Liquid state physics of the magnesium oxide-silicon dioxide system at deep mantle pressures.

摘要

As the primary medium through which planetary differentiation occurs, silicate liquids have a central role in the study of the thermal and chemical evolution of Earth. First principles molecular dynamics simulations were used to study the liquid state physics of the MgO-SiO2 join. We find the structure of liquids to vary continuously upon compression, and to differ markedly from that of the respective isochemical crystalline polymorphs. Liquid structure also depends strongly on composition, with a further notable difference between the structure of magnesio-silicate liquids and that of pure silica. Liquid structure is expressed in the liquid state thermodynamic properties. A density crossover along the forsterite melting curve is found within the stability field of the mineral, a feature which a melting curve computed through the Lindemann criterion from the mean squared atomic displacements in forsterite is unable to reproduce. Composition dependent structural differences within the liquid are expressed as a liquid immiscibility field at low pressure in high silica compositions. We develop a self-consistent thermodynamic description of liquid state thermodynamics relevant to silicate liquids over a large range of pressures and temperatures. To constrain the description, we use simulation results for liquid MgO, MgSiO3, Mg2SiO4 and SiO2, including the thermal electronic contribution to the free energy. With liquid state thermodynamics constrained self-consistently, we investigate the high pressure melting of MgO periclase and MgSiO3 perovskite, with special focus on the changes in density and sound velocity which would be expected during shock melting of periclase and enstatite. We further apply the thermodynamic description to the thermodynamics of mixing along the full extent of the binary. At low pressure the enthalpy of mixing is notably pressure dependent, primarily due to the disappearance of a maximum at high silica compositions with pressure. The assumption of pressure independence in the enthalpy of mixing, with the implication of ideal mixing of liquid volumes, commonly applied in experimental thermodynamic studies is thus found not to hold at low pressures for liquids in the MgO-SiO 2 system. Liquid immiscibility, and its disappearance with pressure, is found to result from significant differences between SiO2 and intermediate composition liquids in liquid structure and its response to compression.