Geocheminar fall-2018

Variable d15N in volcanic gases are typically interpreted as proxies for the contribution of mantle versus slab-derived nitrogen. However, air lies between the mantle and the crust. Hence d15N values alone are blind to air contributions to geological fluids. In order to disambiguate the nitrogen cycle in volcanoes, we determined the N content, d15N and 15N15N excess measured on the Panorama mass spectrometer of 35 N2 samples from a wide variety of crustal and volcanic settings. We find that the low d15N observed in volcanic gases are not mantle contributions but instead must result from isotope fractionation of air-derived N2 in hydrothermal systems. This observation implies that volcanic N2 is dominated by crustal components and air, thus redefining the geodynamic nitrogen sources to the atmosphere.