The phase composition of crystal-fluid nanoinclusions in alluvial diamonds in the northeastern Siberian Platform

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 (δ13Cav = –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 CO2, 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.

The Petyayan-Vara Carbonatite-Hosted Rare Earth Deposit (Vuoriyarvi, NW Russia): Mineralogy and Geochemistry

“…Hypothetically, the neighbouring gneisses could also have been assimilated by carbonatite melts deeper in the complex, increasing Al and Si concentrations. As shown in [131], contamination of the kimberlite melt with crustal components results in the appearance of a Ti-rich potassium-aluminosilicate melt, captured as the inclusions in diamonds. Ti-rich melts of similar compositions were obtained in experiments modelling the melting of carbonatized pelites [132].…”

Phase relationships in the system K2CO3-CaCO3 at 6 GPa and 900-1450 C

“…The specific feature of deep carbonatite melts is high alkali contents (particularly K), determined from studies of the melt inclusions in diamonds (Navon 1991;Schrauder and Navon 1994;Izraeli et al 2004;Klein-BenDavid et al 2004;Tomlinson et al 2006;Weiss et al 2009;Zedgenizov et al 2009Zedgenizov et al , 2011Logvinova et al 2011) and high-pressure experiments on partial melting of carbonatites , kimberlites (Litasov et al 2010b;Sharygin et al 2013), carbonated peridotites (Dasgupta and Hirschmann 2007;Brey et al 2011), eclogites (Dasgupta et al 2004;Yaxley and Brey 2004;Litasov et al 2010a), and pelites (Grassi and Schmidt 2011). Experimental data on synthesis of K-bearing clinopyroxene (Harlow 1997) suggests that clinopyroxenes from inclusions in diamonds and diamondbearing metamorphic rocks with up to 1 wt% K 2 O (Sobolev et al 1972(Sobolev et al , 1991Sobolev and Shatsky 1990;Harlow and Veblen 1991;Shatsky et al 1995) are crystallized from ultrapotassic carbonate-silicate melts containing 15-28 wt% K 2 O.…”

Origin and Evolution of High-Mg Carbonatitic and Low-Mg Carbonatitic to Silicic High-Density Fluids in Coated Diamonds from Udachnaya Kimberlite Pipe

“…The compositions of HDF microinclusions in fibrous diamonds from worldwide deposits are known to vary between three main end-members: silicic, saline, and carbonatitic [6,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. These data were used to constrain many possible scenarios of the origin and evolution of such HDFs.…”

“…The described multiphase assemblage of such microinclusions represents a set of daughter phases of an originally homogenous liquid phase called high-density fluid (HDF), which is similar to supercritical fluids or melts enriched with volatile components [7]. The compositions of microinclusions in fibrous diamonds widely varies between three general end-members: (i) a silicic end-member rich in Si, Al, water, and K; (ii) a saline end-member rich in Cl, water, and K; (iii) a carbonatitic end-member rich in carbonate, Mg, Ca, and K [6,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].…”