Molecular dynamics simulation of structures, bulk moduli, and volume thermal expansivities of silicate liquids in the system CaO-MgO-Al2O3-SiO2

Materials in the system CaO-MgO-Al2O3-SiO2 are important constituents of the Earth's lower crust and mantle. Silicate liquids in this geophysically important system have been studied using molecular dynamics(MD) simulation with an empirical interatomic potential(CMAS94). MD simulations are quite satisfactory in reproducing well the observed structure and pressure-volume-temperature equation-of-state parameters of molten enstatite(MgSiO3), wollastonite (CaSiO3), diopside (CaMgSi2O6), and anorthite (CaAl2Si2O8) al 1900 K and 0 GPa. However, the MD simulated bulk modulus of molten forsterite(Mg2SiO4) at 2300 K and 0 GPa, K-o=18.0(6) GPa, is found to be much smaller than the value, K-o=similar to 60 GPa at similar temperature and pressure conditions, estimated previously based on melting curve analyses of forsterite. In an attempt to investigate the possible occurrence of the density inversion between magmatic liquids and residual crystals in the upper mantle conditions, as proposed by Stolper et al. [1981],we have further applied the MD technique with the CMAS94 potential to the diopside system at 1900 K as an expamle, and have found that the density inversion between crystal and liquid in this system actually occurs at approximately 11 GPa.