Sheared and Granular Peridotites from the Udachnaya–East Kimberlite (Yakutia): Mineralogy, Chemistry, and PGE Patterns

Abstract

Abstract —Mantle xenoliths of fresh sheared and granular peridotites from the Udachnaya kimberlite pipe (Yakutian kimberlite province, Siberia, Russia) are studied in terms of mineralogy, chemistry, and behavior of platinum-group elements (PGE), with a focus on difference between the two types of peridotites and on the role of PGEs in their origin. The analyzed samples of sheared and granular peridotites differ in mineral-liquid equilibrium temperatures and pressures: 1230–1350 °C, 56–67 kbar inferred for the former and 750–1300 °C, 30–67 kbar for the latter. The contents of major oxides, trace elements, and PGEs differ both between sheared and granular peridotites and within each group. Some sheared and all granular peridotites are enriched in incompatible elements, have low percentages of clinopyroxene and contain garnet rich in middle rare-earth elements (MREE). These features result from interaction of the peridotites with melts and related fluids of presumably carbonate compositions containing incompatible elements in high abundances. Sheared peridotites were more strongly enriched and gained more Ca, Al, and Ti than the granular variety. On the other hand, granular peridotites have higher light REE (LREE) and lower heavy REE (HREE) enrichment, which may record compositional difference of metasomatic agents. Metasomatism acted in two stages: first silicate agents caused LREE enrichment and HREE depletion of melts and then the fractionated melts percolated through the mantle and affected the compositions of granular peridotites and, partly, sheared peridotites. The two types of peridotites differ also in PGE patterns, with largely variable Os–Pt concentrations in granular peridotite. This difference may be due to uneven distribution of the Os–Ir and Pt–Os–Ir intermetallic compounds, which formed as sulfide melting residue in the lithospheric mantle. Sheared peridotites show almost uniform Os–Pt distribution and are enriched in Pd, Re, and partly Pt. The enrichment possibly occurred during metasomatism when the percentages of garnet and clinopyroxene increased while sulfide phases precipitated into interstitials.