C and N isotopic composition and the infrared absorption spectra of coated diamonds: evidence for the regional uniformity of CO2H2O rich fluids in lithospheric mantle

Abstract The δ 13 C and δ 15 N values, nitrogen abundances and nitrogen aggregation states of coated diamonds from Botswana, Angola, Sierra Leone and Siberia have been determined. A single cubic micro-diamond from the Northern Territory, Australia was also analysed. The (cubic) coats of the diamonds and the cubic diamond from Australia had a restricted range in isotope composition: δ 13 C= −7.2 to −4.1‰ , δ 15 N= −8.7 to −1.7‰ . In contrast, the cores of the coated diamonds were found to be highly variable: δ 13 C= −21.1‰ to −1.9‰ , and δ 15 N= −2.8‰ to +12.1‰ (with the majority being positive). All of the coats gave the type IaA absorption spectra, together with others due to micro-inclusions dominated by H 2 O and CO 2 , whereas the cores contained nitrogen that was more highly aggregated. The results suggest that coated diamonds were formed following the influx of CO 2 H 2 O rich fluids into diamond-bearing lithosphere. Pre-existing diamonds acted as seeds for renewed growth and these are now the cores of the diamonds. These cores may be very variable in terms of morphology, isotopic composition, age, nitrogen aggregation state and crystallinity depending on the particular history of the source regions and the conditions of diamond growth. It is believed that the influx of volatiles and the growth of the coats was linked to kimberlite magmatism. The δ 13 C and δ 15 N results for the coats indicate that, in terms of C and N isotope composition, the source of CO 2 H 2 O rich fluids is globally quite homogeneous. It is probably located beneath continental lithosphere and, in addition, may have characteristics similar to the source of the ocean island basalts.