Diamond formation and source carbonation: mineral associations in diamonds from Namibia |
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| Authors: | I?Leost T?Stachel Email author" target="_blank">G?P?BreyEmail author J?W?Harris I?D?Ryabchikov |
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| Institution: | (1) Institut für Mineralogie, Universität Frankfurt, Senckenberganlage 28, 60054 Frankfurt, Germany;(2) UMR Geosciences Azur, University of Nice Sophia-Antipolis, Parc Valrose, 06108 Nice Cedex 2, France;(3) Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada;(4) Division of Earth Sciences, Gregory Building, University of Glasgow, Glasgow G12 8QQ, UK;(5) Institute for Geology of Ore Deposits, IGEM, Russian Academy of Sciences, Staromonetny, Pereulok 35, Moscow 109017, Russia; |
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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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