Lessons from geothermal gases at Yellowstone

Abstract:

The magma-hydrothermal system of the Yellowstone Plateau Volcanic Field encompasses over ten thousand individual springs, seeps, and fumaroles spread out over >9000 square kilometers, and produces a range of acid, neutral and alkaline waters. A prominent model (Fournier, 1989 and related papers) concludes that many neutral and alkaline fluids found in hot springs and geysers are derived from a uniform, high-enthalpy parent fluid through processes such as deep boiling and mixing with dilute meteoric groundwater. Acid waters are generally condensates of gas-bearing steam that boils off of subsurface geothermal waters. Our recent studies of gases at Yellowstone (Lowenstern et al., 2015 and references therein) are compatible with such a model, but also reveal that gases are largely decoupled from thermal waters due to open-system addition of abundant deep gas to (comparatively) shallow circulating thermal waters.

Fumarole emissions at Yellowstone range from gas-rich (up to 15 mol%) composed of deeply derived CO₂, He and CH₄, to steam-rich emissions (<0.01% gas) dominated by N₂ and Ar. The clear implication is that deep gas is diluted with atmospheric gas boiled off of geothermal liquids. The general trend is antithetical to that predicted by progressive boiling of a parent fluid (Rayleigh or batch degassing), where decreasing gas content should correlate with increasing proportions of soluble gas (i.e., CO₂). Deep gas at Yellowstone fits into two general categories: 1) mantle-derived CO₂ with a hotspot He isotope signature (>16 R_A) and low CH₄ and He concentrations and 2) mantle-derived CO₂ with much higher CH₄ and/or He concentrations and abundant radiogenic He picked up from crustal degassing. Individual thermal areas have distinct CH₄/He. It remains unclear whether some gas ratios mainly reflect subsurface geothermal temperatures. Instead, they may simply reflect signatures imparted by local rock types and mixing on timescales too fast for reequilibration. Overall, the gas chemistry reflects a broader view of mantle-crust dynamics than can be appreciated by studies of only dissolved solutes in the neutral and alkaline waters from Yellowstone geysers.

Fournier (1989) Ann. Rev. Earth Planet. Sci. v. 17, p. 13-53.

Lowenstern et al. (2015) JVGR, v. 302, 87-101.