The article presents the results of a comprehensive study of diamondiferous lherzolite from the V.Grib kimberlite pipe. The composition of rock-forming minerals (olivine, orthopyroxene, Cr-diopside, Cr-pyrope) in terms of major elements mainly corresponds to minerals from inclusions in diamonds of the lherzolite association and diamondiferous lherzolites of the world. The elevated modal amount of orthopyroxene (18 vol.%) as well as the concentration of FeO (7.5 wt.%) and the value of MgO/SiO2 ratio (0.89) for lherzolite allow assigning it to orthopyroxene-enriched lherzolites. Specific features of the composition of Cr-diopside and Cr-pyrope in respect of rare elements indicate that at the time of capture by kimberlite, lherzolite retained the signs of a slight impact of mantle metasomatism. Modelling results allowed suggesting magnesiocarbonate and silicate high-density fluids (HDF) as the metasomatic agent. No signs of influence of proto-kimberlite melt were found. The degree of nitrogen aggregation in diamond (%B from 6 to 15) indicates a long stay in mantle conditions, which excludes formation shortly before the emplacement of kimberlite. Extremely light values of carbon isotope composition (δ13C = –18.59 ‰) indicate the involvement of organic carbon of subduction origin in diamond formation. Diamond formation could be associated with an ancient metasomatic event occurring with the leading role of low-Mg silicate-carbonate HDF, the source of which were eclogites and/or subducted sedimentary deposits containing organic carbon. The calculated P-T parameters (3.7 GPa, 814 °C) of the last equilibrium of mineral phases of lherzolite point to its capture from a depth of ~118 km, which corresponds to a section of the lithospheric mantle (approximately 95-120 km), within which rocks also demonstrating features of specific transformations under the influence of subduction-related fluids were earlier discovered.

The first results of mineralogical and geochemical studies of a unique xenolith of lithospheric mantle are presented illustrating the earlier non-described mineral association of quartz, Cr-pyrope and Cr-diopside. Structural and textural features of the sample suggest a joint formation of these minerals. The calculated P-T-parameters of the formation of Cr-diopside indicate the capture of xenolith from the depth interval ~ 95-105 km (31-35 kbar) corresponding to the stability field of coesite. This suggests that quartz in the studied xenolith can represent paramorphs after coesite. It was shown that quartz in this rock is not a product of postmagmatic processes. The transformation stage of the source lherzolite into garnet- and clinopyroxene-enriched rock/garnet pyroxenite as a result of exposure to a high-temperature silicate melt was reconstructed. Subsequent stages of the influence of metasomatic agents were identified by the presence of a negative Eu-anomaly in some garnet grains, which could result from the impact of subduction-related fluid and the enrichment of rock-forming minerals with light rare earth elements, Sr, Th, U, Nb and Ta as a consequence of fluid saturated with these incompatible elements. Several models for the formation of SiO2 phase (quartz/coesite) in association with high-chromium mantle minerals are considered including carbonatization of mantle peridotites/eclogites and melting of carbonate-containing eclogites at the stage of subduction and the impact of SiO2-enriched melt/fluid of subduction genesis with peridotites of the lithospheric mantle.