The multistage metasomatized mantle beneath Alakit: evidence from mantle xenoliths from Komsomolskaya kimberlite pipe, Yakutia, stages of mantle evolution

More than 200 metasomatised peridotite xenoliths containing phlogopite, amphibole and ilmenite from the Komsomolskaya pipe are garnet and spinel harzburgites or dunites, and clinopyroxene-enriched lherzolites with garnets (up to 12.5 wt.% Cr₂O₃) and clinopyroxenes (up to 5 wt.% Na₂O). Low-Cr varieties are Fe-enriched pyroxenites, phlogopite metasomatic veins and type A, B eclogites. Minerals were studied by electron microprobe and LA-ICP-MS which revealed their geochemical groupings and their distribution in the mantle section.

     Results indicate that the lithospheric mantle beneath the Komsomolskaya pipe is layered and relatively heated. Heated peridotites at the lithosphere base (7-6 GPa) are enriched in Fe and are porphyroclastic, deformed types and rare polymict breccias. The cold group at 6.0-5.5 GPa (34 mW/m²) are depleted peridotites with sub-Ca garnets. Cpx-fertilized varieties belong to the middle part of the mantle section. Amphiboles range from Cr-hornblendes to edenites (2-6 GPa), showing K-Ti enrichment. Picroilmenites yield two pressure intervals from 6.5 to 5.0 GPa and from 5.0 to 4.0 GPa, forming two differentiation branches. Eclogites are mainly related to the lower part of the section with a peak at pressures of 4-6 GPa.

Trace elements of melts that formed harburgitic garnets-pyroxenes rever to oceanic MORB like melt interaction with peridotites. The subcalcic S-type garnets are similar to subduction-related melts (S-type REE) with troughs in HFSE. Adakite-like hybrid metasomatism formed Na, Al-rich pyroxenes with peaks in Sr and HFSE. K-bearing pyroxenes and amphiboles refer to shoshonitic metasomatism.

Trace elements for Cpx of re-fertilized mantle peridotites have high LREE, Nb-Ta troughs and peaks in Zr, Th, Sr, U and are related to carbonatite –alkaline melts. Protokimberlite (essentially carbonatitic) interaction produced HFSE-enrichment. Type B eclogites show more subduction-related features with HFSE troughs while type A eclogites are closer to hybrid and peridotitic signatures. We suggest six types of major metasomatic agents.  The ⁴⁰Ar/³⁹Ar ages of phlogopites are in the 440-690 Ma range, with some at 1.6 Ga, suggesting multistage metasomatism.  Supported by  RFBR grant 19-05-00788