CONSTRAINTS FROM MOLECULAR DYNAMICS ON THE LIQUIDUS AND SOLIDUS OF THE LOWER MANTLE

Abstract

A method is proposed for calculating freezing point depressions in silicate systems of geologic interest using molecular dynamics simulations. The method directly determines the slope of the liquidus at a given composition and temperature. The construction of a phase diagram is then possible using a fitting procedure, or by inference of a set of liquid phase components whose mixing behavior approximates thermodynamic ideality. Application of this method to the system Mg2SiO4MgSiO3 at low pressures gives results in qualitative agreement with experimentally determined phase diagrams. At 100 GPa this method gives depression curves compatible with curves calculated assuming ideal mixing of the liquid phase species MgOSiO2 or MgOSi12O. This result, together with the melting temperatures of periclase and of MgSiO3 perovskite extrapolated from high-pressure diamond anvil cell experiments, allows the construction of a eutectic phase diagram in the system MgOMgSiO3. Throughout the lower mantle the eutectic temperature is 280–560 K below the melting temperature of MgSiO3 perovskite. Taking account of other oxide components in the mantle drops the eutectic by another 530–730 K and gives tighter upper bounds for the Earth's solidus and geotherm.