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

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 delta13C-values from –5.33per mille sign to –6.63per mille sign, delta15N from –1.0per mille sign to –5.5per mille sign, 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 delta13C ranges (less than 3per ,illes sign from –5 to –8per milles sign), but also in the high levels of nitrogen (~ 1000 ppm), suggesting that the two diamond types are related. Additionally, no delta15N 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 40per mills sign between cube and octahedral growth. The present evidence, therefore, suggests that kinetic fractionation of N-isotopes does not operate during natural diamond formation.