Phase separation, brine formation, and salinity variation at Black Smoker hydrothermal systems

We present the first fully transient 2-D numerical simulations of black smoker hydrothermal systems using realistic fluid properties and allowing for all phase transitions possible in the system H2O-NaCl, including phase separation of convecting seawater into a low-salinity vapor and high-salinity brine. We investigate convection, multiphase flow, and phase segregation at pressures below, near, and above the critical point of seawater. Our simulations accurately predict the range in vent salinities, from 0.05 to 2.5 times seawater salinity measured at natural systems. In low-pressure systems at similar to 1500 m water depth, phase separation occurs in boiling zones stretching from the bottom of the hydrothermal cell to the seafloor. Low-salinity vapors and high-salinity brines can vent simultaneously, and transient variations in vent fluid salinities can be rapid. In high-pressure systems at roughly similar to 3500 m water depth, phase separation is limited to the region close to the underlying magma chamber, and vent fluids consist of a low-salinity vapor mixed with a seawater-like fluid. Therefore, vent salinities from these systems are much more uniform in time and always below seawater salinity as long as phase separation occurs in the subseafloor. Only by shutting down the heat source can, in the high-pressure case, the brine be mined, resulting in larger than seawater salinities. These numerical results are in good agreement with long-term observations from several natural black smoker systems.