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Constraining diamond metasomatic growth using C- and N-stable isotopes: examples from Namibia |
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| Authors: | Pierre Cartigny Thomas Stachel JeffW Harris Marc Javoy |
| Institution: | a IPGP et Universite Paris 7, Laboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe, UMR CNRS 7047, 4 Place Jussieu, T54-64 E1, 75251, Paris cedex 05, France b Department of Earth and Atmospheric Science, University of Alberta, Edmonton, AB, Canada T6G 2E3 c Institut für Mineralogie, Universität Frankfurt, Senckenberganlage 28, 60054 Frankfurt, Germany d Division of Earth Sciences, University of Glasgow, Gregory Building, Glasgow G12 8QQ, Scotland, UK |
| Abstract: | The present paper provides C- and N-stable isotope characteristics, N-contents and N-aggregation states for alluvial diamonds of known paragenesis from placers along the Namibian coast. The sample set includes diamonds with typical peridotitic and eclogitic inclusions and the recently reported “undetermined” suite of Leost et al. Contrib. Mineral. Petrol. 145 (2003) 15] which resulted from infiltration of high temperature, carbonate-rich melts. δ13C-values range from ?20.3‰ to ?0.5‰ (n=48) for peridotitic diamonds and from ?38.5‰ to ?1.6‰ (n=45) for eclogitic diamonds. Diamonds belonging to the “undetermined” suite span a narrower range in δ13C from ?8.5‰ to ?2.7‰ (n=13). When compared with previous studies, diamonds from Namibia are characterised by unusually low proportions of N-free (i.e. Type II) peridotitic and eclogitic diamonds (3% and 2%, respectively) and an unprecedented high proportion of N-rich diamonds (15% and 73%, respectively, have N-contents >600 ppm). δ15N-values for diamonds of the peridotitic, eclogitic and “undetermined” suites range from ?10‰ to +13‰ without correlations with either N-content or δ13C. The similarity in N-isotopic composition and the N-rich character of diamonds belonging to the eclogitic, peridotitic and “undetermined” suites is striking and suggests a close genetic relationship. We propose that a large part of the diamonds mined in Namibia formed during metasomatic events of similar style that introduced carbon and nitrogen into a range of different host lithologies. |
| Keywords: | Diamond Placer deposit Namibia Nitrogen Stable isotopes Metasomatism |
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