Partitioning of Ni and Co between liquid metal and ferropericlase at high pressures

A partitioning behavior of siderophile elements between liquid metal and lower mantle minerals are important to understand the processes occurred in the early Earth. A partitioning behavior between liquid metal and silicate melt or between liquid metal and mantle minerals has been studied to date. However, most of the previous studies were conducted at pressures below 30 GPa, which is lower than the proposed pressure, and those have experimentally ascertained that the partition coefficient varies with pressure of a core-mantle equilibrium in a deep magma ocean (30-60 GPa, >2000 K). Although Bouhifd & Jephcoat (2003) performed partitioning experiments for Ni and Co between liquid metal and silicate melt up to 42 GPa and 2500 K, their results have large uncertainties because of very low contents of Ni and Co in the silicate phase. Ferropericlase which is a major lower mantle phase is considered to be a reservoir of Ni and Co. In this study, we performed partitioning experiments of Ni and Co between liquid metal and ferropericlase at 36 and 40 GPa at 2700-2800 K in order to examine pressure dependence of partition coefficients of Ni and Co. The experiments were performed using a double sided laser heated diamond anvil cell. The starting material was a Fe54Ni29Co17 alloy foils (in wt %) sandwiched between (Mg0.9, Fe0.1)O ferropericlase powder. .The sample was insulated from the diamond surfaces by corundum disks. Textural observation and chemical analysis of the recovered samples were performed by a field emission-SEM EDS. Our results revealed that the exchange partition coefficients of Ni and Fe (KDNi- Fe), and Co and Fe (KDCo-Fe), between liquid metal and ferropericlase are 3.0 (7) and 2.3 (5) at 36 GPa and 2800 K and 4.0 (13) and 2.8 (10) at 40 GPa and 2700 K, respectively. These results revealed that there is no remarkable decrease of the exchange partition coefficients of Ni and Co at pressures between 36 GPa and 40 GPa.