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Phase separation of seawater is an important process controlling the dynamics and chemistry of hydrothermal circulation. We numerically investigate hydrothermal circulation in porous media, including phase separation of seawater. Seawater enters the crust at the seafloor, is heated at depth, and returns to the seafloor as hydrothermal fluids. The seafloor and the bottom of the calculation region are set at depths of 2500 m and 4000 m from the sea surface, respectively. The temperature at the base of the calculation region is set at 600°C. Under these pressure and temperature ranges, supercritical phase separation is inevitable. Here we focus on steady-state conditions, as a first step to investigate the complex process of convection with phase separation. Under these conditions, we demonstrate that phase separation leads to a two-layer structure. Seawater circulates vigorously in the upper layer, and this overlies a stagnant lower layer formed by sinking of dense brine. We find that the key quantity which governs this structure is the ratio of the relative velocity between the two phases to the mean flow velocity in the transition zone between the two layers. As the relative velocity increases, the brine layer becomes thick, and the transition zone becomes thin. Under steady state conditions, the mean salinity at the seafloor should be the same as that of seawater because the total mass of salt should be conserved. Fluids which vent near the ridge axis are more saline than seawater, whereas fluids which vent more than about 100 m away from the axis are less saline than seawater.

Numerical simulations of mid-ocean ridge hydrothermal circulation including the phase separation of seawater