EVALUATING MANTLE-CRUSTAL GAS DYNAMICS OF SOUTHWESTERN US CO2 SPRINGS USING HELIUM ISOTOPES AND HEAVY NOBLE GASES

Recent work identifies a correlation between low-velocity mantle seismic anomalies and the presence of mantle-derived gases, particularly CO₂ and ³He, in geothermal springs across the continental US. Still, there are considerable uncertainties in the mechanisms and timing by which mantle gases reach the crust and the degree to which mantle-lithosphere-crust interactions occur during magmatic gas emplacement and alter composition. These uncertainties limit our ability to determine the implications of low-velocity anomalies or elevated mantle contributions in crustal settings. For example, do elevated mantle contributions reflect an actively degassing mantle in regions with low-velocity anomalies? Or, have the mantle-derived gases been stored in the lithosphere or crust for extended time-periods and are only now migrating to the surface?

Although ³He/⁴He can provide unambiguous indications of mantle-derived gases in continental settings, helium isotope ratios can result from mixing with radiogenic ⁴He or variable mantle or lithospheric contributions. We anticipate that heavier noble gases (Ne, Ar, Kr, Xe) and their isotopes (particularly ²¹Ne, ⁴⁰Ar, ¹²⁹Xe, and ¹³⁶Xe) may provide additional insight on the mantle vs. lithospheric source (e.g., ¹²⁹Xe is dominantly primordial while other isotopes, including ¹³⁶Xe, have mantle and radiogenic contributions) and the relative timing of fluid emplacement.

To address these questions, we analyzed a suite of helium and heavy noble gas isotopic compositions in concert with molecular gas composition (CO₂, N₂, hydrocarbons) and δ¹³C-CO₂ in gas and water samples from geothermal springs from the San Juan mountains (CO), the Valles caldera and Rio Grande rift (NM), and the Colorado Plateau (Grand Canyon, AZ). Site selection was based on the presence of distinct low-velocity mantle anomalies. All samples display resolvable helium isotopic ratios above crustal production values, and the majority of samples also display resolvable enrichments in ¹²⁹Xe relative to air and variable excesses in ²¹Ne*, ⁴⁰Ar*, and ¹³⁶Xe*. Excess ³He and ¹²⁹Xe are consistent with mantle contributions, while variable radiogenic gases reflect the relative mixtures of air-saturated water, mantle, lithosphere, and the crust providing insight on their history during crustal emplacement.