NI-RICH SULFIDE MELT INCLUSIONS IN YAKUTIAN DIAMONDS

Sulfide mineral inclusions are common among inclusions in diamonds and provide unique information about the distribution of chalcophile elements in the mantle and are intensively studied over many years. Sulfide inclusions in diamonds are pyrrhotite, pentlandite, chalcopyrite, pyrite, troilite, K-bearing Fe- sulfides and monosulfide solid solution (Mss). At that similar to the other diamond inclusions they belong to two types of paragenesis: eclogite (0-12% Ni) and peridotite (22-36% Ni) (Bulanova et al, 1996). We present the first report of sulfide melt inclusions significantly enriched in Ni (> 48%). Using FIB/TEM method in 5 alluvial and 10 diamonds from kimberlite pipes of Yakutia have been identified individual Ni-rich sulfide inclusions, similar in chemical composition to heazlewoodite and millerite. The inclusions are located in the central zone of the diamond and their sizes vary from 10 to 200 nm. The Ni/(Ni+Fe) ratio of the inclusions ranges between 0,64 and 0,92, but within the same diamond-matrix of inclusion have a similar composition. Of greatest interest are the inclusions of sulfides, which were found in diamonds of peridotite xenolith from the Udachnaya kimberlite pipe. All of identified sulfides are characterized by a high-Ni composition and classified as similar to millerite. Inclusions have a pure nickel (or nickel oxide?) shell. They are associated with silicates, carbonates, halides and fluid. The nitrogen content measurements showed that the diamonds, containing high-Ni sulfides, practically does not contain nitrogen, or in small amounts (over 100 ppm). The wide S/(S+Ni+Fe) range, the constant and high Ni/(Ni+Fe), and association with carbonatitic melts suggest that the sulfide-bearing micro-inclusions do not contain a mineral phase but rather, a Ni-rich sulfide melt. The sulfide melts adds another branch to the diamond forming fluid system encountered in diamonds. Carbonatitic melts were found before with hydrous-silicic melts and brine; here we show they can coexist with anomalously Ni-rich sulfide melts as well.

This study is done on state assignment of IGM SB RAS, financial support was providing by Russian Science Foundation (19-17-00128).