The Geochemical features of the garnets from peridotites of Udachnaya pipe (Yakutia)

Study of mantle xenoliths from the kimberlite pipes allows to get an important information about the origin and transformations of the lithosphere substance. The most deep-seated rocks of the lithospheric mantle are deformed peridotites, having the features of recrystallization and structures of the flow. These rocks undergo significant transformations and enrichment with different components under the effect of asthenospheric melts penetrating by the cracks and faults. The most depleted rocks which are not affected by the enrichment processes are supposed to be the megacrystalline dunites. The garnets from 5 deformed lherzolites (mineral composition: olivine, orthopyroxene, clinopyroxene, garnet), and 20 megacrystalline harzburgite-dunites (mineral composition: olivine, garnet, sometimes chromite, orthopyroxene) from the kimberlite pipe Udachnaya (Yakutia) were analyzed by the LAM-ICPMS. The chondrite normalized REE patterns of garnets from deformed lherzolites are mostly characterized by smooth increasing of LREE values with gradation in the field of HREE. The distribution curves of REE in garnets of megacrystalline dunites have highly sinusoidal character. The very big difference in REE values is seen in the width of dunite field. It shows the different degree and time of their enrichment. The most of distribution curves for garnets of megacrystalline dunites have the peaks at the LREE and MREE showing increased Ce (7 samples), Pr (4 samples), Nd (7 samples), Sm (2 samples) concentrations proving early enrichment. The sharp sinusoidality of REE distribution curves, early enrichment with the LREE prove that some garnets of studied dunites were not in the equilibrium with the melt. In general these garnets could be formed as a result of transformation of earlier chromites, which is confirmed by the increased Cr-concentration in them (for example, garnet UV167/09 contains 11.84 wt.% Cr₂O₃), either as a result of exsolution of primary Al, Cr-containing orthopyroxene, presenting in garzburgites.1 page(s