Geochemical heterogeneity of sublacustrine hydrothermal vents in Yellowstone Lake, Wyoming

ROV-based submersible operations in Yellowstone Lake (Yellowstone National Park, Wyoming, USA) have identified two locations with chemically and mineralogically distinct sublacustrine hydrothermal fluids and deposits: The Deep Hole east of Stevenson Island (SI Deep Hole) and the SE West Thumb Deep Vent field (SE WT Field). The SI Deep Hole is the deepest part of Yellowstone Lake (120 m), and hosts up to 174 °C fluids heated by steam that condenses on contact with cold lake water. The resulting hot fluids exit through a clay alteration cap that largely consists of kaolinite. The SI Deep Hole hydrothermal vents are analogous to steam-heated hot springs elsewhere in Yellowstone National Park (YNP) (e.g. the Mud Volcano area), which are typified by fumaroles and acid-sulfate fluids. These deep sublacustrine vent fluids are more aptly termed “carbonic-acid-sulfide”, owing to a lack substantial sulfate from H₂S oxidation and acidity attributed to dissolved CO₂. At 53 m depth, vent fluids at the SE WT Field achieve temperatures of up to 141 °C. These neutral-chloride fluids are largely similar to those that produce siliceous deposits in many subaerial YNP geyser basins. Geothermometry calculations and binary mixing relationships indicate the SE WT Field fluids equilibrated at 207 to 224 °C following mixing between oxygenated dilute cold groundwater and a fluid with T = 345 to 364 °C, Cl = 400 ppm, and δD = −149 ‰ (VSMOW); broadly similar to the putative deep parent fluid that supplies most geyser basins in YNP. The δD values of SE WT Field fluids cannot be derived from lake water, implying km-scale lateral hydrothermal fluid flow from outside the boundary of Yellowstone Lake. The fluid compositions and hydrothermal processes operating in the two sublacustrine vent systems are distinct but the overarching influence of magmatic heating, complex degassing, and alteration mineralization effects are broadly comparable to those that affect their subaerial counterparts. The high temperatures and hydrostatic pressures of the sublacustrine vents, however, provide a window into the evolution of fluid chemistry in more deeply seated and less accessible regions of the subsurface YNP and similar volcanically active hydrothermal systems elsewhere.