Trace Element Study of Clinopyroxenes from Garnet and Spinel Peridotite Xenoliths of the Burkal River

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Trace element chemistry of clinopyroxene in the mantle xenoliths from melanephelinites of the Burkal volcanic group has been studied. The Burkal group is composed of several local outcrops of 5-8 Ma melanephelinites within the Khentey domal uplift near the Russia/Mongolia boundary. Cr-diopside group xenoliths include garnet and spinel lherzolite, spinel harzburgite and dunite, and garnet and spinel pyroxenites. Hydrous minerals were not detected, however shallow mantle feldspatic metasomatism is present. Clinopyroxene from garnet lherzolites has high TiO_2, Al_2O_3, and Na_2O relative to clinopyroxene from spinel lherzolites. Olivine has composition of Fo_<90-92>. Spinel has Mg#=60-80 and contains 10-46 wt.% Cr_2O_3. Clinopyroxene from garnet lherzolites has REE patterns typical for fertile peridotites. Trace element patterns of clinopyroxene from depleted spinel peridotites show progressive depletion in HREE and HFSE and enrichment in LREE toward more depleted varieties of harzburgites and dunites. REE patterns of clinopyroxene in harzburgites are strongly U-shaped and have (La/Sm)n=5-36 and (Sm/Yb)n=0.4-2.1. Clinopyroxene in harzburgites has also extremely low Zr content (0.4-3.4ppm) and high Ti/Zr ratio ranged in 190-240. The patterns of clinopyroxene in depleted peridotites are indicative of significant partial melting (up to 15-20%) of the primary substrate followed by cryptic metasomatic enrichment by silicate or carbonatitic melt. Estimation of T-P parameters for garnet lherzolites reveals equilibration at 17-23 kbar (60-90km depths) and 1050-1150℃. T-estimations for harzburgites and dunites indicate, that they may form veins at 50-70km depth, whereas shallow mantle (low-T) depleted peridotites were not detected. The uppermost mantle may be composed of fertile spinel lherzolites.Article

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