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

D. C. Petts*, T. Chacko, T. Stachel, R. A. Stern, L. M. Heaman

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

27 Citations (Scopus)

Abstract

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 ion mass spectrometry. Combined C- and N-isotope and N-abundance measurements were made across four distinct growth zones and show the following range of values: δ13C=-5.7 to -2.1‰ δ15N=-7.0 to +5.5‰ [N]=104 to 5420at.ppm. The core zone displays a continuous, rimward increase in δ13C and δ15N 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 (KN) of 4.4 and a N-isotope fractionation factor (15Ndiam-fluid) of -4.0±1.2‰ (2σ) at ~1100°C, for precipitation from a pure carbonate fluid. Calculated KN and 15Ndiam-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 (N2, NH3 or NH4 +) and the CN- molecule, as an analogue for the carbon-nitrogen bond in diamond, yield the following 15Ndiam (CN)-fluid estimates at 1100°C: -3.6‰ for NH4 +, -2.1‰ for N2 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 15N 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 15Ndiam-fluid, intracrystalline N-isotope variations in diamond should provide a sensitive test for fluid-related, fractional crystallization processes. Furthermore, the large magnitude of 15Ndiam-fluid could be reflected in the wide range of δ15N values for natural diamonds and the absence of clearly defined modes for the N-isotope compositions of peridotitic and eclogitic diamonds.

Original languageEnglish
Pages (from-to)188-200
Number of pages13
JournalChemical Geology
Volume410
DOIs
Publication statusPublished - 2 Sep 2015
Externally publishedYes

Keywords

  • Carbon isotopes
  • Diamond
  • Fractionation factor
  • Mantle fluids
  • N, NH, NH
  • Nitrogen isotopes
  • Reduced partition function ratios
  • SIMS

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