AN IMPROVED THERMODYNAMIC MODEL OF METAL-OLIVINE-PYROXENE STABILITY DOMAINS

Abstract

Interactions between several silicate and metallic phases are studied by applying a self consistent thermodynamic approach and using recent thermodynamic data. We compute proportions and compositions of oxidized silicates and of reduced metallic phase in equilibrium at various temperatures and oxygen fugacities. The empirically observed activity-composition relationships for ternary metallic alloys are used and their applications to a general thermodynamic expression for a 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 reducing conditions are directly related to the presence of nickel impurities. We precisely evaluate the modifications 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 xNi= Ni/(Ni+Fe+Mg) ranging from 10 ppm to 1% by: ln fO2=-39.83+ 7.86 ln xNi, ln fO2 =-14.68+6.21 ln xNi, at 900 K and 1600 K, respectively.