The evolution of diamond-forming fluids indicating a pre-kimberlitic metasomatic event in the mantle beneath the Mirny field (Siberian craton)

A suite of coated diamonds from the International’naya (n = 63) and Mir (n = 5) kimberlite pipes (Mirny field, Siberian craton) was studied to track the most recent metasomatic events in the sub-continental lithospheric mantle (SCLM). The diamonds consist of two contrasting domains: an older monocrystalline core, and a younger fibrous coat, the latter of which formed just prior to the kimberlite eruption. Microinclusions in the coats indicate two types of growth media: (i) dominant, silicic to low-Mg carbonatitic high-density fluids (HDFs), and (ii) minor, high-Mg carbonatitic HDFs. Eclogitic mineral inclusions were identified in the cores of the diamonds containing silicic to low-Mg carbonatitic HDFs in the coats, while peridotitic mineral inclusions were found in the cores of the diamonds with high-Mg carbonatitic HDFs in the coats. The chemistry of the high-Mg carbonatitic HDFs suggests that they originated from partial melting of a carbonated peridotite. Major components and trace-element patterns of most silicic to low-Mg carbonatitic HDFs support an eclogitic source with accessory rutile and variable ratios of CO2/H2O. In many coats, the HDFs evolve from silicic to more carbonatitic, and rarely from carbonatitic to more silicic. The scenario of both evolutionary trends could have involved the passage of peridotite-derived carbonatitic HDFs into eclogites, which triggered their partial melting, followed by the appearance of immiscible, highly silicic and Ca-rich carbonatitic fluids/melts. The immiscible fluids evolved toward a single-silicate–carbonate melt, whose cooling resulted in the precipitation of silicate minerals.