Mg and Fe-rich carbonate–silicate high-density fluids in cuboid diamonds from the Internationalnaya kimberlite pipe (Yakutia) D.A. Zedgenizov a, ⁎, A.L. Ragozin a , V.S. Shatsky a , D. Araujo b , W.L. Griffin b , H. Kagi c a Institute of Geology and Mineralogy, 3 Koptyuga Avenue, Novosibirsk, 630090, Russia b GEMOC ARC National Key Centre, Macquarie University, NSW 2109, Australia c Geochemical Laboratory, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan abstract article info Article history: Received 8 September 2008 Accepted 10 May 2009 Available online 7 June 2009 Keywords: Diamond microinclusions Mantle high-density fluids Microinclusions in 44 cuboid diamonds from the Internationalnaya kimberlite pipe (Yakutia) show wide compositional variations. In comparison with the available worldwide database most microinclusions in diamonds from Internationalnaya define a continuous range of compositions from carbonatitic to hydrous- silicic high-density fluids (HDFs). The hydrous-silicic compositional range is reported here for the first time in Yakutian diamond-forming fluids. The hydrous-silicic end-members are rich in water, SiO 2 , Al 2 O 3 ,K 2 O and P 2 O 5 . Carbonatitic end-members are rich in carbonate, CaO, MgO, Na 2 O and FeO. Microinclusions in diamonds from Internationalnaya show a range from magnesian to extremely iron-rich compositions. There are two compositional arrays of microinclusions: (i) carbonate-rich with SiO 2 b10 wt.% where FeO decreases without any correlation with sulfur, water, carbonate or chlorine and (ii) carbonatitic to hydrous-silicic where FeO generally decreases as SiO 2 contents increase. The trace-element patterns of the microinclusions are generally similar to those of kimberlites and carbonatites, but there are significant differences in the major elements. The relative abundance of K in the microinclusions is significantly higher. The microinclusions have smooth patterns for the LILE, normalized to a primitive mantle composition. HFSE patterns in the microinclusions show some depletion in Ti, Zr and Hf relative to Ta, Nb and Mo. Primitive mantle normalized REE patterns reveal low abundances of the heavy REE and high concentrations of light REE. The La/Dy ratio of microinclusions varies widely, decreasing from carbonatitic to hydrous-silicic compositions. A general correlation of δ 13 C of diamonds with the relative abundance of carbonates in the microinclusions suggests that carbon isotope compositions are related to the evolution of the parental media. The carbonate–silicate range of high-density fluids observed in diamonds from Internationalnaya may be explained by fractional crystallization of mantle fluids/melts, or mixing between liquids with different compositions. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Diamonds are “pristine time capsules” from Earth's mantle. The strength of diamond and its low reactivity in the silicate environment help to shield the trapped material from changing conditions in the mantle environment and during ascent. Mineral inclusions show that most diamonds are derived from the subcontinental lithospheric mantle (up to 250 km depth; Richardson et al., this issue ; Aulbach et al., this issue; Araújo et al., this issue). A compositional range of peridotitic to eclogitic host-rocks, where most diamonds occur, has been ascribed to varying degrees of melt extraction from a fertile rocks and refertilisation of mantle protoliths by metasomatic fluids/melts. The metasomatic origin of diamonds is further suggested by the observation that many diamond-bearing xenoliths have carbon contents higher than any known silicate melts (Schulze et al., 1996), and by the relationship of diamonds to metasomatic veins in mantle xenoliths (Spetsius et al., 2002; Taylor and Anand, 2004; Shatsky et al., 2008). Sub-micrometer inclusions in fibrous diamonds carry high-density fluids (HDF) from which the diamonds are inferred to have precipitated (e.g. Navon et al., 1988; Klein-BenDavid et al., this issue). At the pressures and temperatures of the diamond stability field, many systems that carry silicate melts, carbonatitic melts and hydrous fluids are beyond a second critical point and are fully miscible (Wyllie and Ryabchikov, 2000). It seems likely that the fluids were trapped as uniform, highly concentrated, high-density fluids that were similar to sub-critical melts with high volatile contents. It is suggested that the multi-phase assemblage of such microinclusions represents a mantle- derived fluid or volatile-rich melt, which was trapped in the diamonds as they grew and subsequently crystallised a range of daughter minerals (Chrenko et al., 1967; Navon et al., 1988; Guthrie et al., 1991; Zedgenizov et al., 2004; Izraeli et al., 2004; Klein-BenDavid et al., 2006; Logvinova et al., 2008). Thus, microinclusions in fibrous diamonds provide well- preserved samples of the deepest fluids available for research. Lithos 112S (2009) 638–647 ⁎ Corresponding author. E-mail address: zed@uiggm.nsc.ru (D.A. Zedgenizov). 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.05.008 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos