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 N₂ (Δ₃₀), along with ultrahigh precision measurements of Ar, Kr, and Xe isotopes, of hydrothermal gas samples from Yellowstone National Park. We show that δ¹⁵N 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 CO₂. Notably, a similar correlation exists regardless of the degree of atmospheric contamination, suggesting that the δ¹⁵N of the Yellowstone mantle source is similar to the atmosphere (i.e., ∼0‰). Two component mixing models between Δ₃₀ and noble gases demonstrate that N₂/³⁶Ar (5.3 ± 0.7 × 10⁵) and ³⁶Ar/¹³⁰Xe (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 N₂- and Xe-rich recycled material. Conversely, we suggest that the similarity in δ¹⁵N and N₂/³⁶Ar 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.