Partitioning of Re and Os between liquid metal and magnesiowüstite at high pressure

In order to study the partitioning of Re and Os between liquid iron-rich alloy and magnesiowüstite at high pressure, multi-anvil experiments have been performed on samples of Fe–Ni–Os–Re–O (4–8 wt.% Os and 4–12 wt.% Re) metal contained in MgO single crystal capsules. The range of pressure–temperature conditions was 5–10 GPa and 1900–2200 °C with experimental run durations of 6–30 min. During the experiments, the MgO reacted with the liquid metal to form magnesiowüstite. Compositions of the quenched liquid metal and the FeO, MgO and NiO contents of magnesiowüstite were determined by electron microprobe. Re and Os concentrations in magnesiowüstite were determined by LA–ICP–MS using a Re–Os-doped silicate glass standard. Based on the experimental results and assuming a valence of +2 for both Re and Os in magnesiowüstite, liquid metal–magnesiowüstite distribution coefficients (KDmet/mw) are 60–240 for Re and 1.3×104 to 3.1×104 for Os. Within the uncertainties, there is no observable effect of either temperature or pressure on the partitioning of Re and Os over the range of experimental conditions. However, the values are very low compare to metal–silicate KDmet/mw values determined at 1 bar and 1400 °C (3×109 for Re and 7×106 for Os [Geochim. Cosmochim. Acta 65 (2001) 2161; Am. Mineral. 85 (2000) 912]). KDmet/mw values, assuming core–mantle equilibrium, are estimated to be ∼68 for both elements. Thus, although mantle concentrations of Re may be explained by core–mantle equilibration at high pressure and temperature, the experimentally determined distribution coefficients for Os are several orders of magnitude too high. Our results are therefore consistent with the “late veneer” hypothesis as an explanation for the mantle concentrations of highly siderophile elements. However, a consequence of the late veneer would be domains in the deep mantle with suprachondritic Re/Os ratios.