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Cohenite, native iron and troilite inclusions in garnets from polycrystalline diamond aggregates

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Abstract

Syngenetic garnet of eclogitic/pyroxenitic composition included in a polycrystalline diamond aggregate from the Venetia kimberlite, Limpopo Belt, South Africa shows multiple inclusions of spherules consisting of 61±5 vol% Fe3C (cohenite), 30±2 vol% Fe-Ni and 9±3 vol% FeS (troilite). Troilite forms shells around the native iron-cohenite assemblage, implying that both compositions were immiscible melts and were trapped rapidly by the silicate. It is proposed that this polycrystalline diamond-silicate-metallic spherule assemblage formed in very local pressure and fO2 conditions in cracks at the base of the subcratonic lithosphere from a C-H-O fluid that reacted with surrounding silicate at about 1,300–1,400 °C. In a mantle fluid consisting of CH4>H2O>H2 near fO2=IW, the H2 activity increases rapidly when carbon from the fluid is consumed by diamond precipitation, driving the oxygen fugacity of the system to lower values along the diamond saturation curve. Water from the fluid induces melting of surrounding silicate material, and hydrogen reduces metals in the silicate melt, reflected by an unusually low Ni content of the garnet. The carbon isotopic composition of δ13C=−13.69‰ (PDB) and the lack of nitrogen as an impurity is consistent with formation of the diamond from non-biogenic methane, whereas δ18O=7.4‰ (SMOW) of the garnet implies derivation of the silicate from subduction-related material. Hence, very localized and transient reducing conditions within the subcratonic lithosphere can be created by this process and do not necessarily call for involvement of fluids derived from subducted material of biogenic origin.

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Acknowledgments

We thank Pierre Cartigny (IPGP Paris) and Nathalie Grassineau (University of London) for providing first class stable isotope analyses. Cyrena Goodrich provided a Disko cohenite standard and gave valuable insights into the metallurgical literature and discussions with Astrid Holzheid and Sharon Webb helped to shape thought regarding oxygen fugacity estimations. Earlier versions of this article profited from informal and formal reviews by Dave Green, Nick Sobolev, Alan Woodland, Ben Harte and Jeff Harris. D.J. gratefully acknowledges financial support from the Deutsche Forschungsgemeinschaft.

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  1. D. E. Jacob

    Present address: Max-Planck Institut für Chemie, Abt. Geochemie, Postfach 3060, 55020 Mainz, Germany

Authors and Affiliations

  1. Institut für Geologische Wissenschaften, Universität Greifswald, F.L. Jahnstr. 17a, 17487 Greifswald, Germany

    D. E. Jacob

  2. Geowissenschaftliches Zentrum der Universität Göttingen, Goldschmidt-Str. 1, 37077 Göttingen, Germany

    A. Kronz

  3. Geoscience Center, DeBeers Consolidated Mines, PO Box 82232, Southdale 2135, South Africa

    K. S. Viljoen

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  1. D. E. Jacob
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Correspondence to D. E. Jacob.

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Jacob, D.E., Kronz, A. & Viljoen, K.S. Cohenite, native iron and troilite inclusions in garnets from polycrystalline diamond aggregates. Contrib Mineral Petrol 146, 566–576 (2004). https://doi.org/10.1007/s00410-003-0518-2

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