Age of rounded diamonds of the variety V from placers of the north-east of the Siberian platform
Reutsky V.N.1, Zedgenizov D.A.2, Ragozin A.L.1, Kalinina V.V.1, Zemnukhov A.L.3
1 Sobolev Institute of Geology and Mineralogy Siberian Branch of Russian Academy of Sciences, Novosibirsk, Russia.
2 Zavaritsky Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences, Ekaterinburg, Russia.
3 Almazy Anabara AO, Yakutsk, Russia.
Keywords: diamond, carbon isotopes, mineral inclusions, zircon, oxygen isotopes, U-Pb dating, kimberlites, placer deposits, Siberian craton
Abstract
Rounded diamonds of the variety V (Yu. L. Orlov’s classification) are endemic to placers in the northeastern Siberian Craton. Their primary source has not yet been established, making it difficult to estimate the age of these crystals. Geochemical features (δ13C from -17 to -25‰, Nppm from 1200 to 2000) indicate that subducted material contributed to their formation. Studied diamond intergrowth consist of crystals with δ13С = -20.8 and -20.7‰ and nitrogen content of 1693 - 2036 and 1568 - 1953 at. ppm, correspondingly. Zircon inclusions with δ18O = 7.9-8.8‰ found in one of these diamonds have a U-Pb age of 262.5 ± 2.7 and 232.6 ± 3.0 Ma. The zircon inclusion with an older age may belong to the early stages of formation of the intergrowth or to the diamond crystal fragments that are part of the intergrowth. Alternatively, it may be protogenetic. We suggest that the crystallization substrate and source of the carbon of this diamond was a fragment of subducted oceanic crust containing organic sediments and hydrothermally altered basalts. The diamond studied was brought to the surface no earlier than the Triassic episode of kimberlite magmatism on the Siberian craton.
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