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    Multiple fluids in diamond coat and their role in diamond growth

    Tomlinson, E.L. and Jones, A.P. and Milledge, J. and Beard, A.D. (2003) Multiple fluids in diamond coat and their role in diamond growth. In: Eighth International Kimberlite Conference, 2003, Victoria, Canada. (Unpublished)

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    Abstract

    This work is an assessment of the role of fluids in diamond growth using mantle fluid compositions measured in micro-inclusions in coated diamond. Coated diamonds comprise a clear octahedral core surrounded by an overgrowth of inclusion-rich octahedral diamond. Navon and Hutcheon [1] recognized that micro-inclusions contain fluids rich in H2O, CO32-, SiO2, K2O, CaO and FeO, present in bulk proportions that resemble potassic magmas. Schrauder and Navon [2] studied the composition of individual inclusions, noting that they vary linearly between a carbonatitic and a hydrous end-member. A third fluid type, a KCl brine, was identified in cloudy diamonds [3], this fluid was thought to be distinct from the two end-members described in fibrous diamonds. EMPA AND FTIR DATAElectron-microprobe and infrared spectroscopy results for coated diamonds from the Democratic Republic of Congo, indicate that all 3 fluids are present in the diamond micro-inclusions. The fluid compositions lie along two continua (figure 1): Carbonatite-Silicate Series: Span a continuum between 1) a carbonatite-like (rich in CaO, P2O5, FeO and MgO) and 2) an alkali silicate (rich in SiO2, Al2O3, K2O and TiO2) end-member with roughly K-feldspar stoicometry. SiO2 correlates with the other major elements, suggesting a mixing relationship. The presence and abundance inclusions without K-Cl indicates that these end-members mixed before the involvement of K-Cl. K-Cl fluid-Silicate Series: Span a continuum between 1) the alkali silicate end-member and 2) an end member rich in K and Cl. There is a positive correlation between Cl and K2O (also between Cl and Na2O), but the trends for the other major elements are the same as the carbonatite-silicate series. The calculated Cl end-member composition is similar to the ìbrineî identified in cloudy diamonds [3]. These inclusions are preferentially located at the core-coat boundary, suggesting that the K-Cl fluid is involved early in coat growth. A fourth fluid end-member, rich in sulfur, has also been identified (figure 1). Diamonds whose bulk inclusion compositions fall towards the silicate end-member contain more H2O than the carbonatite-like end-member. Figure 1: Individual inclusion compositions grouped into series. ♦- Carbonatite-silicate series; ! - Silicate K-Cl series; ! - S-rich. Also shown, - calculated series end-member compositions, and published data for inclusions in fibrous diamond: ♦- Zaire and Botswana [1]; " - Jwaneng, Botswana [2]; ♦- Brine in cloudy diamond, Koffiefontein, South Africa [3].

    Metadata

    Item Type: Conference or Workshop Item (Paper)
    School: Birkbeck Schools and Departments > School of Science > Earth and Planetary Sciences
    Depositing User: Sarah Hall
    Date Deposited: 10 Jun 2019 15:17
    Last Modified: 11 Jun 2019 08:35
    URI: http://eprints.bbk.ac.uk/id/eprint/27775

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