Properties of Silicate Melts at High Pressure and Temperature from Ab Initio Calculations

Abstract:

The evolution of planetary interiors is intrinsically connected to the behavior and properties of silicate melts at high pressures and temperatures. Our work comes as a complement to existing data expanding the pressure, temperature, and compositional ranges.

We used the V.A.S.P. code to perform NVT Molecular Dynamics simulations on two basic compositions: Mg₂SiO₄ and MgSiO₃. All calculations are done within augmented planar wave formalism of the Density Functional Theory. Supercells of 160 atoms clino-enstatite and 112 atoms forsterite were melted at 5000K and then cooled and thermalized, using the Nose-Hoover thermostat, at temperatures more representative of Earth’s interior (3000 and 4000K). The pressure range of our investigations spans from 0 to approximately 160GPa. Since important properties, density and magnetism, are dependent on the presence of iron we also created (Fe_x-1,Mg_x)SiO₃ and (Fe_x-1,Mg_x)₂SiO₄melts from the thermalized pure compositions by replacing the desired amount of magnesium atoms with iron. Because other transitional elements present similar behavior as iron, and nickel is an important element in the core, compositions containing different amounts of nickel were also created by adding extra Ni atoms in the system.

We analyze in detail the behavior with pressure of the density, clustering and coordination, total magnetization, and thermodynamical parameters of the melts.

Our results indicate that changes in the structure and magnetic moment of the Forsterite melt begin at relatively low pressure. As an application of our data to the Earth’s present deep interior we analyzed in great detail various possible mixtures of Fe bearing melt and solid mantle in an attempt to fit the density estimated for the Ultra Low Velocity Zones.