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On the possibility of a kinetic fractionation of nitrogen stable isotopes during natural diamond growth |
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| Authors: | Pierre Cartigny Jeff W Harris Randi Davies Marc Javoy |
| Institution: | 1 Laboratoire de Géochimie des Isotopes Stables, UMR CNRS 7047, Institut de Physique du Globe & Université Paris 7, 2 Place Jussieu, T54-64 E1, 75251 Paris, Cedex 05, France 2 Division of Earth Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK 3 Diamond Trading Company Research Centre, Belmont Road, Maidenhead, SL6 6JW, UK 4 Geochemical Evolution and Metallogeny of Continents (GEMOC), School of Earth Sciences, Macquarie University, NSW 2109, Sydney, Australia |
| Abstract: | In a diamond from New South Wales (Australia), cubic and octahedral growth sectors, as identified by cathodoluminescence (CL), show slight differences in N-contents of 29 and 42 ppm respectively but no significant differences in either δ13C, δ15N and nitrogen aggregation state with values at +1.96‰, +19.4‰, and 25% Type IaAB aggregation, respectively.Two gem cubes from the Orapa kimberlite (Botswana) were studied by CL revealing a nonfaceted cubic growth. Accordingly, nine other gem cubes were combusted and yielded δ13C-values from -5.33‰ to -6.63‰, δ15N from -1.0‰ to -5.5‰, and nitrogen contents from 914 to 1168 ppm, with nitrogen aggregation state being only Type IaA (zero % B). The gem cubes show striking similarities to fibrous/coated diamonds, not only in both δ13C ranges (less than 3‰ from -5 to -8‰), but also in the high levels of nitrogen (≈ 1000 ppm), suggesting that the two diamond types are related. Additionally, no δ15N variation was detected between the cube and octahedral growth sectors of the Australian diamond, in the cube sectors of the nine gem cubes from Botswana, nor in fibrous/coated diamonds previously studied. These analyses contrast with an earlier study on a synthetic diamond, which reported a strong kinetic fractionation of N-isotopes of about 40‰ between cube and octahedral growth. The present evidence, therefore, suggests that kinetic fractionation of N-isotopes does not operate during natural diamond formation. |
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