Pressure-induced coordination changes of transition-metal ions in silicate melts

Abstract

THE strong partitioning of cobalt and nickel from silicate melts into solid silicate and metal phases^1–3 during the large-scale fractionation processes that took place early in Earth history^4–9 determines their abundance in mantle-derived magmas and in the metallic core. The global distributions of these elements have therefore been used to constrain models of the Earth's chemical evolution^4–8, as well as that of the Moon^10,11. But in virtually all model calculations^4,6,7,10,11 the effect of pressure on partitioning has been neglected — these models have used partition coefficients determined at very low pressures. Here we present crystal-field spectra of doped silicate glasses quenched from high-pressure, high-temperature melts, which show that the coordination of cobalt and nickel ions in silicate melts changes at high pressures. Our results suggest that the partitioning of these species into the melt phase increases markedly with pressure, such that the crystal/melt and metal/melt partition coefficients decrease by more than an order of magnitude at lower-mantle conditions.