Olivine‐modified spinel‐spinel transitions in the system Mg₂SiO₄‐Fe₂SiO₄: Calorimetric measurements, thermochemical calculation, and geophysical application

Abstract

The olivine(α)‐modified spinel(β)‐spinel (γ) transitions in the system Mg 2 SiO 4 ‐Fe 2 SiO 4 were studied by high‐temperature solution calorimetry. Enthalpies of the β‐γ and a α‐γ transitions in Mg 2 SiO 4 at 975 K and of the α‐γ transition in Fe 2 SiO 4 at 298 K were measured. The γ solid solution showed a positive enthalpy of mixing. Phase relations at high pressures and high temperatures were calculated from these thermochemical data including correction for the effect of nonideality of α, β, and γ solid solutions. The calculated phase diagrams agree well with those determined experimentally by Katsura and Ito very recently. The α − (Mg 0.89 , Fe 0.11 ) 2 SiO 4 transforms to β through a region of α+β without passing through the α+γ phase field at around 400 km depth in the mantle with an interval of about 18(±5) km. Temperatures at 390 and 650 km depths are estimated to be about 1673 and 1873 K, respectively, assuming an adiabatic geotherm.