A thermodynamic explanation for black smoker temperatures

There is a remarkable difference between the maximum temperature of black smoker effluent (350 °C–400 °C) and the temperature of the solidifying magma which heats it (∼1,200 °C)1,2,3. It has been suspected4 for some time that the nonlinear thermodynamic properties of water5 might be responsible for this discrepancy. Here, we translate this hypothesis into a physical model, by examining the internal temperature structure of convection cells in a porous medium. We demonstrate that, at pressures appropriate to seafloor crust, plumes of pure water form naturally at ∼400 °C for any heat source with temperature greater than ∼500 °C. Higher temperatures are confined to a boundary layer at the base of the convection cell, where the flow is horizontal. The phenomenon is explained analytically using the thermodynamic properties of water, and is illustrated by numerical simulations. Our model predicts the existence of the high-temperature ‘reaction zone’ found in ophiolites6 and suggests that vent temperatures will remain steady as magma chambers solidify and cool7.