Simultaneous partitioning of silicon and oxygen into the Earth's core during early Earth differentiation

Abstract

AbstractSilicon and oxygen are potential light elements in the Earth's core and may be involved in metal‐silicate reactions at the present day core‐mantle boundary. We have performed multianvil experiments at 25 GPa and 2770–3080 K to understand the simultaneous partitioning of these elements between liquid iron–rich metal and silicate melt. The presence of O in liquid Fe at high temperatures influences the partitioning of Si, causing more Si to partition into the metal than would be expected based on lower temperature measurements. Although Si and O are mutually exclusive in Fe metal at <3000 K, the level at which both element concentrations are similar in the liquid metal rises above 1 wt % at >3000 K. We have developed a thermodynamic model based on these experiments that accounts for the interaction between O and Si in the liquid metal. Comparison between this model and the previous results of diamond‐anvil cell experiments up to 71 GPa indicates very little pressure dependence but a strong temperature dependence for O and Si partitioning. Our model predicts that subequal concentrations of Si and O, sufficient to explain the outer core density deficit, would have partitioned into core‐forming metal if equilibration occurred between the metal and a magma ocean with a bulk silicate Earth composition at an average depth of ~1200 km (~50 GPa and ~3300 K). An O‐ and Si‐enriched buoyant layer may have developed at the top of the outer core as a result of subsequent equilibration with the overlying mantle.