Thermodynamics, diffusion and structure of NaAlSi2O6liquid at mantle conditions: A first-principles molecular dynamics investigation

Abstract

[1] We present results from first-principles molecular dynamics simulations of NaAlSi2O6 liquid at mantle conditions. Liquid state thermodynamics and diffusion are evaluated at temperatures of 3000–6000 K and pressures of 0–144 GPa, and interpreted in the context of the underlying liquid structure and dynamics. Consistent with previous first-principles studies of polymerized liquids, we find that a fourth order finite strain expansion is required to account for the pressure-volume equation of state. Thermodynamic properties for NaAlSi2O6 liquid derived from our results using a self-consistent thermodynamic relation agree well with experimental measurements. Over the pressure range considered, isochoric heat capacity decreases from ∼5.3 NkB to ∼4.2 NkB, while the Gruneisen parameter increases from ∼0.3 to ∼1.1. Liquid structure changes are continuous with compression, showing trends in mean coordination numbers and relative abundances of distinct coordination species consistent with first-principles studies of other silicate liquid compositions. Computed species self-diffusivities generally decrease with pressure, except that those for Si and O increase with pressure below 16 GPa at 3000 K. While self-diffusivities for Na at low pressures are almost an order of magnitude higher than other species, this contrast in species mobility disappears at above 20 GPa. Consequently, alkali-rich silicate liquids associated with low degree melting in Earth's interior are unlikely to have high electrical conductivity and will be difficult to detect by magnetotelluric sounding.