Olivine-modified spinel-spinel transitions in the system Mg₂ SiO₄ -Fe₂SiO₄: calorimetric measurements, thermochemical calculation, and geophysical application

The olivine(α)-modified spinel(β)-spinel (γ) transitions in the system Mg₂SiO₄-Fe₂SiO₄ were studied by high-temperature solution calorimetry. Enthalpies of the β-γ and α-γ transitions in Mg₂SiO₄ at 975 K and of the α-γ transition in Fe₂SiO₄ 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)₂SiO₄ 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. -Authors