CaSiO₃ and CaTiO₃ perovskite-melt partitioning of trace elements: Implications for gross mantle differentation

We report the results of CaSiO₃ perovskite-melt partitioning experiments conducted at conditions relevant to the uppermost lower mantle. Partition coefficients, measured by LA-ICPMS, show that CaSiO₃ perovskite has a strong affinity for REE, Y, Th and U, making CaSiO₃ the most likely host for these elements in the lower mantle. Comparison with the low-pressure analogue CaTiO₃ shows that large ions partition similarly into the Ca-sites of the two perovskites. Thus CaTiO₃ may be used as a model to investigate the factors controlling the partitioning of many trace ions into CaSiO₃. Combined fractionation of MgSiO₃ and CaSiO₃ perovskites during an early magma ocean has been suggested as a means of enriching the lower mantle in silica. Our results show, however, that the chondritic upper mantle ratios of refractory lithophile elements would be lost after ∼ 17% of perovskite fractionation. A large perovskitic layer 'hidden' in the lower mantle is therefore improbable.