A nitrogen isotope fractionation factor between diamond and its parental fluid derived from detailed SIMS analysis of a gem diamond and theoretical calculations

To determine the magnitude of N-isotope fractionation between diamond and its parental fluid, detailed C- and N-isotope analyses of a complexly-zoned, eclogitic diamond (JDE-25) were undertaken using secondary ionmass spectrometry. Combined C- and N-isotope and N-abundance measurements were made across four distinct growth zones and show the following range of values: delta C-13 = -5.7 to -2.1 parts per thousand; delta N-15 = -7.0 to + 5.5 parts per thousand; [N] = 104 to 5420 at. ppm. The core zone displays a continuous, rimward increase in delta C-13 and delta N-15 values and decreases in N-abundance, and is interpreted to have formed by fractional crystallization of diamond from a single pulse of fluid (i.e., closed system). Modelling of the isotopic and abundance data from the core zone yields a diamond-fluid nitrogen partition coefficient (K-N) of 4.4 and a N-isotope fractionation factor (Delta N-15(diam-fluid)) of -4.0 +/- 1.2 parts per thousand (2 sigma) at similar to 1100 degrees C, for precipitation from a pure carbonate fluid. Calculated K-N and Delta N-15(diam-fluid) values would have larger magnitudes if JDE-25 formed from a more complex fluid, in which the carbonate species formed only a minor component. Theoretical calculations of N-isotope fractionation between the principal N-species associated with upper mantle fluids (N-2, NH3 or NH4+) and the CN- molecule, as an analogue for the carbon-nitrogen bond in diamond, yield the following Delta N-15(diam) ((CN)-fluid) estimates at 1100 degrees C: -3.6 parts per thousand for NH4+, -2.1% for N-2 and -1.4% for NH3. The theoretical calculations provide only minimum estimates of the true diamond-fluid N-isotope fractionation factor, given that the C-N single bond in diamond would have a lower affinity for N-15 than the stronger C-N triple bond in the CN- molecule. Accordingly, the theoretical N-isotope fractionation factors are consistent with the empirical fractionation factor derived from diamond JDE-25. As a consequence of the large magnitude of Delta(15)N(diam-)fluid, intracrystalline N-isotope variations in diamond should provide a sensitive test for fluid-related, fractional crystallization processes. Furthermore, the large magnitude of Delta N-15(diam-fluid) could be reflected in the wide range of delta N-15 values for natural diamonds and the absence of clearly defined modes for the N-isotope compositions of peridotitic and eclogitic diamonds. (C) 2015 Elsevier B.V. All rights reserved.