On the possibility of a kinetic fractionation of nitrogen stable isotopes during natural diamond growth

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 delta(13)C, delta(15)N and nitrogen aggregation state with values at + 1.96parts per thousand, + 19.4parts per thousand, 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 delta(13)C-values from -5.33parts per thousand to -6.63parts per thousand, delta(15)N from -1.0parts per thousand to -5.5parts per thousand, 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 delta(13)C ranges (less than 3parts per thousand from -5 to - 8parts per thousand), but also in the high levels of nitrogen (- 1000 ppm), suggesting that the two diamond types are related. Additionally, no delta(15)N 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 40parts per thousand between cube and octahedral growth. The present evidence, therefore, suggests that kinetic fractionation of N-isotopes does not operate during natural diamond formation. Copyright (C) 2003 Elsevier Science Ltd.