MOLECULAR-DYNAMICS SIMULATION OF PHASE-TRANSITIONS AND MELTING IN MGSIO3 WITH THE PEROVSKITE STRUCTURE

The high pressure phase transitions and melting of the mantle mineral MgSiO3 with the perovskite structure were investigated using molecular dynamics (MD) simulations of a large system of atoms on a parallel co mputer. The simulations reveal an orthorhombic to cubic transition acc ompanied by a sharp increase in diffusion of the O atoms. The phase tr ansition and melting temperature depend sensitively on the level of de fects in the solid. At pressures of the Earth's lower mantle, the tran sition is found to occur at temperatures substantially higher than the mantle temperatures. Therefore, any seismic discontinuity in the lowe r mantle may not be related to a phase transition of the perovskite st ructure, but instead may be due to chemical changes at that depth, in contrast to currently accepted mineralogical models assuming chemical homogeneity of the lower mantle.