Element partitioning during core-mantle differentiation in a magma ocean: New high-pressure data on CI- and EH-chondrite model compositions

High-pressure metal-silicate partitioning experiments demonstrate that the mantle contents of some siderophile elements are consistent with equilibration between molten silicate and molten metal at high pressures and high temperatures. This conclusion is a solid argument for the `magma ocean theory'. This study is aimed at constraining the factors affecting major and trace element partitioning during core formation in a magma ocean. We report on a series of partitioning experiments between molten metal and silicate melt on CI- and EH-chondrite model compositions. Two CI-model compositions are used, one S-free and one with 2 wt% S. Oxidation state ranges from IW-2.2 in runs using CI chondrites to IW-4.6 in the runs using EH chondrite. Data were obtained by EPMA and LA-ICPMS at 3.6 and 7.7 GPa. As expected, partition coefficient (D) for S increases with increasing pressure (DS=15 at 3.6 GPa, 90 at 7.7 GPa). Fe, Si, Ti, Cr, Mn, Ga, Nb and Ta have Ds significantly higher at lower oxygen fugacity, with DSi=0.4 and DFe=200 at IW-4.6 (DSi<0.0004 and DFe=10 at IW-2.2). Cr, Mn, Nb, and Ta become compatible (D>1) in molten metal at IW-4.6. At both 3.6 and 7.7 GPa, DCr=25, DMn=1.7 and DTa=10. In contrast DNb increases with increasing pressure at IW-4.6: DNb=130 at 3.6 GPa and DNb=240 at 7.7 GPa. Similarly, at IW-4.6, DGa increases with increasing pressure: DGa=50 at 3.6 GPa and DGa=100 at 7.7 GPa. This behavior is opposite to the one observed at higher oxygen fugacity (IW-2.2) where DGa decreases with increasing pressure: DGa=5-6.5 at 3.6 GPa and DGa=1.3-1.5 at 7.7 GPa, for S-bearing and S-free CI-chondrite respectively. Also, Ti is affected by the oxidation state and pressure conditions: DTi<0.1 at IW-2.2, but at IW-4.6 DTi=0.4 at 3.6 GPa and DTi=0.25 at 7.7 GPa. Our results show that over the conditions of pressure-temperature-oxygen fugacity of this study refractory lithophile elements are highly incompatible in molten metal: DAl<0.004, DMg<0.0004, DCa<0.01, DSc<0.05, DNd<0.02, DSm<0.04, DLu<0.01, DHf<0.02, DTh<0.02 and DU<0.02. Partitioning experiments are currently underway to extend the data set to higher pressures. Efforts are also being made to provide absolute Ds rather than upper bounds for refractory lithophile trace elements. In the discussion, we will focus on the trace element composition of the core and the oxidation state during core formation in a magma ocean.