Heat and Mass Transport in a Vapor‐Dominated Hydrothermal Area in Yellowstone National Park, USA: Inferences From Magnetic, Electrical, Electromagnetic, Subsurface Temperature, and Diffuse CO₂ Flux Measurements

Abstract

Vapor-dominated hydrothermal systems are characterized by localized and elevated heat and gas flux. In these systems, steam and gas ascend from a boiling water reservoir, steam condenses beneath a low-permeability cap layer, and liquid water descends, driven by gravity ("heat pipe" model). We combine magnetic, electromagnetic, and geoelectrical methods and CO 2 flux and subsurface temperature measurements in the Solfatara Plateau Thermal Area in the Yellowstone Caldera to address several fundamental questions: (1) What are the structural and/or lithological controls on heat and mass transport in vapor-dominated areas? (2) What is the geometry and size of convecting multiphase thermal plumes? (3) Are thermal plumes associated with subsurface rock alteration and demagnetization? Magnetic and electromagnetic data inversions suggest an asymmetric 50-to 100-m thick basin of glacial deposits with the thickest part adjacent to the margin of a rhyolite flow. The 3-D electrical conductivity model in the glacial basin reveals a narrow vertical conductor interpreted as a focused multiphase plume, which coincides at the ground surface with the heat and CO 2 flux maxima. The magnetic data suggest that destruction of magnetic minerals due to rock alteration associated with the hydrothermal plume occurs mainly near the ground surface. We propose a model where the buoyant multiphase plume forms in response to decompression, boiling, and phase separation of pressurized thermal groundwater that discharges from the brecciated base of a rhyolite flow into the basin of glacial deposits. Results from multiphase groundwater flow and heat transport numerical simulations corroborate the first-order characteristics of this model.

Geological and Thermal Control of the Hydrothermal System in Northern Yellowstone Lake: Inferences From High‐Resolution Magnetic Surveys

“…Along the lake's western shore are a set of prominent sinuous‐shaped, north‐south oriented, high‐amplitude, positive magnetization anomalies limited on their eastern side by a sharp gradient (violet line on Figure 7) and a negative anomaly. Such strong magnetization and sharp magnetic gradients also are observed along the leading edge of the Elephant Back flow (Qpce, northwestern corner of Figures 2a and 4a) and recently have been observed by Bouligand et al (2019) at the margin of an upper flow unit part of the Solfatara Plateau rhyolite flow. We propose that these sinuous‐shaped magnetization highs are due to vertical edges of upper flow units of the West Thumb rhyolite flow that are exposed along the western shoreline and partly buried in the lake beneath the lake sediments.…”

“…Such an outer ring of decreased magnetization has not been previously observed over any of the terrestrial vapor‐dominated venting areas of YNP, possibly due to the lower spatial resolution of previously available magnetic data. However, a recent high‐resolution ground magnetic survey of the on‐land, vapor‐dominated Solfatara Plateau thermal area in YNP (Bouligand et al, 2019) showed that a decrease in magnetization was restricted to the near surface and in a few areas offset from the buoyant gas and heat plume.…”

“…In the latter case, demagnetization by alteration appears to be absent from the vapor‐dominated reservoir. Alteration in subaerial vapor‐dominated systems is restricted to the near surface where ground or surface waters mix with acid‐sulfate steam‐condensate and intensely alter the rock and its magnetic minerals (Bouligand et al, 2019; Hochstein & Soengkono, 1997). Nevertheless, such magnetization differences may be erased through time as a hydrothermal system evolves through several stages of alteration including both vapor‐ and liquid‐dominated phases (Hochstein & Soengkono, 1997).…”

Heat Flux From a Vapor‐Dominated Hydrothermal Field Beneath Yellowstone Lake

“…Conductive heat transfer through the cap condenses vapor, and the heavier condensate descends in an annulus around the vapor-rich zone, forming a narrow convection cell ("heat pipe," White et al, 1971). Where fractures penetrate the low-permeability cap, some of the steam and gas are emitted to the atmosphere through fumaroles (Bouligand et al, 2019;Hurwitz & Lowenstern, 2014). This model requires some modifications in order to apply it to the sublacustrine Deep Hole hydrothermal field (Figure 3).…”

Geophysical Imaging of Yellowstone’s Hydrothermal Plumbing System

“…Current models for the YNP hydrothermal system are based upon geochemistry and the spatial relation between thermal features and surface geology. Permeable basal breccias 8,20 guide thermal uid ow and are thought to localize thermal features at lava ow fronts 1,2,21 . At higher elevations, the mixing of groundwater with steam and gas boiled out of deep alkaline waters create acid-sulfate pools, fumaroles and mud pots 1,14 .…”