Geochemistry of vapor-dominated hydrothermal vent deposits in Yellowstone Lake, Wyoming

Highlights

• Yellowstone Lake vent mineral deposits reveal complex alteration processes.

• Hydrogen isotope data for kaolinite indicates high formation temperatures

• Reaction transport modeling constrains temporal evolution of the vent system.

• Decreases in magnetic intensity are consistent with sulfidation of magnetite at vent sites.

Abstract

Yellowstone Lake hydrothermal vent systems have been studied using ROV assets to better understand the chemical and mineralogical evolution of the sublacustrine sediments through which the hot spring fluids discharge to the lake floor. Here we focus on the deposits/alteration and coexisting vent fluid chemistry associated with the Deep Hole on the lake floor, east of Stevenson Island. Remote in its location, at 120 m below the lake surface, this region in the northeast portion of Yellowstone Lake is associated with numerous hydrothermal vents and hot springs, providing evidence of high-temperature fluid-mineral interaction and phase separation phenomena. Vapor-dominated hydrothermal fluids issuing from Deep Hole vents attain temperatures in excess of 150 °C and are enriched in magmatically derived H₂S and CO₂. Upon mixing with lake water in the root zone of the hydrothermally active vents, the dissolved gases render the mixed fluid, both acidic and reducing, effectively transforming diatomaceous sediment, with detritally sourced Al and Fe components, to an alteration assemblage dominated by kaolinite, pyrite, and lesser boehmite. These alteration processes have been modeled by computer based simulations, coupling fluid flow and mineral dissolution kinetics, to provide insight on the temporal evolution of the vent system. Results predict rapid dissolution of amorphous silica. The magnitude and rate of silica loss, facilitated by the continuous influx of acidic source fluids, yields an increasingly silica poor alteration mineral sequence with time, characterized by quartz, followed by kaolinite and ultimately boehmite. These data are consistent with the observed decrease in SiO₂/Al₂O₃ ratio of the vent deposits with increasing abundance of trace immobile elements, suggesting significant mass loss with reaction progress. Pyrite is predicted to form from sulfidation of magnetite, with noteworthy decrease in magnetic intensity, as measured for hydrothermally altered sediment in the near-field vent environment. Moreover, hydrogen isotope compositional data for kaolinite, together with δD vent fluid data, suggest temperatures in keeping with the high temperatures measured for the vent deposits and discharging fluid, while supporting the potential use of kaolinite as a geothermometer. The predicted and observed transformation of silica-rich protolith to kaolinite, boehmite, and pyrite underscores the large scale dissolution and removal of silica, with possible implications for the temporal evolution of vent deposits on the lake floor in the Stevenson Island Deep-Hole region.

Introduction

Yellowstone National Park (YNP) (Wyoming, USA) has long been recognized as a region of unusually intense volcanic and tectonic activity that is currently centered above the Yellowstone hotspot (Morgan et al., 2009a). Concentrated within the boundaries of the most recent expression of this hotspot—0.64 Ma Yellowstone Caldera—heat and non-condensable gases from the underlying crystallizing magma reservoir interact with deeply circulating meteoric water to produce extensive and geochemically diverse expressions of hydrothermal activity (Fournier, 1989; Chiodini et al., 2012; Hurwitz and Lowenstern, 2014). Largely recognizable, the subaerial geothermal systems of YNP have been extensively studied; however, less is known of the sublacustrine hydrothermal systems of Yellowstone Lake (Shanks III et al., 2007; Fowler et al., 2019b).

