Metasomatic parageneses in deep-seated xenoliths from pipes Udachnaya and Komsomol'skaya-Magnitnaya as indicators of fluid transfer through the mantle lithosphere of the Siberian craton

Abstract

The petrography, major element, and trace element (TE) compositions of minerals from two types of modal metasomatites (metasomatized peridotites and pyroxenites) from kimberlite pipes Udachnaya and Komsomol'skaya-Magnitnaya, Yakutia, have been studied. It is shown that texturally and chemically equilibrated metasomatites A consist of a set of superimposed minerals: phlogopite + diopside ± ilmenite ± apatite ± sulfides ± graphite. Their major and trace element compositions have specific features. The contents of TEs in garnet and clinopyroxene from these metasomatites are close to those in garnet and clinopyroxene from low-temperature coarse-grained peridotites richest in TEs. The distribution of a significant portion of TEs between garnet and clinopyroxene from A-type metasomatites and from coarse-grained lherzolites rich in TEs is close to experimental values reported for minerals coexisting with carbonatitic and basaltic fluids. We assume that this metasomatic process was nearly synchronous with the global metamorphism and cratonization of the mantle lithosphere and that high-density silicate–carbonate fluidmelts were metasomatizing agents.Another large mantle metasomatism process in the lithosphere of the Siberian craton was associated with the Middle Paleozoic kimberlite magmatic event, induced by the Yakutian thermochemical plume. Metasomatic minerals (Mg phlogopite + Cr diopside + chromite ± sulfides ± graphite) intensely replaced the minerals of the primary paragenesis, particularly, garnet. These reaction metasomatites show a sine-shaped REE pattern in garnet and disequilibrium between garnet and clinopyroxene. It is supposed that the reaction metasomatism in the mantle lithosphere of the Siberian craton was associated with ingress of reduced asthenospheric fluids at early stages of the kimberlite formation cycle. Metasomatic graphite formed in metasomatites of both types, and this fact evidences for two diamond formation epochs in the history of the mantle lithosphere of the Siberian craton.