Uranium partitioning between liquid iron and silicate melt at high pressures: implications for uranium solubility in planetary cores

Abstract

We have investigated the partitioning of U between silicate melt and Fe liquid at pressures of 3.0 to14.5 GPa and temperatures of 1660 to 2500 oC. The solubility of U in liquid Fe is in the range of 0.6 to 800 ppm and increases with temperature (T) and pressure (P). When P = or > 7 GPa and T > Tmelt of the silicate phase (olivine), the U concentration in Fe is 3 to 5 times greater than for run products where T < Tmelt of the silicate phase. Correspondingly, partitioning coefficient DU values can reach 0.031 at 8.5 GPa (using BN sample container) and 0.036 at 14.5 GPa (using graphite sample container). This implies that if a terrestrial-type planetary core with a pure Fe composition formed from a deep magma ocean (T > Tmelt of the silicate phase), then > 2.4 ppb U could have entered the core. Alternatively, if a core with same composition formed by percolation (T < Tmelt of the silicate phase), then based on the experimental results indicating DU increases with increasing P, 1 to 4 ppb U may have entered the core. The concentration of Si in liquid Fe also increases with pressure. The concentration of Ca is < 0.44 wt % for most samples and no relation with U concentration is found, which indicates that U may alloy with Fe directly. If Si concentration in the Fe phase can be used as an indicator of oxygen fugacity, then the increase in Si and U with pressure suggests a pressure dependent decrease in oxygen fugacity. This supports U (and possibly also Si) inclusion in the the core of Earth at the time of core formation. The implications for radioactive heating in the planetary cores are briefly discussed. Keywords: Uranium; partition coefficients; high pressure; dynamos; planetary cores; heat sources, LA-ICP-MS.