Metal-silicate Partitioning of Uranium and Thorium up to 138 GPa and 5500 K and Implications for Stratified Layer at the Top of the Outer Core

Abstract:

The excess abundances of siderophile elements in the mantle can be explained by metal-silicate equilibrium at mid-mantle depths in magma ocean of the growing Earth. The final equilibrium pressure and temperature would reach 37-60 GPa and melting temperature of the mantle at this pressure (Wade et al. 2012; Siebert et al., 2013). Much severe conditions (>6000 K) have been supposed at the final stage of the Earth’s formation immediate aftermath the moon-forming giant impact (e.g. Canup, 2004), evoking the additional chemical equilibrium between core materials of the giant impactor and the surrounding silicate materials.

Previous studies on partitioning of U up to 20 GPa and 2700 K by multi-anvil press have shown very small partition coefficients (D ~10^-5) in S-poor system with oxygen fugacity at around IW-1.5 (Wheeler et al., 2006; Bouhifd et al., 2013). Such a very small D make it difficult to examine the partitioning at higher P-T using laser-heated diamond anvil cell (LH-DAC) and electron microprobe since the small size of each phase introduce artificial error by such as secondary fluorescent effect (Wade and Wood, 2012). 1% contamination from surrounding silicate may increase D by three orders (i.e. D =10^-2), artificially. One solution is to use laser ablation ICP-MS by carefully ablating only a metallic portion.

Here, we introduced FIB to isolate the metallic phase from the surrounding silicate melt by slicing off surrounding silicate potion. Consequently, we have successfully obtained the metal-silicate partitioning data of U and Th up to 138 GPa and 5500 K in S-free/S-poor system using LH-DAC. The results show a large temperature dependence of partition coefficient of uranium and thorium, approaching to 0.1~1 at temperature near 5500 K. The pressure dependence was not observed clearly. The large temperature dependence suggests that only the core material of the giant impactor can be enriched in U and Th, which may stratify at the top of the liquid core.