Thermal convection in a vertical permeable slot: Implications for hydrothermal circulation along mid-ocean ridges

Hydrothermal vents along unsedimented portions of mid-ocean ridges are fed by flow confined to a 1-3 km high percolating system of fissures subparalle l to the spreading center. We have solved the coupled thermal and flow equa tions in such a system. We assume that hydrothermal circulation occurs in a vertical porous slot 1.5 km high, 2.3 km long, and at most 100 m wide. In this model we take into account the heat transported by the hydrothermal fl ow inside the porous slot as well as the heat conducted through the surroun ding impermeable crustal layer. The fluid is free to enter or leave the top of the slot. The fluid enters the slot at a temperature of 2 degrees C, wh ile the bottom of the porous slot is held at a constant temperature of 420 degrees C. We first consider a vertical slot with a horizontal base. Our ca lculations show the development of tall and narrow unsteady convective cell s. Because of the nonlinear relationship of the viscosity and density of se awater with temperature, the upper thermal boundary layer is much thicker t han the lower one. The fluid advected in the hot plumes tends to remain con fined inside the slot because the upper boundary is thick and viscous. In h igh Rayleigh number experiments, some very active hot plumes are able to th in and pierce the upper cold boundary layer, and vent hot fluids at the sea floor. The maximum temperature at:the exit of these plumes reaches a value at most equal to 270 degrees C. The presence of an axial magma chamber alon g most of the length of the East Pacific Rise may justify the use of a hori zontal base to model hydrothermal systems along fast spreading ridges. Howe ver, where the hydrothermal systems develop above a short-lived magmatic in trusion a few kilometers long, the lower interface of the percolating syste m of fissures is inclined. In that case our calculations show that the conv ective flow tends to be unicellular and steady state with a wide region of downwelling and a narrow upwelling zone. The temperature of the fluid exiti ng the slot at the top reaches a maximum value of 360 degrees C. These resu lts lead us to suggest that hydrothermal plumes generated inside a fracture with a horizontal base give rise to vents which wander along the fracture plane with a maximum temperature of about 270 degrees C. However, when the base of the fracture plane is inclined, a large hydrothermal steady plume i s generated which can give rise, at its exit, to a black smoker with fluid temperatures in excess of 350 degrees C.