Modeling the Melting of MgSiO3-perovskite and MgO-MgSiO3 under Lower Mantle Conditions

Abstract:

The melting temperatures of lower mantle minerals under high-pressure conditions are of importance for understanding the evolution of the interiors of the Earth and other planets, and to constrain whether deep melts exist in the Earth now. The melting temperatures of the Mg-end-member perovskite and periclase phases are very high under deepest lower mantle conditions and so experimental data are difficult to obtain and somewhat uncertain. We have, therefore, investigated the melting behaviour of MgSiO₃-perovskite and MgO-MgSiO₃ at pressures of 25 and 120 GPa via molecular dynamics simulations. All the simulations are performed adopting a liquid-solid (2-phase) model of about 750-900 atoms and using a combination of ab-initio and empirical pair-potential approaches. As expected, we find that the time required for the MgSiO₃ system to melt increases substantially near the melting temperature. This requires, therefore, very long simulations times (hundreds of picoseconds) to melt the system near its melting temperature. These times are too long for ab initio simulations, but we are able to use the empirical potentials to correct the ab initio results to the true melting temperature. We find that the melting temperature of MgSiO₃ at 25GPa is in very good agreement with previous experimental data (~2800 K). At 120 GPa the melting T is about 5500 K, again in agreement with previous shock-wave results. We also calculate the melting temperature depression at an approximately eutectic composition (MgO-MgSiO₃). We find that the melting temperature decreases by about 100 K at 25 GPa, and 500 K at 120 GPa.