The electrical conductivity of silicate liquids at extreme conditions

Abstract:

Could the Earth have had a silicate dynamo early in its history? One requirement is that the electrical conductivity of silicate liquids be sufficiently high. However, very little is known about this property at the extreme conditions of pressure and temperature that prevailed in the magma ocean. We have computed from first principles molecular dynamics simulations the dc conductivity of liquid Silica SiO2 at pressure and temperature conditions spanning those of the magma ocean and super-Earth interiors. We find semi-metallic values of the conductivity at conditions typical of the putative basal magma ocean in the Early Earth. The variation of the conductivity with pressure and temperature displays interesting behavior that we rationalize on the basis of the closing the pseudo-gap at the Fermi level. For temperatures lower than T < 20,000 K electrical conductivity exhibits a maximum at intermediate compressions. We further explain this behavior in terms of stuctural changes that occur in silica liquid at high pressure; we find that the structure approaches that of the iso-electronic rare earth element Ne. We compare with Hugoniot data, including the equation of state, heat capacity, and reflectivity. The behavior of the heat capacity is different to that inferred from multiple Hugoniot experiments. These differences and the effect of including exact exchange on the calculations are discussed. Our results have important consequences for magnetic field generation in the early Earth and super-Earths.