Partitioning of nickel and cobalt between silicate perovskite and metal at pressures 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 have a much lower abundance of both Ni and Co and a considerably lower Ni/Co ratio owing to the preferential partitioning of these elements into the iron core. Two models have been put forward to explain these discrepancies: homogeneous accretion, (involving changes of the Ni and Co partition coefficients with oxygen and sulphur fugacities, pressure and temperature) and heterogeneous accretion, (the addition of chondritic meteorites to the mantle after core formation was almost complete). Here we report diamond-cell experiments on the partitioning of Ni and Co between the main lower-mantle mineral ((Mg,Fe)SiO3-perovskite) and an iron-rich metal alloy at pressures up to 80 GPa (corresponding to a depth of ∼1,900 km). Our results 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 observed in samples derived from the Earth's mantle appear to indeed be consistent with a homogeneous accretion model.