PARTITIONING OF RARE EARTH ELEMENTS BETWEEN CASIO3 PEROVSKITE AND COEXISTING PHASES: CONSTRAINTS ON THE FORMATION OF CASIO3 INCLUSIONS IN DIAMONDS

Abstract

Minerals with CaSiO3 composition were found as inclusions in diamonds, and are considered to be originally of perovskite structure. To constrain their genesis and consequently the extent of circulation of mantle material, rare earth element (REE) partitioning between CaSiO3 perovskite and coexisting majoritic garnet (20 GPa, 1520°C) or MgSiO3 perovskite (25 GPa, 1600°C) was determined by combining the technologies of high-pressure synthesis and trace-element analysis using ion probe. It is consistent with previous experiments and confirms that CaSiO3 perovskite is the main REE depository, especially of the light-REE. KCe(CaPv/Gt) is ∼1900, and decreases gradually to 18.5 of KYb(CaPv/Gt). For CaPv/MgPv, it decreases gradually from 57.7 of KCe to about 10 of KYb. Estimated REE concentrations in the source lithology of the CaSiO3 inclusions according to these partitioning coefficients, either peridotitic or eclogitic paragenesis, show strong enrichment in light-REE (e.g. Cen 18–163), very different from normal mantle peridotite and subducted oceanic crust. It is proposed that interaction with carbonatic melt in the deep mantle may have played an important role in the formation of these CaSiO3 inclusions in diamonds, as well as in their ascending transportation.