The origin and dynamics of nitrogen in the Earth's mantle constrained by 15N15N in hydrothermal gases

The development of high-resolution gas source mass spectrometry has permitted entirely new types of measurements of multiply-substituted isotopologues in gas species of geochemical significance. Here, we present recent advances afforded by measurements of (NN)-N-15-N-15 in natural samples, together with (NN)-N-14-N-14 and (NN)-N-15-N-14. We show that the abundance of the doubly-substituted (NN)-N-15-N-15 isotopologue in hydrothermal gases, often mixtures of volatiles of widely different origins, allows tracing the provenance of nitrogen. The approach is based on the recent finding that atmospheric N-2 has a substantial enrichment in (NN)-N-15-N-15 of nearly 20 parts per thousand relative to any other source of N-2. This is particularly useful for the study of hydrothermal gases, where characterizing the isotopic composition and provenance of volcanic N-2 is important for a wide range of applications in high-temperature geochemistry, but where air-derived N-2 is unavoidable. In this review, we summarize the evidence that (NN)-N-15-N-15 is an unambiguous tracer of air contamination. We compare two sets of published (NN)-N-15-N-15 data acquired on gases from plume and arc volcanoes. We show how different sources of volcanic N-2 may be in plume versus arc environments, and discuss the first-order constraints on the deep N cycle that are provided by the new (NN)-N-15-N-15 data. Important findings include that the delta N-15 tracer, used alone or in conjunction with N-2/Ar and N-2/He ratios, can be surprisingly deceiving. Isotope fractionation of atmospheric nitrogen occurs within hydrothermal systems, resulting in negative delta N-15 values similar to estimates for mantle values, yet with (NN)-N-15-N-15 values that preclude a mantle origin. The (NN)-N-15-N-15 data show that the true delta N-15 of volcanic components is positive in arcs but near-zero at the Yellowstone plume. In other words, atmospheric N-2 can mimic mantle delta N-15, and mantle delta N-15 can look like the value of air. Without (NN)-N-15-N-15, the apportioning of mantle and atmospheric N-2 in mixed gases can easily be wrong. With (NN)-N-15-N-15, we also determine the true N-2/He-3 and N-2/Ar-36 ratios of volcanic components in hydrothermal systems. Results inform our understanding of the deep nitrogen cycle. Plume and arc volcanic end-members show distinct isotope and elemental ratios, consistent with sub-arc sources being overwhelmed by near-quantitative slab devolatilization, while the Yellowstone plume source is not reflecting volatile subduction.