Pyropes from China: Peridotite Xenoliths from Kimberlites versus Megacrysts in Basalts

The properties of garnets in peridotite xenoliths from kimberlites in China are compared with garnets occurring as megacrysts (disaggregated peridotites) in alkali basalts. The pyropes in xenoliths from kimberlites in the Shandong and Liaoning provinces are distinctly purple, whereas the megacryst pyropes in basalts are considerably darker. Kelyphitic reaction rims are common on pyropes in peridotite xenoliths from kimberlites, but are rare on pyropes in basalts. The classification of pyropes according to the scheme of Dawson and Stephens (1975) demonstrates that pyropes in peridotite xenoliths from kimberlites are mainly Group-9 garnets, whereas those in basalts typically belong to Group 3 (calcic pyrope-almandine) and Group 1 (titanian pyrope). In China, pyropes in peridotite xenoliths of kimberlites, as compared with those in basalts, are richer in Cr2O3, Cr/(Cr + Al), knorringite, uvarovite, and Cr-component, but lower in Fe2O3 + FeO, Fe/(Fe + Mg), grossularite, and almandine molecules. Unlike the situation in basalts, there is a distinct negative correlation between Cr2O3 and Al2O3 contents of the pyropes in peridotite xenoliths from kimberlites. The formation pressures of pyropes in the peridotite xenoliths mainly plot within the diamond stability field (i.e., more than 4GPa). These garnets may be useful as an indicator mineral for diamond exploration.