Thermodynamics, diffusion and structure of NaAlSi2O6 liquid at mantle conditions: A first-principles molecular dynamics investigation

Abstract

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 Grneisen 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. Copyright 2011 by the American Geophysical Union.