Properties of Hydrous Aluminosilicate Melts at High Pressures

In this study, we use first-principles molecular dynamics simulations to explore the behavior of anhydrous aluminosilicate melt with a stoichiometry of NaAlSi2O6 up to pressures of ～30 GPa and temperatures between 2500 and 4000 K. We also examine the effect of water (～4 wt % H2O) on the equation of state and transport properties of the aluminosilicate melt and relate them to atomistic scale changes in the melt structure. Our results show that water reduces the density and bulk modulus of the anhydrous melt. However, the pressure derivative of the bulk modulus of the hydrous melt is larger than that of the anhydrous melt. The pressure dependence of the transport property exhibits an anomalous behavior. At a pressure of ～12 GPa, anhydrous aluminosilicate melts exhibit maxima in diffusion and minima in viscosity. Dissolved water in melts also affects both diffusion and viscosity. In hydrous aluminosilicate melts, the maxima in diffusion and the minima in viscosity occur at ～14 GPa. The anomalous behavior of transport properties is related to the pressure-induced changes in the melt structure. At shallower depths, i.e., up to 100 km, relevant for subduction zone settings, the lower density compounded by the lower viscosity of hydrous aluminosilicate melts is likely to provide buoyancy for upward migration. At greater depths of ～180–200 km, greater compressibility of the hydrous aluminosilicate melts together with the minimum viscosity could hinder magma migration and may explain the presence of a partial melt layer at the lithosphere–asthenosphere boundary