Atomic scale view on partially molten rocks: Molecular dynamics simulations of melt-wetted olivine grain boundaries

Abstract

[1] Partial melting is an important geological process that affects physical, chemical and rheological properties of rocks. Here, atomic scale simulations are used to study the structure and transport properties of ultrathin melt films between olivine grains, which is a simple model system of a partially molten peridotite. The model systems consist of 0.8 to 7.0 nanometer thick layers of magnesium silicate melt with a composition close to MgSiO3 confined between Mg2SiO4 forsterite crystals. We examine how the atomic structure, the chemistry and the self-diffusion coefficients vary across the interface and investigate their dependence on the thickness of the melt layer and the crystal orientation. Interfacial layers of up to 2 nm thickness show distinctly different physical behavior than the bulk melt and the bulk mineral. The simulation results indicate that for crystal orientations with higher surface energy, the self-diffusion coefficients of all ionic species in the melt decrease. A solid-like charge ordering in the melt is observed close to the crystal-melt interface, which leads to a decrease in mobility of all species. For modeling the petrophysical behavior of partially molten rocks, the effective diameter for the conducting channels is reduced by up to two nanometers, which may effect the rheological and transport properties of the partially molten rocks, especially in the presence of ultrathin melt films in well wetted systems. In the latter case, the electrical conductivity of the confined melt in a partially molten rock could be reduced up to a factor of two due to interfacial effects.