Origins of geothermal gases at Yellowstone

Gas emissions at the Yellowstone Plateau Volcanic Field (YPVF) reflect open-system mixing of gas species originating from diverse rock types, magmas, and crustal fluids, all combined in varying proportions at different thermal areas. Gases are not necessarily in chemical equilibrium with the waters through which they vent, especially in acid sulfate terrain where bubbles stream through stagnant acid water. Gases in adjacent thermal areas often can be differentiated by isotopic and gas ratios, and cannot be tied to one another solely by shallow processes such as boiling-induced fractionation of a parent liquid. Instead, they inherit unique gas ratios (e.g., CH₄/He) from the dominant rock reservoirs where they originate, some of which underlie the Quaternary volcanic rocks. Steam/gas ratios (essentially H₂O/CO₂) of Yellowstone fumaroles correlate with Ar/He and N₂/CO₂, strongly suggesting that H₂O/CO₂ is controlled by addition of steam boiled from water rich in atmospheric gases. Moreover, H₂O/CO₂ varies systematically with geographic location, such that boiling is more enhanced in some areas than others. The δ¹³C and ³He/CO₂ of gases reflect a dominant mantle origin for CO₂ in Yellowstone gas. The mantle signature is most evident at Mud Volcano, which hosts gases with the lowest H₂O/CO₂, lowest CH₄ concentrations and highest He isotope ratios (~16Ra), consistent with either a young subsurface intrusion or less input of crustal and meteoric gas than any other location at Yellowstone. Across the YPVF, He isotope ratios (³He/⁴He) inversely vary with He concentrations, and reflect varied amounts of long- stored, radiogenic He added to the magmatic endmember within the crust. Similarly, addition of CH4 from organic-rich sediments is common in the eastern thermal areas at Yellowstone. Overall, Yellowstone gases reflect addition of deep, high-temperature magmatic gas (CO₂-rich), lower-temperatures crustal gases (⁴He- and CH₄-bearing), and those gases (N₂, Ne, Ar) added principally through boiling of the meteoric-water-derived geothermal liquid found in the upper few kilometers. We also briefly explore the pathways by which Cl, F, and S, move through the crust.