Title: "15N15N as a tracer of atmospheric and magmatic N2 in fumarolles: new insights on the N/Ar ratio of plume sources."
Abstract: "The volatile element composition of Earth’s mantle is the direct consequence of planetary formation, but significant uncertainty remains about the relative importance of solar contributions, versus additions from asteroids or comets. The ratio of nitrogen (N) to non-radiogenic argon (36Ar) is markedly different in solar (N2/36Ar = 15.5±0.2) vs. asteroidal (N2/36Ar=1.0±0.7x106) sources, thus making it a powerful tracer of volatile origins. Here we show that the N2/36Ar of mantle reservoirs are chondritic, and thus asteroidal in origin. We derive new mantle N2/36Ar values by measuring the rare 15N15N isotopologue of N2 gases in volcanic fumaroles from three volcanic regions: Iceland, Eifel (Germany), and Yellowstone (Wyoming, USA). We use that the abundance of 15N15N as a unique tracer of air contamination and show that negative d15N values observed in fumaroles, although mimicking mantle values, in fact represent air-derived N2 that experienced 15N/14N fractionation in sub-surface hydrothermal systems. Using a two-component mixing model we demonstrate that gases from Eifel come from a mixture of subducted crust and the ambient mantle sources, where the recycled component must carry relatively high N2/36Ar and d15N values. In contrast, we show that the 3He-rich plume source of Yellowstone has a N2/36Ar ratio virtually indistinguishable from both chondrites and the convective mantle. These results show that despite potential volatile losses caused by exposure of magma oceans to space during planet formation, the modern terrestrial mantle retains primordial gases acquired during planetary growth. A corollary is that subduction of atmospheric-derived N and Ar (possibly cometary) suggested by earlier studies is ruled out . Instead, we conclude that the modern mantle hosts N and Ar likely delivered to Earth’s mantle by chondritic asteroids during accretion rather than by dissolution from protoplanetary nebular gas or by subduction of cometary components."