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; the shape of the grains is often complicated by hooked and elongated formations. Garnets from the V. Grib pipe have a more rounded shape and range in size from 1 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 10 or more times lower than in the composition of chondrite. 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. While metasomatized garnets show an increase in Y and Zr contents from the cneter to the edge, in depleted garnets the peripheral regions of garnet grains are depleted in Y, Zr, Ti and LREE, in contrast to their central parts. These features of the chemical composition and morphology of garnets indicate their restite origin as a result of partial melting. For the first time, it has been shown that restitic lherzolites have been preserved in the upper horizons of the lithospheric mantle, the garnets in which have not been subjected to metasomatic changes.

pyrope, lithospheric mantle, kimberlite pipe, lherzolite, REE, mantle metasomatism