Diamonds from the Poiskovaya, Zapolyarnaya, and Leningrad Emberlite Pipes, Northern Yakutia: Correlation of Carbon Isotopic Composition and Nitrogen Content As an Indicatorof Fluid Diamond Formation

摘要

Compared to most studied kimberlite pipes of the Yakutian province, diamonds from the Poisk-ovaya, Zapolyarnaya, and Leningrad pipes (about 100 determinations) are characterized by a decrease in the average δ~(13)C (-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 δ~(13)C and the nitrogen content. This correlation is considered to be a result of synchronous and progressive loss of nitrogen and the light isotope ~(12)C 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 + C_(diam)-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.