Carbon and nitrogen isotope systematics in diamond: Different sensitivities to isotopic fractionation or a decoupled origin?

Using stable isotope data obtained on multiple aliquots of diamonds from worldwide sources, it has been argued that carbon and nitrogen in diamond are decoupled. Here we re-investigate the carbon-nitrogen relationship based on the most comprehensive microbeam data set to date of stable isotopes and nitrogen concentrations in diamonds (n = 94) from a single locality. Our diamond samples, derived from two kimberlites in the Chidliak Field (NE Canada), show large variability in delta C-13 (-28.4 parts per thousand to -1.1 parts per thousand, mode at -5.8 parts per thousand), delta N-15 (-5.8 to + 18.8 parts per thousand, mode at -3.0 parts per thousand) and nitrogen contents ([N]; 3800 to less than 1 at ppm). In combination, cathodoluminescence imaging and microbeam analyses reveal that the diamonds grew from multiple fluid pulses, with at least one major hiatus documented in some samples that was associated with a resorption event and an abrupt change from low delta C-13 and [N] to mantle-like delta C-13 and high [N]. Overall, delta C-13 appears to be uncorrelated to delta N-15 and [N] on both the inter- and intra-diamond levels. Co-variations of delta N-15-log[N], however, result in at least two parallel, negatively correlated linear arrays, which are also present on the level of the individual diamonds falling on these two trends. These arrays emerge from the two principal data clusters, are characterized by slightly negative and slightly positive delta N-15 (about -3 and +2 parts per thousand, respectively) and variable but overall high [N]. Using published values for the diamond-fluid nitrogen isotope fractionation factor and nitrogen partition coefficient, these trends are perfectly reproduced by a Rayleigh fractionation model. Overall, three key elements are identified in the formation of the diamond suite studied: (1.) a low delta C-13 and low [N] component that possibly is directly associated with an edogitic diamond substrate or introduced during an early stage fluid event. (2.) Repeated influx of a variably nitrogen-rich mantle fluid (mildly negative delta C-13 and delta N-15). (3.) In waning stages of influx, availability of the mantle-type fluid at the site of diamond growth became limited, leading to Rayleigh fractionation. These fractionation trends are clearly depicted by delta N-15 -[N] but are not detected when examining co-variation diagrams involving delta C-13. Also on the level of individual diamonds, large (>= 5 parts per thousand) variations in delta N-15 are associated with delta C-13 values that typically are constant within analytical uncertainty. The much smaller isotope fractionation factor for carbon (considering carbonate- or methane-rich fluids as possible carbon sources) compared to nitrogen leads to an approximately one order of magnitude lower sensitivity of delta C-13 values to Rayleigh fractionation processes (i.e. during fractionation, a 1 parts per thousand change in delta C-13 is associated with a 10 parts per thousand change in delta N-15). As a consequence, even minor heterogeneity in the primary isotopic composition of diamond forming carbon (e.g., due to addition of minor subducted carbon) will completely blur any possible co-variations with delta N-15 or [N]. We suggest this strong difference in isotope effects for C and N to be the likely cause of observations of an apparently decoupled behaviour of carbon and nitrogen isotopes in diamond. (C) 2016 Elsevier B.V. All rights reserved.