Constant-pressure constant-temperature ab initio molecular dynamics simulations at high temperatures have been used to study MgSiO3, the major constituent of the Earth's lower mantle to conditions of the Earth's core-mantle boundary. The calculated equilibrium volumes and densities are compared with simulations using an orthorhombic perovskite configuration under the same conditions. For molten MgSiO3, we have determined the diffusion coefficients and shear viscosities at different thermodynamic conditions. Our results provide new constraints on the properties of molten MgSiO3 at conditions near the core-mantle boundary. The volume of the liquid is greater than that of the solid throughout the pressure-temperature conditions examined, and the volume change on fusion ranges from 5% at 88 GPa and 3500 K to 2.9% at 120 GPa and 5000 K. Existing experimental constraints on solid-liquid partition coefficients for Fe suggest that Fe is preferentially partitioned into the liquid. Such enrichment of Fe increases the density of the liquid, thus allowing the possibility of negatively buoyant melts from (Mg,Fe)SiO3 perovskite compositions at deep lower mantle conditions for plausible values of solid-liquid partition coefficients for Fe. At 120 GPa and 4500-5000 K, the diffusion coefficient of liquid MgSiO3 is 2-3 × 10-5 cm 2/s and the diffusion rates of the different chemical species are similar. The shear viscosity is estimated using Zwanzig's formula to be 19-31 cP under these conditions. On the basis of our calculated diffusivities, MgSiO3 is above the glass transition temperature at 120 GPa and 4500 K.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science