Structure and Density of H2O-Rich Mg2SiO4 Melts at High Pressure From Ab Initio Simulations

Abstract

Water has a strong effect on silicate melt properties, yet its dissolution mechanism in depolymerized melts, typical for mantle composition, remains poorly understood. Here we report results of first-principles molecular dynamics simulations for hydrous Mg2SiO4 melts with 6, 16, and 27 wt% H2O at pressure and temperature conditions relevant to the upper mantle and mantle transition zone. The results show that hydrogen bonds not only to the network-forming cation Si but also to Mg which—nevertheless—remains the most important network modifier. There is no evidence to support the hypothesis based on experimental data that water may cause an increase in melt polymerization for ultramafic magmas; the ratio of non-bridging oxygen per Si increases with the addition of the oxygen from H2O. The partial molar volume of water is independent on concentration in our simulations which allows us to examine the density of hydrous melt systematically. The critical water content—at which melts are neutrally buoyant compared to the surrounding mantle—is ~4 wt% H2O for a pyrolite composition, much lower than the high water content (>10 wt%) observed in petrological experiments and estimated thermodynamically for low-degree partial melts formed in the vicinity of the mantle transition zone.