Post-spinel transformations in the system Mg2SiO4-Fe2SiO4:: Element partitioning, calorimetry, and thermodynamic calculation

Compositions of coexisting spinel and magnesiowustite in the system Mg2SiO4-Fe2SiO4 have been experimentally determined at 18.5 and 20.4 GPa at 1873 K to constrain equilibrium boundaries of the postspinel transitions in relatively Fe2SiO4-rich composition. Calorimetric measurements of pyroxene and perovskite solid solutions in the system Mg2SiO3-Fe2SiO3 have been performed by a differential drop-solution method in a controlled atmosphere. Using the above data with published thermodynamic data on the high-pressure phases in the system MgO-FeO-SiO2, phase relations of the postspinel transitions have been thermodynamically calculated. The calculated boundaries are generally consistent with the experiments by Ito and Takahashi. The calculated transition interval of spinel to perovskite + magnesiowustite is about 0.01-0.18 GPa for mantel spinel composition. The perovskite-magnesiowustite field expands to the Fe2SiO4-rich side with increasing temperature, in contrast to previous thermodynamic calculations. The present thermodynamic data show stability of an assemblage of magnesiowustite-stishovite in the Mg2SiO4-rich composition of the Mg2SiO4-Fe2SiO4 system between the fields of spinel and of perovskite + magnesiowustite at relatively low temperatures.