Straddling the southeast margin of the 0.64 Ma Yellowstone Caldera, the northern two-thirds of Yellowstone Lake (Fig. 1A), is hydrothermally active, and accounts for ~10% of the total flux of hydrothermally derived components into Yellowstone National Park (YNP) overall (Morgan and Shanks, 2005). The 341 km² basin is composed largely of Quaternary rhyolitic flows, glacial deposits, and lake sediment; with the upper sequence of unaltered basin floor primarily consisting of laminated fine-grained lacustrine sediments. These sediments contain significant amounts of diatomaceous material (Johnson et al., 2003; Morgan et al., 2009b), and include evidence of hydrothermal explosions and hydrothermal activity sufficient to alter the basin morphology (Shanks III et al., 2007). As recently emphasized from high-resolution, multi-scale surveys, hydrothermal activity within the lake is well recognized by magnetic lows within lake floor vent depressions, including linear fissures southeast of Stevenson Island (Bouligand et al., 2020). These northwestern trending fissures, also recognized from ROV observations, and earlier from high-resolution bathymetric data (Johnson et al., 2003; Morgan et al., 2009b), likely focus flow of more deeply sourced hydrothermal fluids, enhancing near-surface alteration, especially in the Deep Hole area, east of Stevenson Island (SI Deep Hole) (Fig. 1B, C). At ~120 m depth, fluids issuing from this region of the lake floor produce a recognizable thermal anomaly in the lake water column, which suggests a hydrothermal fluid influx of ~1.4 × 10³ kg/s and heat flux of 20–50 MW (Sohn et al., 2019). The individual hydrothermal vents present within SI Deep Hole, are among the hottest sub-lacustrine vents to be reported anywhere in the world, with in-situ temperatures of 174 °C (Fowler et al., 2019b). Described as 5–10 cm non-constructional orifices (Fig. 2A), SI Deep Hole hydrothermal vents issue fluids that not only achieve high temperatures but are also CO₂ saturated, as suggested by thermodynamic calculations and the observations of CO₂ bubbles separating from fluids issuing from vents on the lake floor (Fowler et al., 2019b; Tan et al., 2017).

The unique nature of the composition and temperature of the venting fluid is suggestive of a steam-heated hydrothermal system (White et al., 1971; Fowler et al., 2019b). Thus, the SI hydrothermal vent fluids in the Deep Hole region are mixtures of high enthalpy steam and lake bottom water (Fowler et al., 2019b) (Fig. 3). The occurrence of a vapor-dominated hydrothermal system is possible due to an overlying low-permeability substrate acting as a sediment cap to a more permeable reservoir. In general, a sediment cap of low permeability allows steam to escape, while minimizing the influx and quench effects of the ambient lake water (Schubert et al., 1980; Raharjo et al., 2016). Thus, the high enthalpy vapor transfers heat by conduction and advection to the overlying lake water, broadly similar to heat transfer processes of subaerial fumaroles observed elsewhere in YNP (Hurwitz and Lowenstern, 2014). Vapor-dominated subaerial systems are often recognized by noteworthy acidity and high sulfate concentrations (Rowe et al., 1973; Truesdell and White, 1973; Hurwitz and Lowenstern, 2014;. This is not the case, however, for sub-lacustrine Stevenson Island vent fluids owing to the general absence of dissolved O₂, and thus, acidity is provided not from sulfide oxidation, but rather hydrolysis of dissolved CO₂ (Sohn et al., 2017; Fowler et al., 2019b). Accordingly, sub-lacustrine hydrothermal systems at Stevenson Island in Yellowstone Lake might provide a better representation of chemical reactions that may occur within subaerial systems, but at depths hundreds of meters below the land surface (Fowler et al., 2019b, Fowler et al., 2019c).

Here we focus on the composition of minerals sampled from active and inactive vents in the SI area on the floor of Yellowstone Lake. To accomplish this, we use chemical, mineralogical, isotopic, magnetic, and in-situ temperature data, as well as results of 1D numerical simulations, depicting mineral alteration processes and rates of mass transfer. In combination with previously reported vent chemistry (Fowler et al., 2019b, Fowler et al., 2019c), these data provide a comprehensive picture of the chemical and mineralogical evolution of the Stevenson Island sub-lacustrine hydrothermal system in time and space.