YELLOWSTONE LAKE HYDROTHERMAL FLUIDS: SYSTEMATIC VARIATIONS IN CHEMICAL AND PHYSICAL PROPERTIES CONTROL SEDIMENT ALTERATION AND HYDROTHERMAL EXPLOSIONS

Yellowstone Lake is an extremely active hydrothermal area on the Yellowstone Plateau with very high heat flow, multiple large (>100 m diameter) hydrothermal explosion craters, dozens of hydrothermal domes, and hundreds of active and currently inactive subaqueous hydrothermal vents. Hydrothermally altered muds from the active and deepest (~119 m depth) hydrothermal vent area in the lake (the Stevenson Island Deep Hole) and an active hydrothermal dome in the northern lake both have low-Cl vent fluids, less than average lake water at 140 µm/L (5 mg/L). Their alteration mineral assemblages of smectite, kaolinite, chlorite, boehmite, pyrite, anhydrite, and quartz are typical of vapor-dominated alteration in Yellowstone. Stable isotope (δD and δ¹⁸O) values of vent and pore fluids in the Deep Hole vent field are consistent with hot, ascending vapor-dominated fluids (H₂O, CO₂, and H₂S) mixing with lake water trapped during sedimentation or entrained during venting.

In contrast, hydrothermal explosion deposits from the large Elliott’s Crater and Mary Bay explosion craters are related to liquid-dominated, Cl-rich fluids (called alkaline-chloride) that carry high dissolved silica concentrations due to water-rock interaction at high temperatures at depth. The explosion deposits contain chalcedony, quartz, albite, smectite, chlorite, pyrite, and anhydrite and lack kaolinite. The δ¹⁸O values of bulk sediment indicate pre-explosion alteration temperatures of 150-300^oC.

Hydrothermal alteration studies indicate low- and high-Cl fluids produce distinctly different mineral assemblages and produce hydrothermal explosions of systematically variable power. Alkaline-Cl fluids flashing from liquid to steam in the Elliott’s Crater and Mary Bay systems produced the largest hydrothermal explosion craters and deposits known on Earth. Vapor-dominated systems in Yellowstone Lake explode by simple expansion of the steam phase, producing less voluminous and less extensive deposits.