YELLOWSTONE LAKE: AN EXTREME LAKE INFLUENCED BY SUBLACUSTRINE HOT SPRINGS AND THE YELLOWSTONE MAGMATIC SYSTEM

Bathymetric, seismic reflection, and aeromagnetic mapping of Yellowstone Lake shows that the area inside the 0.64-Ma Yellowstone Caldera hosts multiple post-caldera rhyolitic lava flows, active faults and fissures, hot spring vents and sinter deposits, hydrothermal explosion craters, and hydrothermally altered sediments. Yellowstone hydrothermal fluids originate predominantly from deep circulating meteoric waters above the magmatic system and are probably depth-limited by the brittle-ductile transition at temperatures of 370-400°C, at depths of about 3-4 km. Brines (generated by phase separation near the magma chamber) and degassed magmatic volatiles (CO₂, H₂S, HCl) mix with circulating meteoric waters, resulting in a 350°C, CO₂- and H₂S-rich, deep thermal-reservoir fluid with about 310 mg/kg Cl. The deep reservoir fluids boil during ascent due to depressurization and may mix with local fluids. Sublacustrine hydrothermal vent fluids, sampled using a submersible remotely operated vehicle (ROV), show δD-Cl systematics that indicate the ascending fluids boil to ≥220°C with Cl increasing to ≤570 mg/kg and then mix with Yellowstone Lake water at or just below lake-floor hydrothermal vent sites.

The geochemical composition of Yellowstone Lake water is strongly influenced by sub-lacustrine hydrothermal activity and magmatic volatiles, with strong enrichment in dissolved As, B, Cl, Cs, Ge, Li, Mo, Sb, and W relative to the weighted average of inflowing stream waters. Geochemical reaction modeling indicates that the composition of ascending hydrothermal fluids is controlled by reaction with sub-lacustrine volcanic rocks and diatom-rich sediments, equilibrating with alteration minerals at 190-280ºC. Yellowstone Lake water is a mixture of inflowing surface water, hydrothermal fluid, and magmatic volatiles. Magmatic H₂S is the dominant sulfur source in the lake as indicated by sulfur isotope values of hydrothermal vent H₂S (δ³⁴S = -0.5 to 3.6) and SO₄, derived from H₂S oxidation, in lake waters (δ³⁴S = 2.8 to 3.9). Similarly, carbon isotope values of dissolved HCO₃ in lake water and vent fluids (δ¹³C = -1.5 to -8.8) indicate a strong component of magmatic CO₂. Hydrothermal flux estimates based on Cl indicate that ~10% of the total hydrothermal fluid flux in Yellowstone National Park occurs in Yellowstone Lake.