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

摘要

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.