Modeling extreme nitrogen isotope variability in Neoarchean diamonds from Knee Lake, Superior Craton

The nitrogen (N) isotope composition of world-wide diamonds shows considerable variability that is uncorrelated with carbon isotopes. Most of this variability is currently explained by different compositions of the N sources, although fractionation processes can nonetheless play a role. Because N in Earth’s mantle is overprinted by subduction as time progresses, we examined the N isotope composition of the Knee Lake diamond suite, hosted in volcaniclastic rocks erupted in the Neoarchean at ∼2.73 Gy, to try to obtain an older glimpse of N isotope variability. Secondary ion mass spectrometry analyses of 186 Knee Lake diamonds revealed extraordinarily varied N isotope compositions, expanding from the lowest (−46.8 ‰) to the highest (+31.8 ‰) values measured for terrestrial diamonds, strongly clustered around a median of −0.7 ‰. The median value indicates a heavier-than modern δ 15 N composition of the Archean mantle, suggesting a secular change from a 15 N-enriched primordial composition. Whether some of the δ 15 N variability in the Knee Lake diamonds could have been inherited by crustal recycled or unmixed primordial sources, the range and discontinuous distribution of the most extreme values rule out an overall explanation as being formed by mixing of heterogeneous N sources and suggests the involvement of fractionation processes. The systematics of N concentration ([N]) and δ 15 N variations for most of the Knee Lake diamonds are permissive of a Rayleigh distillation process. Rayleigh distillation during kinetic isotope fractionation associated with the decomposition of N-phases could even produce the most 15 N-enriched compositions and the uncorrelated [N]-δ 13 C variability. However, Ryleigh fractionation cannot produce the most 15 N-depleted compositions. The extreme N isotope compositions in the Knee Lake diamond suite are correlated with crystal shape, the most negative δ 15 N values occurring in cuboid and the most positive in octahedral crystals. This suggests the operation of kinetic N fractionation processes related to different growth-surface structures or growth conditions. Despite the recognition of distinct N isotope compositions in different growth sectors in synthetic crystals, two mixed-habit Knee Lake diamonds show only a small growth-sector related N isotope variability. The only process we can model that is capable of producing the most 15 N-depleted values is diffusion along chemical gradients, whether pre-existing in the growth environment or generated by crystal growth under diffusion-limited conditions as indicated by the re-entrant, fibrous shape of some cuboid crystals. These observations highlight the complexity of nitrogen isotope geochemistry in the Earth mantle, and suggest that various kinetic effects may play a role in fractionating the N isotope compositions of some diamonds during their growth. The possibility of N isotope fractionation during diamond growth suggests caution in the use of isotopic tracers in presence of extreme values, even at the high temperatures of the Earth mantle.