Partitioning of nickel and cobalt between silicate perovskite and metal atpressures up to 80 GPa

The high abundance of both nickel and cobalt and the chondritic Ni/Co ratio found in samples derived from the Earth's mantle are at odds with results from laboratory-based partitioning experiments conducted at pressures up to 27 GPa (refs 1,2), The laboratory results predict that the mantle should h ave a much lower abundance of both Ni and Co and a considerably lower Nil C o ratio owing to the preferential partitioning of these elements into the i ron core. Two models have been put forward to explain these discrepancies: homogeneous accretion(3-6) (involving changes of the Ni and Co partition co efficients with oxygen and sulphur fugacities, pressure and temperature) an d heterogeneous accretion(7-9) (the addition of chondritic meteorites to th e mantle after core formation was almost complete), Here we report diamond- cell experiments on the partitioning of Ni and Co between the main lower-ma ntle mineral ((Mg,Fe)SiO3-perovskite) and an iron-rich metal alloy at press ures up to 80 GPa (corresponding to a depth of similar to 1,900 km), Our re sults show that both elements become much less siderophilic with increasing pressure, such that the abundance of both Ni and Co and the Ni/Co ratio ob served in samples derived from the Earth's mantle appear to indeed be consi stent with a homogeneous accretion model.