N-rich fluid inclusions in octahedrally-grown diamond

Abstract

Nitrogen is generally considered a trace component in the Earth's mantle. Mantle volatiles sampled from mid-ocean ridge basalts or preserved as fluid inclusions in xenoliths are dominated by CO2, with only traces of nitrogen. Here we describe CO2–N2 fluid inclusions with 40 mol% N2 and silicate melt inclusions with ∼0.1 wt% dissolved N2 in mantle-derived diamonds from 3 different cratons. The diamonds are octahedrally-grown, which is the most common growth habit and rarely reported to contain fluid inclusions. The inclusions have C/N ratios <1, more than two orders of magnitude below typical mantle values. We propose that nitrogen can become concentrated to high degrees by processes related to diamond growth. A growing diamond does not readily consume the nitrogen available, suggesting incompatible behaviour. Residual N2 from this process represents a concentrated nitrogen flux escaping the convecting mantle. Hidden nitrogen fluxes like this might be significant in counteracting the large apparent net influx by subduction. Based on nitrogen and carbon speciation in the inclusions, we propose that redox processes control the liberation of nitrogen from the convecting mantle, by the oxidation of NH+4 in silicates to mobile N2, concurrent with a parallel oxidation process releasing carbonate melt from the asthenosphere. Isotopic fractionation accompanying changes in nitrogen speciation could help account for the apparent isotopic mismatch between surficial and mantle nitrogen.