Postspinel transformations in the system Mg₂SiO₄‐Fe₂SiO₄ and some geophysical implications

Abstract

The high‐pressure transformation in MgSiO 3 and those in the spinel phases of compositions from Mg 2 SiO 4 to (Mg 0.5 Fe 0.5 ) 2 SiO 4 in the Mg 2 SiO 4 ‐Fe 2 SiO 4 system were investigated using a uniaxial split‐sphere apparatus. The phase boundaries between ilmenite‐perovskite in MgSiO 3 and between Mg 2 SiO 4 spinel and the assemblage of MgSiO 3 perovskite and MgO periclase were determined to be P (GPa) = 26.8–0.0025 T (°C) and P (GPa) = 27.6–0.0028 T (°C), respectively, in the temperature range 1000–1600°C. The pseudobinary diagrams for the system Mg 2 SiO 4 ‐Fe 2 SiO 4 were determined at temperatures of 1100°C and 1600°C. It was demonstrated that the magnesian spinel (with Fe/Mg + Fe < 0.22 at 1100°C and <0.26 at 1600°C) dissociates into perovskite and magnesiowüstite within an extremely narrow pressure interval (<0.15 GPa at 1600°C). The dissociation pressure was found to be almost independent of iron content and to coincide to that at 670 km depth within experimental uncertainties. These experimental results indicate that the sharpness of the 670‐km discontinuity may indeed be due to this dissociation in a peridotitic or pyrolitic mantle. The current status of our understanding of deep mantle mineralogy and chemistry is discussed based on recent high‐pressure and high‐temperature experiments.