An analysis of the hydrothermal fluid chemistry and isotopic data of Yellowstone Lake vents

Abstract

Yellowstone National Park is a dynamic environment home to an array of geysers, hot springs, and hydrothermal vents fueled by the underlying continental magmatic intrusion. Yellowstone Lake vent fluids accounts for approximately 10% of the total geothermal flux for all of Yellowstone National Park. Though studying this remote hydrothermal system poses severe challenges, it provides an excellent natural laboratory to research hydrothermal fluids that undergo higher pressure and temperature conditions in an environment largely shielded from atmospheric oxygen. The location of these vents also provides chemistry that is characteristic of fluids deeper in the Yellowstone hydrothermal system. In August 2016 and 2017, hydrothermal fluids were collected from the Stevenson Island vents in collaboration with the Hydrothermal Dynamics of Yellowstone Lake (HD-YLAKE) project using novel sampling techniques and monitoring instrumentation. The newly built ROV Yogi was deployed to reach the vents in-situ with temperatures in excess of 151oC at 100-120 m depth, equipped with a 12-cylinder isobaric sampler to collect the hydrothermal fluids. Analyses of the Yellowstone Lake hydrothermal fluid revealed chemistry almost identical to that of the lake water, with the exception of an abundance of dissolved gases, such as CO2 and H2S. Dissolved H2 and CO are also present, suggesting more reducing conditions at elevated temperatures with high fractions of hydrothermal source fluid. Reducing conditions are also indicated by high H2S/SO4 ratio, and in-situ chemical sensor data. A particularly abnormal feat of these fluids is the dissolved silica concentrations, which are well below saturation with respect to quartz and amorphous silica, in spite of the silica-rich substrate which the hydrothermal fluids vent through. One explanation for this chemical data is influx of high enthalpy steam from a boiling zone immediately beneath the lake floor. Mass-balance calculations indicate the collected sample contain 27% vapor to mix with lake water in order to achieve the observed temperatures of the vent fluids. However, this interpretation is a paradigm shift from the previous models, which entail mixing of a chloride rich, isotopically heavy deep thermal reservoir liquid with lake water.