Carbon isotopic variation within individual diamonds

Abstract

Various regions of type I diamonds have been analysed to determine intra-specimen variation in carbon isotopic abundance. We report here that generally, although not in all cases, a trend was observed from the centre to the exterior. The cores of some samples were isotopically light (enriched in 12C) whereas the edges became progressively heavier. For the specimen showing the greatest spread of δ13C values, the range observed covered almost 4‰, from −11.01 to −7.32‰. Such changes could be interpreted according to one or a combination of fractionation processes. Since diamonds exhibit a wide range of δ13C values (+2.7 to −34.4‰)1,2 and many specimens show evidence of heterogeneity, we have looked for isotopic differences between areas in individual diamonds which might reasonably be expected to have been formed in different conditions. The small scale of such internal structures in diamond requires the accurate excision of selected volumes. Such a requirement can now be met by controlled laser dissection. As a preliminary to realizing the full potential of such an approach, we have examined the exteriors and cores of coated diamonds, as representing one of the most conspicuous instances in which two distinct epochs of diamond genesis might be involved.