www.elsevier.com/locate/rgg The phase composition of crystal-fluid nanoinclusions in alluvial diamonds in the northeastern Siberian Platform A.M. Logvinova a, * , R. Wirth b , A.A. Tomilenko a , V.P. Afanas’ev a , N.V. Sobolev a a V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b GFZ, German Research Centre for Geosciences, Chemistry and Physics of Earth Materials, Telegrafenberg, C-120, D-14473 Potsdam, Germany Received 9 March 2011; accepted 5 April 2011 Abstract The phase composition of crystal-fluid nanoinclusions in two types of placer diamonds of unknown genesis from the northeastern Siberian Platform (Ebelyakh diamondiferous region) has been first studied by transmission electron microscopy including electron diffraction, analytical electron microscopy (AEM), electron energy loss spectroscopy (EELS), and chromatography. The type I diamonds are transparent dodecahedroids, and the type II ones, widespread in this region, are dark rounded crystals assigned to variety V according to Orlov’s mineralogical classification. Isotopic and IR-Fourie spectroscopic studies showed that the type II diamonds have a strongly light carbon isotope composition (δ 13 C av = –22.4‰) and high concentrations of nitrogen admixture (1100–1800 ppm). Nitrogen is present mainly as an aggregate. It is shown that all inclusions no larger than 400 nm are polyphase particles consisting of solid (silicate, oxide, carbonate, salt) and fluid phases. The type I diamonds bear high-Mg carbonatite inclusions (up to 100 nm) consisting of magnesite, dolomite, Fe-spinel, and clinohumite. The fluid phase has high concentrations of K, Cl, and O. The inclusions are similar in composition to the near-solidus melts of saturated carbonatized peridotites; thus, they might have resulted either from the crystallization of the parental melt or from the quenching and crystallization of deep-seated carbonate-silicate melt. The type II diamonds bear low-Mg carbonatite polyphase nanoinclusions consisting of Ba-, Sr-, and Ca,Fe-carbonatites, K,Ba-phosphates, Ti,Si- and Ti,Al-phases, and abundant fluid segregations filled mainly with CO 2 , N, and hydrocarbons. These melts/solutions might have been supplied from subducted rocks of the oceanic and, partly, continental Earth’s crust. The enrichment of these inclusions in incompatible elements might evidence the percolation of salt fluids enriched in Ba, Sr, P, Ti, K, and Cl through carbonatized eclogites. © 2011, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: diamond; nanoinclusions; fluid; carbonates; spinel; clinohumites; subduction Introduction The northeastern Siberian Platform is famous by unique diamond placers localized mainly in modern alluvial deposits. Some of these placers are mined. The richest ones occur along the Ebelyakh, Billyakh, Mayat, and Molodo Rivers (Afanas’ev et al., 2011; Dobretsov and Pokhilenko, 2010; Grakhanov, 2006). The problem of determining the kind of diamond orebodies is still debatable. Placer diamonds in the north- eastern Siberian Platform show the greatest diversity. There are some types of crystals that were not found in famous kimberlite bodies or their portion in kimberlites is much smaller than that in the placers. Hundreds of kimberlite bodies, mainly of Mesozoic age, are either not diamondiferous in this area or are poor in diamonds to form placers. The Northeast Siberian diamonds are characterized by different formation conditions and carbon sources. Diamonds of ultrabasic par- agenesis in commercial Yakutian kimberlite bodies of Middle Paleozoic age amount to >90% (Yefimova and Sobolev, 1977), whereas the Northeast Siberian diamonds are both of peridotitic and eclogitic parageneses, with the latter being predominant (Afanas’ev et al., 2000a,b, 2009; Sobolev et al., 1979, 1999, 2009b; Tomilenko et al., 2001; Zinchuk and Koptil’, 2003). An eclogite paragenesis (coesite + garnet + omphacite) with a high-pressure quartz modification was found for the first time in the northeastern Siberian Platform (Sobolev et al., 1976). This is reflected in the carbon isotope composition of diamonds: Commercial kimberlites are char- acterized mainly by a heavy carbon isotope composition typical of the mantle carbon source, whereas nearly a half of placer diamonds (30 of analyzed 66) (Galimov, 1984; Russian Geology and Geophysics 52 (2011) 1286–1297 * Corresponding author. E-mail address: logv@igm.nsc.ru (A.M. Logvinova) doi:10.1016/j.rgg.201 . 00 1068-7971/$ - see front matter D 201 IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. V S. . S bolev 1, 1 o . 10 2