Restitic origin of garnets and lherzolites containing them from the Mir and V. Grib kimberlite pipes

The paper presents the results of studying the chemical composition of "depleted" garnets from lherzolite xenoliths of the highly diamondiferous Mir and V. Grib kimberlite pipes. Garnets from the Mir pipe xenoliths are distinguished by their rather large sizes, up to 15 mm, and their morphology is complicated by hooked and elongated shapes. Garnets from the V. Grib pipe are isometric and have sizes of up to 4 mm. Based on the content of CaO and Cr2O3, the garnets belong to the lherzolite paragenesis. Garnets from the studied xenoliths have low contents of incompatible elements and have fractionated REE distributions with a sharp decrease from heavy to light. The LREE concentrations are less than 0.1 chondrite unit. Variations in Y and Zr contents form a trend from "depleted" peridotites to "metasomatized" lherzolites, which can be observed both within a single xenolith and within a single grain. Peripheral areas of garnet grains are depleted in Y, Zr, Ti and LREE, in contrast to the central parts, showing the reverse zoning observed in metasomatized garnets. The features of the chemical composition and morphology of Cr-pyropes are a consequence of partial melting of peridotites at the last stage of their formation. Thus, it is shown for the first time that restitic lherzolites have been preserved in the upper horizons of the lithospheric mantle, the garnets in which were not subjected to metasomatic changes.