Simulation of the pre-melting behaviour of MgSiO₃ perovskite at high pressures and temperatures

Abstract

MAGNESIUM-rich silicate perovskite is thought to be the dominant mineral phase in the Earth's lower mantle. The behaviour of MgSiO₃ perovskite at high temperatures and pressures is therefore important for a wide range of geophysical problems, including the chemical and thermal evolution of the Earth, mantle convection, the thermal gradient in the mantle and the secular variations of the Earth's magnetic field. Experimental investigations at lower-mantle conditions are, however, difficult. We have performed computer simulations of MgSiO₃ perovskite under typical lower-mantle pressures and temperatures using the constant-temperature and constant-pressure molecular dynamics (MD) method. At pressures above 10 GPa, our simulations suggest that orthorhombic MgSiO₃ perovskite undergoes a temperature-induced phase transformation to a cubic (or pseudo-cubic) phase before melting, and that the cubic phase is a solid electrolyte. The MD method tends to overestimate the temperature of melting and related phenomena, but should provide a reliable qualitative description of the ionic-conductivity behaviour. Quantitative determination of the structure and ionic conductivity of MgSiO₃ perovskite at lower-mantle conditions must, however, await improvements in both experimental and simulation techniques.