A fundamental event in Earth's history was its early differentiation into a silicate mantle and dense metallic core. Whether there has been significant chemical exchange between the core and mantle subsequently is a long-standing question that has broad implications for geochemistry and geophysics. It is possible that complex seismic structures observed at the base of the mantle, and possibly also near the top of the core, relate to chemical reactions at the core-mantle boundary. Geochemical signatures observed in ocean island basalts, based primarily on osmium isotopes, also have been interpreted to reflect material contributions from the outer core. This study will address the importance of chemical exchange between the core and mantle through two different lines of experimental investigation, one aimed toward resolving whether the outer core is a plausible source of the radiogenic osmium isotope signature in ocean island basalts, and the other at constraining the kinetics of chemical exchange across the core-mantle boundary.
The hypothesis that the outer core is a source of radiogenic osmium is based on the idea that Pt/Os and Re/Os ratios in the outer core are raised significantly by crystallization of the inner core. The proposed study will expand on previous experimental work that has cast doubt on this notion, because the differences in solid-metal/liquid-metal partition coefficients among Os, Re and Pt are too small and decrease with pressure. The new study will investigate partitioning of these elements in the Fe-FeO and Ni-NiO systems at high pressures, to determine whether oxygen-bearing systems may be capable of producing the appropriate partition coefficients. Experimental studies of reactions at the core-mantle boundary have focused primarily on equilibrium phase relations and element partitioning. The kinetics of the reactions are equally important for evaluating their relevance to the Earth. It is planned to pursue experiments to determine the mechanisms and rates of reactions between lower-mantle analog materials and liquid metals, and to determine the diffusion rates of transition metals through periclase. The proposed study will further test the results of recent experimental work by this group that indicates that the crystal field effect has a strong influence on the diffusion rates of transition metals with partially filled d orbitals, and it is a central goal of the project to investigate this effect in detail.