Age of Round Diamonds of Variety v From Placers of the Northeast of the Siberian Platform

Round diamonds of Variety V according to Yu.L. Orlov are endemic representatives of placers of the northeast of the Siberian Platform. Their primary source has not yet been identified which makes it difficult to estimate the age of these crystals. Geochemical features (δ13С from –17 to –25‰, Nppm from 1200 to 2000) indicate a contribution of subducted materials in their formation. Two diamond crystals that constitute the intergrowth studied in this work have δ13C of –20.8 and –20.7‰ and nitrogen impurity concentrations of 1693– 2036 and 1568–1953 at. ppm, respectively. Zircon inclusions with δ18O = 7.9–8.8‰ found in one of these crystals have a U-Pb age of 262.5 ± 2.7 and 232.6 ± 3.0 Ma. The zircon inclusion with an older age may reflect early stages of the intergrowth formation or its constituent diamond crystal fragments, or it could be protogenetic in origin. We argue that the crystallization substrate and source material for this sample originated from a fragment of subducted oceanic crust containing organic sediments and hydrothermally altered basalts. The studied diamond was transported to the surface no earlier than the Triassic episode of kimberlite magmatism on the Siberian craton.