Geochemical evolution of a sublacustrine hydrothermal vent system: Temporal constraints imposed by reactive transport modeling

Studies of heat and mass transport in hydrothermal systems are challenging owing to the complex feedback that exists between chemical and physical processes. Taking full advantage of the existing geochemical framework recently recognized for sublacustrine vent fluids in the Stevenson Island Deep Hole, Yellowstone Lake, here we apply a multiphase reactive transport model, coupling hydraulic and thermal effects, to simulate time series changes in mineralization associated with venting of vapor-dominated hydrothermal fluids. This distinctive region of the lake floor is characterized by a series of overlapping depressions hosting sites of active venting. Model results show that the addition of high enthalpy steam enriched in CO2 and H2S, creates acidic and reducing conditions, while heating entrained lake water and coexisting sediment to temperatures in excess of 174 °C. The diatomaceous silica-rich substrate is predicted to dissolve, being replaced largely by pyrite, quartz, and kaolinite, in good agreement with the observed vent mineralization and fluid chemistry. Data suggest that vent-related mineral dissolution and transformation processes occur rapidly, on the order of 1 ka for the spatial scale of the modeled-up flow zone considered. The accuracy of time series changes predicted however, is explicitly linked to a number of physical and chemical factors imposed on the model, including source fluid, flow rate and gas enrichment, and sediment thermal conductivity, permeability, and reactive surface area, as well as mineral reaction rate data for primary and secondary alteration phases. When combined with robust field observations, as is the case here, reactive transport models can elucidate important, if not fundamental, controls on the temporal and spatial evolution of the geochemistry of modeled system, challenging chemical, and physical conditions notwithstanding.