The α, β, γ phase relations in Fe2SiO4‐Mg2SiO4 and Co2SiO4‐Mg2SiO4: Calculation from thermochemical data and geophysical applications

Abstract

On the basis of thermochemical data for olivine (α), modified spinel (β), and spinel (γ) polymorphs of Mg2SiO4, Fe2SiO4, and Co2SiO4, the binary Mg2SiO4‐Fe2SiO4 and Mg2SiO4‐Co2SiO4 phase diagrams are calculated. From Mg/Fe partitioning data between olivine‐garnet, β‐phase‐garnet, olivine‐pyroxene, and β‐phase‐pyroxene, thermochemical parameters for β‐Fe2SiO4 are estimated. The β‐Fe2SiO4 is highly metastable with respect to γ‐Fe2SiO4. The calculated binary Mg2SiO4‐Fe2SiO4 phase diagram shows a narrow field of α+β and a broad field of β+γ, in reasonable agreement with experimental phase relations. The composition range over which β phase solid solution is stable expands with increasing temperature but is generally limited to Mg/(Fe + Mg) > 0.8. In the Mg2SiO4‐Co2SiO4 system, the β phase is calculated to occur only at Mg/(Co + Mg) < 0.05 and Mg/(Co + Mg) > 0.6, with α and γ coexisting at intermediate compositions. Using the above phase relations in Mg2SiO4‐Fe2SiO4 and assuming an adiabatic geotherm, temperatures are calculated to range from about 1573 to 1700 K at depths of 400–520 km for a mantle of Iherzolite composition.