First-Principles Simulations of Structure, Density, and Diffusivity of Silicate and Oxide Melts: Implications for Deep Mantle Magma

Abstract

Structural, thermodynamic, and transport properties of several geologically important silicate liquids have been investigated at pressures relevant to deep Earth via first-principles molecular dynamics (FPMD) simulations. Results from CaO, CaSiO3, model basalt (eutectic mixture of 36 wt. % anorthite and 64 wt. % diopside), hydrous model basalt (model basalt with 5 wt. % water), and mid-oceanic ridge basalts (with 9.9 wt. % FeO and 2.4 wt. % Na2O) are presented. This work is a milestone in the density functional theory based calculations of compositionally complex systems including molten iron bearing silicates and a major step to understanding the natural magma. Simulation results show that liquid structure changes considerably on compression with all of the mean cation-oxygen coordination numbers increasing nearly linearly with compression. Most coordination changes occur at pressures below 30 GPa, which are accountable for rapid initial increase of melt density on compression. Melt compositions studied here along with other silicate melts remarkably show the same pressure evolution for Si-O coordination increasing from 4- to 6-fold over the entire mantle regime, but considerably differing in O-Si coordination. This change implies that the coordination polyhedra serve as building blocks of all silicate melts but how they act together to control the melt behavior might vary among different melts. CaO and CaSiO3 liquids are much more compressible than their solid counterparts indicating the possibility of liquid-solid density crossovers at high pressure. In basaltic melts, the magnitudes of density changes due to the Fe and H2O components are such that the melts including hydrous melt may be buoyantly stable at one or more depths in the mantle. Calculated self-diffusion and viscosity coefficients of the basaltic melts at zero pressure closely follow an Arrhenian law with activation energies ranging from 79 to 158 kJ/mol. However, the pressure variations of these coefficients requires a non-Arrhenian representation with variable activation volume. Predicted differences in viscosity of all basaltic melts with other silicate melts are subtle at shallow depths (up to ~20 GPa), suggesting the viscosity of major magma-forming silicate melts might not change much over that regime. Studies of hydrous model basalt reveals that the speciation of the H2O component consists of mostly hydroxyls and molecular water at lower pressure, which change to more complex extended forms at higher pressure. Our calculation also shows that the volume of mixing in a melt-water system is nearly zero for most of the mantle pressure regime with negative