Nitrogen isotope systematics and origins of mixed-habit diamonds

Abstract

Nitrogen isotope values from mantle diamonds are a commonly used tracer in the quest to track volatiles within the Earth’s mantle through deep time. Interpretations of this isotope data are valid so long as stable isotope fractionation processes in the mantle are understood. The fractionation of nitrogen isotopes between {111} and {100} growth sectors is well documented for high-pressure high-temperature (HPHT) synthetic diamonds, but there is little data on whether it also occurs in natural mixed-habit diamonds. We present 91 in-situ nitrogen isotope (δ15N) measurements, along with carbon isotope (δ13C) values and nitrogen abundances [N], obtained from three mixed-habit diamonds by secondary ion mass spectrometry (SIMS). While the well-documented enrichment of nitrogen concentrations in octahedral sectors compared to contemporaneous cuboid sectors is observed, a similarly clear disparity is not obvious in the δ15N data. Whereas HPHT synthetic diamonds exhibit 15N enrichment in the {100} sectors by ∼+30‰, the mixed-habit diamonds studied here show enrichment of the octahedral sectors in 15N by only 0.4–1‰. This major difference between HPHT synthetic and natural mixed-habit diamonds is proposed to be the result of different physical properties of the growth interfaces. The smooth interfaces of the octahedral sectors are the same in both types of crystal, but the outermost atoms on the smooth cube interfaces of an HPHT synthetic diamond behave differently to those on the rough cuboid interfaces of the natural mixed-habit diamonds, resulting in different δ15N values. Both the δ13C (average of ∼−8.7‰) and δ15N (average of ∼0‰) data show only minor offsets from the typical mantle values (δ13C=−5±3‰, δ15N=−5±4‰). This may indicate diamond formation from a mantle derived fluid/melt containing a minor subducted component (lowering δ13C values and elevating δ15N) or relate to moderate degrees of isotopic fractionation of a pure mantle fluid/melt by prior diamond precipitation. The homogeneous nature of both the carbon and nitrogen isotopic compositions of all three diamonds, however, documents continuous and unlimited supply of diamond forming fluid/melt, with a constant composition. Such homogenous isotopic compositions exclude fluid mixing or isotopic fractionation close to the site of diamond formation and preclude distinguishing between these two processes based on diamond analyses alone.