Volatile emissions and gas geochemistry of Hot Spring Basin, Yellowstone National Park, USA

Abstract

We characterize and quantify volatile emissions at Hot Spring Basin (HSB), a large acid-sulfate region that lies just outside the northeastern edge of the 640 ka Yellowstone Caldera. Relative to other thermal areas in Yellowstone, HSB gases are rich in He and H 2, and mildly enriched in CH 4 and H 2S. Gas compositions are consistent with boiling directly off a deep geothermal liquid at depth as it migrates toward the surface. This fluid, and the gases evolved from it, carries geochemical signatures of magmatic volatiles and water–rock reactions with multiple crustal sources, including limestones or quartz-rich sediments with low K/U (or 40⁎Ar/ 4⁎He). Variations in gas chemistry across the region reflect reservoir heterogeneity and variable degrees of boiling. Gas-geothermometer temperatures approach 300 °C and suggest that the reservoir feeding HSB is one of the hottest at Yellowstone. Diffuse CO 2 flux in the western basin of HSB, as measured by accumulation-chamber methods, is similar in magnitude to other acid-sulfate areas of Yellowstone and is well correlated to shallow soil temperatures. The extrapolation of diffuse CO 2 fluxes across all the thermal/altered area suggests that 410 ± 140 t d − 1 CO 2 are emitted at HSB (vent emissions not included). Diffuse fluxes of H 2S were measured in Yellowstone for the first time and likely exceed 2.4 t d − 1 at HSB. Comparing estimates of the total estimated diffuse H 2S emission to the amount of sulfur as SO 4 2− in streams indicates ~ 50% of the original H 2S in the gas emission is lost into shallow groundwater, precipitated as native sulfur, or vented through fumaroles. We estimate the heat output of HSB as ~ 140–370 MW using CO 2 as a tracer for steam condensate, but not including the contribution from fumaroles and hydrothermal vents. Overall, the diffuse heat and volatile fluxes of HSB are as great as some active volcanoes, but they are a small fraction (1–3% for CO 2, 2–8% for heat) of that estimated for the entire Yellowstone system.