Stability of MgSiO₃ Perovskite in the Lower Mantle

As it is expected to be the dominant phase in the lower mantle, any pressureinduced phase changes in MgSiO₃ could require significant modifications in current models of the dynamics and structures of Earth's deep mantle. Studies to date have yielded discrepant results regarding the high-pressure stability of MgSiO₃ perovskite. Understanding the source of discrepancy is essential, both to resolving the stability of perovskite and to developing more reliable techniques for understanding the Earth's deep interior. In this report, we give an overview of previous studies on the stability of MgSiO₃ perovskite and recent observations on the post-perovskite transition. We also summarize our measurements on MgSiO₃ perovskite to core-mantle boundary pressure-temperature (P-T) conditions using in-situ X-ray diffraction. Major peaks in our diffraction patterns are best explained by those of MgSiO₃ perovskite at 1200-2500-km depth conditions. No evidence of dissociation to MgO + SiO₂ and a transition to a cubic perovskite structure has been found to core-mantle boundary P-T conditions. We have also observed a new peak at 2.62-2.57 Å at 88-145 GPa, the existence of which may be relevant to a modification in perovskite crystal structure. However, the possibility that this peak may be from a chemical reaction among gasket, anvil materials, and sample cannot be ruled out. More significant changes are observed during heating above 2500 K at 135-145 GPa: appearance of new peaks, splitting of a peak, and intensity changes of some diffraction peaks. The recently proposed post-perovskite phase explains dominant new diffraction features. Based on the results available as of May 2005, the post-perovskite transition appears to be relevant to the D" seismic discontinuity. Furthermore, depth of the post-perovskite transition may be very sensitive to variations in chemical composition as well as temperature.