Diamonds from the Poiskovaya, Zapolyarnaya, and Leningrad kimberlite pipes, Northern Yakutia: Correlation of carbon isotopic composition and nitrogen content as an indicator of fluid diamond formation

Abstract

Compared to most studied kimberlite pipes of the Yakutian province, diamonds from the Poiskovaya, Zapolyarnaya, and Leningrad pipes (about 100 determinations) are characterized by a decrease in the average δ13C (−4.59, −4.50,−4.04‰) and by relatively low average nitrogen contents (93, 254, 304 ppm, respectively). These pipes also differ in the relative abundance of alpha-1 group crystals in Galimov’s classification. Based on a stable combination of morphology and properties, we distinguished diamond populations of the common origin. All of them have significant a negative correlation between δ13C and the nitrogen content. This correlation is considered to be a result of synchronous and progressive loss of nitrogen and the light isotope 12C in the medium during the diamond formation. The conclusion is drawn that the alpha-1 crystals were formed in the open fluid system. The pyrolysis of ethane—C2H6 → CH4 + H2 + Cdiam—is assumed to be a model of diamond precipitation from fluid. The pressure release due to extension deformation of the mantle under tectonic action might be a factor shifting this reaction toward diamond formation.