivines [Fe/Fe+Mg) = 0.1] the stability limits are given for values of x Ni  Ni/Ni+Fe+Mg) ranging from 10 ppm to 1% by: ln f O 2 39:83 7:86ln x Ni ; ln f O 2 14:68  6:21ln x Ni , at 900 K and 1600 K, respectively. Introduction Olivine is the major mineral of the Earth's upper mantle; a precise knowledge of its stability ®eld is therefore important, especially for understanding laboratory measurements performed on this mineral. This stability domain not only depends on pressure and temperature but also on oxygen fugacity, f O 2 , e.g., Nitsan 1974). At low enough oxygen fugacities, metallic iron is extracted from forsterite±fayalite solid solutions as predicted Nitsan 1974) and experimentally observed Boland and Duba 1981, 1986; Lemelle et al. 1999). In addition to its potential importance for core formation in the Earth, this phenomenon might explain some characteristics of olivine-metal interactions in meteorites e.g., dusty oli- vines). The observation of Elligham diagrams suggests that the presence of minor components can signi®cantly modify the olivine stability ®eld with respect to oxygen fugacity. The purpose of this paper is to present a computation method for calculating the stability limit of natural olivines with respect to oxygen fugacity, taking into account the presence of minor components. We compute the stability limit under reducing conditions of mantle olivines modeled as a solid solution of forsterite Mg 2 SiO 4 ) and fayalite Fe 2 SiO 4 ) containing nickel as a major impurity. Similar calculations were performed by Arculus et al. 1990); in this study, we extend their ap- proach using a free-energy minimization procedure with updated thermodynamic data. In particular, this method allows us to take into account non-ideal activity-com- position relationships for minor elements such as silicon in the metallic phase. The solubility of silicon in metal can become important at very low oxygen fugacities and this phenomenon can have important implications for enstatite chondrites and possibly for the early Earth. Calculation procedure All computations are performed within the temperature range 700 K±2100 K at 1 bar total pressure. Under these temperature and pressure conditions, precise thermodynamic data can be used. The involved phases are olivine Mg 2 SiO 4 , Fe 2 SiO 4 , Ni 2 SiO 4 endmembers), orthopyroxene MgSiO 3 , FeSiO 3 , NiSiO 3 ), a, b quartz and cristobalite SiO 2 ), metallic Fe, Ni, Si) with bcc and Contrib Mineral Petrol 2000) 140: 73±83 Ó Springer-Verlag 2000 J. Matas &) 1  Y. Ricard E Â cole normale supeÂrieure de Lyon, UMR/CNRS F-5570, Lyon, France e-mail: jmatas@ens-lyon.fr; Fax: 1-510-643-9980 L. Lemelle 2  F. Guyot Laboratoire de MineÂralogie-Cristallographie, Institut de Physique du Globe, Paris, France Present addresses: 1 Department of Geology and Geophysics, University of California, Berkeley, Calif., USA 2 E Â cole normale supeÂrieure de Lyon, UMR/CNRS F-5570, Lyon, France Editorial responsibility: T. L. Grove