Insights from fumarole gas geochemistry on the origin of hydrothermal fluids on the Yellowstone Plateau

The chemistry of Yellowstone fumarole gases shows the existence of two component waters, type MC, influenced by the addition of deep mantle fluid, and type CC, influenced by crustal interactions (CC). MC is high in ³He/⁴He (22 Ra) and low in ⁴He/⁴⁰Ar (∼1), reflecting input of deep mantle components. The other water is characterized by ⁴He concentrations 3–4 orders of magnitude higher than air-saturated meteoric water (ASW). These high He concentrations originate through circulation in Pleistocene volcanic rocks, as well as outgassing of Tertiary and older (including Archean) basement, some of which could be particularly rich in uranium, a major ⁴He source. Consideration of CO₂–CH₄–CO–H₂O–H₂ gas equilibrium reactions indicates equilibration temperatures from 170 °C to 310 °C. The estimated temperatures highly correlate with noble-gas variations, suggesting that the two waters differ in temperature. Type CC is ∼170 °C whereas the MC is hotter, at 340 °C. This result is similar to models proposed by previous studies of thermal water chemistry. However, instead of mixing the deep hot component simply with cold, meteoric waters we argue that addition of a ⁴He-rich component, equilibrated at temperatures around 170 °C, is necessary to explain the range in fumarole gas chemistry.