The silicate and carbonatite metasomatic alterations in lithospheric mantle under Siberian craton (the evidence of mantle xenoliths from Udachnaya pipe)

The modal metasomatic alteration for lithosphere mantle may be investigated using mantle xenoliths from kimberlite pipes. The pyroxenite xenoliths from diamondiferous upper-Devonian Udachnaya kimberlite pipe (Daldyn field, Yakutia) were studied, mineral chemical composition was researched. The several stages of metasomatic processes were observed in the lithospheric mantle under the Udachnaya kimberlite pipe.

The most garnets from studied xenoliths demonstrated so-called “normal” trace element distribution that closed to the element coefficients the basalt - silicate melt. All garnets are characterized by a maximum in Ta, a minimum in La-Ce, and a minimum in Ti. Eclogites (especially with a mosaic texture) are characterized by a small minimum in Eu, which may indicate the presence of Pl in the initial rock. The presence of eclogitic xenoliths (with Eu-minimum in garnet) indicates the influence of the subduction component. The pyroxenite xenoliths with narrow variations in the composition of minerals indicates their crystallization from melts, accompanied later by metamorphic recrystallization. The 10% studied xenoliths are observed the secondary metasomatic amphibole (pargasite - taramite) - the evidence of modal Amph metasomatism. The Ti/Eu – La/Yb distribution for clinopyroxene and amphibole demonstrate the influence of silicate (not carbonatite) melts.The increasing concentrations of HFSE and REE group elements can also be traced by the Cpx and Grt chemical composition, it may be indicating the asthenospheric melts. Several garnet samples from high-Zr (80–100 ppm) eclogites demonstrated the features of fluid metasomatism (fluid-produced, accompanied by phlogopite crystallization). Another sign of the alkaline melts influence is secondary Na silicates crystallization (according reaction Grt + Omphacite + K - Fluid = Amph + Сpx2 ± Cal ± Sodalite).

Thus, several stages of metasomatic processes were observed in the lithospheric mantle under the Udachnaya kimberlite pipe. The presence of eclogite xenoliths (with a Eu minimum) indicates the influence of a subduction component. The presence of pyroxenite xenoliths with narrow variations in mineral composition indicates their crystallization from melts - the second stage of metasomatic processes, subsequently be accompanied by metamorphic recrystallization. The content of trace elements in garnets indicates the asthenospheric nature of the melts. Such melts brought elements of the HFSE and REE groups, as well as Pt, Pd and Re. The last stage is most clearly traced using modal Amph metasomatism. The presence of secondary amphibole indicates widespread occurrence of silicate pre-kimberlite metasomatism in the lithospheric mantle beneath the center of the Siberian craton.

The research was supported by Russian Science Foundation grant № 22-77-10073.