Effects of Fe-enrichment on the equation of state and stability of (Mg,Fe)SiO3 perovskite

Fe-rich natural orthopyroxenes with compositions of (Mg0.61Fe0.38Ca0.01)SiO3 (Fe#38) and (Mg0.25Fe0.74Ca0.01)SiO3 (Fe#74) were studied at pressures up to 155 GPa and temperatures up to 3000 K. Single-phase orthorhombic GdFeO3-type perovskite was synthesized by heating to ∼ 2000 K at 63 GPa for the Fe#38 composition and at 72 GPa for the Fe#74 composition. At lower pressures, heating both compositions resulted in a mixture of perovskite, SiO2 and (Mg,Fe)O. These measurements provide new constraints on the dependence of (Mg,Fe)SiO3 perovskite stability on pressure and composition. Upon further compression and heating at 89 and 99 GPa, Fe#38 and Fe#74 perovskites transformed to two-phase mixtures of perovskite and post-perovskite, consistent with previous findings that increasing Fe content lowers the transition pressure. The volume of (Mg,Fe)SiO3 perovskites increases linearly with Fe-content. Volume data were fit to the Birch–Murnaghan equation of state. The bulk modulus at 80 GPa is 550–560 GPa for both Fe-rich perovskites, comparable to or slightly higher than the values measured for MgSiO3 perovskite at this pressure, indicating that the bulk modulus of perovskite is not strongly sensitive to Fe content.