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.