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.

This article presents the results of studying microinclusions of fluids/melts in diamonds from the placers of the Krasnovishersky District (western slope of the Middle/Northern Urals), which make it possible to establish the evolution of diamond-forming media in the subcontinental lithospheric mantle of the eastern margin of the East European craton. Impurity composition of the studied crystals reveals three different types of diamonds, the formation of which was associated with separated metasomatic events. Microinclusions in B-type diamonds containing A and B nitrogen defects reflect an older metasomatic stage characterized by the leading role of silicic and low-Mg carbonatitic fluids/melts. The second stage is associated with the growth of A-type diamonds containing nitrogen exclusively in the form of A-centers. At this stage, the formation of diamonds was related with low-Mg carbonatitic media, more enriched in MgO, CaO, CO2, and Na2O compared to B-type diamonds. The third stage probably preceded the eruption of the transporting mantle melt and led to the formation of C-type diamond containing A and C nitrogen defect centers and microinclusions of silicic to low-Mg carbonatitic composition. The recorded trend in the evolution of diamond-forming fluids/melts is directed towards more carbonatitic compositions. Fluids/melts are probably sourced from eclogitic and pyroxenitic mantle substrates.