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

A method is proposed for calculating freezing point depressions in sil icate systems of geologic interest using molecular dynamics simulation s. The method directly determines the slope of the liquidus at a given composition and temperature. The construction of a phase diagram is t hen possible using a fitting procedure, or by inference of a set of li quid 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 experimental ly 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-Si1/2O. This result, together wit h the melting temperatures of periclase and of MgSiO3 perovskite extra polated from high-pressure diamond anvil cell experiments, allows the construction of a eutectic phase diagram in the system MgO-MgSiO3. Thr oughout the lower mantle the eutectic temperature is 280-560 K below t he melting temperature of MgSiO3 perovskite. Taking account of other o xide components in the mantle drops the eutectic by another 530-730 K and gives tighter upper bounds for the Earth's solidus and geotherm. C opyright (C) 1997 Elsevier Science Ltd.