An improved thermodynamic model of metal-olivine-pyroxene stability domains

Interactions between several silicate and metallic phases are studied by ap plying a self consistent thermodynamic approach and using recent thermodyna mic data. We compute proportions and compositions of oxidized silicates and of reduced metallic phase in equilibrium at various temperatures and oxyge n fugacities. The empirically observed activity-composition relationships f or ternary metallic alloys are used and their applications to a general the rmodynamic expression for a non-regular ternary system is explicitly discus sed. We show that the stability limits of olivines and pyroxenes with respe ct to precipitation of metallic phases under reducing conditions are direct ly related to the presence of nickel impurities. We precisely evaluate the modifications of the stability limits asa function of nickel content. For t ypical mantle olivines[Fe/(Fe + Mg) = 0.1] the stability limits are given f or values of x(Ni) = Ni/(Ni+Fe+Mg) ranging from 10 ppm to 1% by: In f(O2) = -39.83+7.86 ln x(Ni), In f(O2) = -14.68 + 6.21 ln x(Ni), at 900 K and 1600 K, respectively.