Metasomatic clinopyroxene inclusions in diamonds from the Liaoning province, China

Major and trace element compositions were determined for 6 clinopyroxene (cpx) inclusions in diamonds from a kimberlite pipe in northeastern China. The observed high mg# ratios [100*Mg/(Mg+Fe)] (92.8-94.3), strong enrichment in light rare earth elements, and the coexistent mineral assemblage suggest that the inclusions do not represent the compositions of the original mantle cpx, but rather may have formed by a metasomatic reaction between mantle peridotite and carbonatic melt prior to enclosure in diamonds. The trace element compositions of the metasomatic melts in equilibrium with the inclusions were calculated from the trace element concentrations of the inclusions using experimentally determined element-partition coefficients of cpx/melt. With the exception of Sr, the obtained concentrations are in good agreement with the trace element contents of natural erupted carbonatites. The results show that metasomatism by carbonatic melts could cause significant changes in the major and trace element composition of the deep mantle, and is highly active even at the depths corresponding to the base of the continental lithosphere (150-200 km). The carbonatic melts could also be the source of carbon for the crystallization of diamond. Considering the large variations in mineral chemistry of the cpx inclusions from different localities worldwide (e.g., mg#, K2O content), a variety of processes could be responsible for their origin. Copyright (C) 2001 Elsevier Science Ltd.