Carbon and nitrogen isotope systematics within a sector-growth diamond from the Mir kimberlite, Yakutia

Abstract

A single Yakutian octahedral diamond, displaying striking internal growth structure whereby cubic and octahedral growth sectors are inter-grown and surrounded by an octahedral rim, has been analysed for carbon and nitrogen isotopic compositions (by secondary ion mass spectrometry, SIMS), and for nitrogen concentration (by SIMS and Fourier transform infrared spectroscopy, FTIR) and nitrogen aggregation state (by FTIR). A graphite “seed” inclusion identified within the diamond is enriched in K, Ca, Ti, Rb, and Sr, providing evidence that the diamond may have grown from a carbonate melt/fluid interacting with upper mantle rocks. Carbon and nitrogen isotope compositions become progressively heavier from the core region (δ 13C=−7‰ to −5‰ and δ 15N=−3‰) towards the inner rim zones (δ 13C=−3‰ and δ 15N=+8.9‰ to +5‰) of the diamond. Nitrogen concentration and aggregation measurements show corresponding decreases that generally correlate with the isotopic variation. These systematic changes within the core and intermediate regions of the diamond are consistent with their formation during diamond growth from CO 2-rich fluids as a continuous event, accompanied by slight progressive isotopic fractionation of carbon and nitrogen. However, the observed isotope and nitrogen abundance trends differ from those predicted from thermodynamic modelling of fluid–solid equilibria in a C–N–O–H-bearing system due to changes in parameters such as f O 2 . Within the diamond octahedral rim region, grown by a layer by layer mechanism, nonsystematic variations in nitrogen abundance, nitrogen aggregation, and nitrogen and carbon isotope ratios were observed. Several interpretations are given for this phenomenon, including kinetic effects during growth of the diamond rim under different conditions from those of the core-intermediate regions; or rapidly changing fluid sources during the growth; or post-growth processes of nitrogen aggregation. No fractionation of nitrogen isotopes between cubic and octahedral growth zones was identified within the diamond, in contrast to the fractionation phenomena found in synthetic diamonds of mixed growth. Our results illustrate the potential C–N isotopic variation within individual diamonds and highlight the need for more studies of this type if the origin of isotopic variations in diamond suites is to be understood.