High pressure equilibrium of nickel and cobalt between metal and mantle minerals

The differentiation of a Fe-rich metallic core from the mantle silicates has been one of the most important events in the early history of Earth. The abundances of siderophile elements (i.e., mostly incorporated into the metal phase) remaining in the mantle after the formation of the core provide strong constraints on the differentiation mechanism. During these last years, it has been demonstrated that the absolute and relative abundances of these elements in the Earth's upper mantle cannot be explained by metal-silicate exchange processes based on low pressure (1 bar) and moderate temperature (1000-2000 C) partition coefficients. This paper reports analytical transmission electron microscopy (ATEM) results on metal/silicate or oxide assemblages obtained from diamond anvil cell (DAC) experiments at 40, 50, and 70 GPa, using Ni- and Co-bearing synthetic olivines as starting materials. Those silicates are reduced at high pressure and high temperature, explaining the coexistence of metal and silicates in the run products. These experiments, therefore, provide new data on partitioning of Ni and Co between metal and silicates or oxides at very high pressures. The ATEM results show that Ni and Co remain siderophile at least up to 70 GPa, but that their siderophile character decreases with pressure, as already observed in previous studies at lower pressures. The results also suggest that the abundances of Ni and Co observed in the Earth's upper mantle cannot be explained by very high pressure equilibrium between silicate perovskite, magnesiowuestite, and metal. (AIAA)