Residual Origin of Garnets and Their Host Lherzolites from the Mir and V. Grib Kimberlite Pipes

The results of study of the chemical composition of “depleted” garnets from xenoliths of lherzolites from the Mir and V. Grib highly diamondiferous kimberlite pipes are presented. The garnets from the xenoliths of the Mir pipe are characterized by a relatively large size (3–15 mm) and a hook-shaped or elongated grain morphology. The garnets from the V. Grib pipe are rounder, and their size ranges from 1 to 4 mm. Based on the content of CaO and Cr2O3, the garnets belong to the lherzolite paragenesis. The garnets of the studied xenoliths have low contents of incompatible elements and are characterized by fractionated rare earth element (REE) patterns with a strong decrease from heavy to light REEs (LREEs). The LREE contents are ten or more times lower in comparison with chondrite. The Y and Zr contents form a trend from “depleted” peridotites to “metasomatized” lherzolites, which can be observed in both within a single xenolith and within a single grain. In contrast to metasomatized garnets with an increasing from core to rim Y and Zr content, the peripheral parts of the depleted garnets are depleted in Y, Zr, Ti, and LREEs in comparison with the central parts. These peculiarities of the chemical composition and morphology of garnets indicate their restite origin as a result of partial melting. It is shown for the first time that the upper horizons of the lithospheric mantle preserve restite lherzolites, the garnets in which were unaffected by metasomatic processes.