Noble Gas Isotopes and Nitrogen Isotopologues Reveal Deep Sources and Subsurface Fractionation in Yellowstone Gases.

Nitrogen plays a critical role in maintaining Earth's hospitable surface environment over geological time. Despite our atmosphere being dominated by nitrogen, our understanding of how nitrogen was delivered to Earth and how subsequent planetary processes modified Earth's nitrogen budget through time is currently lacking. Here, we report measurements of isotopologues of N (Δ), along with ultrahigh precision measurements of Ar, Kr, and Xe isotopes, of hydrothermal gas samples from Yellowstone National Park. We show that δN variations are correlated with nonradiogenic Ar, Kr, and Xe isotope ratios, indicating that groundwater-derived nitrogen and noble gases in hydrothermal samples are fractionated by the same process as they diffuse through a rising column of magmatic CO. Notably, a similar correlation exists regardless of the degree of atmospheric contamination, suggesting that the δN of the Yellowstone mantle source is similar to the atmosphere (i.e., ∼0‰). Two component mixing models between Δ and noble gases demonstrate that N/Ar (5.3 ± 0.7 × 10) and Ar/Xe (1611 ± 212) in the Yellowstone mantle source are lower and greater than the MORB mantle source, respectively, suggesting that contrary to previous findings, the plume mantle source has not been more efficiently overprinted by the addition of N- and Xe-rich recycled material. Conversely, we suggest that the similarity in δN and N/Ar between the Yellowstone mantle source and chondritic meteorites indicates that nitrogen and noble gases in the deep mantle reflect the composition of the material that initially formed Earth.