First-principles study of density, viscosity, and diffusion coefficients of liquid MgSiO3at conditions of the Earth's deep mantle

摘要

Constant-pressure constant-temperature {it ab initio} molecular dynamicssimulations at high temperatures have been used to study MgSiO$_3$ liquid, themajor constituent of the Earth's lower mantle to conditions of the Earth'score-mantle boundary (CMB). We have performed variable-cell {it ab initio}molecular dynamic simulations at relevant thermodynamic conditions across oneof the measured melting curves. The calculated equilibrium volumes anddensities are compared with the simulations using an orthorhombic perovskiteconfiguration under the same conditions. For molten MgSiO$_3$, we havedetermined the diffusion coefficients and shear viscosities at differentthermodynamic conditions. Our results provide new constraints on the propertiesof molten MgSiO$_3$ at conditions near the core-mantle boundary. The volumechange on fusion is positive throughout the pressure-temperature conditionsexamined and ranges from 5% at 88 GPa and 3500 K to 2.9% at 120 GPa and 5000 K.Nevertheless, neutral or negatively buoyant melts from (Mg,Fe)SiO$_3$perovskite compositions at deep lower mantle conditions are consistent withexisting experimental constraints on solid-liquid partition coefficients forFe. Our simulations indicate that MgSiO$_3$ is liquid at 120 GPa and 4500 K,consistent with the lower range of experimental melting curves for thismaterial. Linear extrapolation of our results indicates that the densities ofliquid and solid perovskite MgSiO$_3$ will become equal near 180 GPa.