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Diamond formation and source carbonation: mineral associations in diamonds from Namibia

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Abstract

Mineral inclusions in diamonds from Namibia document a range of mantle sources, including eclogitic, websteritic and peridotitic parageneses. Based on unusual textural features a group of inclusions showing websteritic, peridotitic and transitional chemical features is assigned to an 'undetermined suite' (12% of the studied diamonds). The mutual characteristic of this group is the occurrence of lamellar intergrowths of clinopyroxene and orthopyroxene. In addition, the 'undetermined suite' is associated with a number of uncommon phases: in one diamond MgCO3 is enclosed by clinopyroxene. Other minerals that form touching inclusions with the pyroxene lamellae are (1) a SiO2 phase observed in three diamonds, together with CaCO3 in one of them, (2) phlogopite and a Cr-rich 'titanate' (probably lindsleyite). The inclusions document a metamorphic path of decreasing pressures and temperatures after entrapment in diamond. First, homogeneous low-Ca clinopyroxenes were entrapped at high temperatures. They subsequently exsolved orthopyroxene and probably also SiO2 (coesite) on cooling along a P,T trajectory that did not allow garnet to be exsolved as well. Phlogopite, carbonates and LIMA phases are the result of overprint of a peridotitic source rock by a carbon-rich agent. The resulting unusual, olivine-free mineral association and the host diamonds are interpreted as products of extensive carbonation of the peridotite.

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Acknowledgements

We gratefully acknowledge financial support of this project through the German Research Foundation (DFG) and additional support through De Beers Consolidated Mines Ltd. T.S. acknowledges the Canada Research Chairs program (CRC). We thank A. Girnis for helpful comments and suggestions and M. Okrusch and L. Franz for thoughtful reviews of the manuscript.

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Authors and Affiliations

  1. Institut für Mineralogie, Universität Frankfurt, Senckenberganlage 28, 60054 Frankfurt, Germany

    I. Leost, T. Stachel, G. P. Brey, J. W. Harris & I. D. Ryabchikov

  2. UMR Geosciences Azur, University of Nice Sophia-Antipolis, Parc Valrose, 06108 Nice Cedex 2, France

    I. Leost

  3. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada

    T. Stachel

  4. Division of Earth Sciences, Gregory Building, University of Glasgow, Glasgow G12 8QQ, UK

    J. W. Harris

  5. Institute for Geology of Ore Deposits, IGEM, Russian Academy of Sciences, Staromonetny, Pereulok 35, Moscow 109017, Russia

    I. D. Ryabchikov

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  1. I. Leost
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  2. T. Stachel
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  3. G. P. Brey
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  4. J. W. Harris
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  5. I. D. Ryabchikov
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Correspondence to G. P. Brey.

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Editorial responsibility: J. Hoefs

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Leost, I., Stachel, T., Brey, G.P. et al. Diamond formation and source carbonation: mineral associations in diamonds from Namibia. Contrib Mineral Petrol 145, 15–24 (2003). https://doi.org/10.1007/s00410-003-0442-5

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  • DOI: https://doi.org/10.1007/s00410-003-0442-5

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