Permeability changes due to mineral diagenesis in fractured crust: implications for hydrothermal circulation at mid-ocean ridges

The hydrothermal processes at ridge crests have been extensively studied du ring the last two decades. Nevertheless, the reasons why hydrothermal field s are only occasionally found along some ridge segments remain a matter of debate. In the present study we relate this observation to the mineral prec ipitation induced by hydrothermal circulation. Our study is based on numeri cal models of convection inside a porous slot 1.5 km high, 2.25 km long and 120 m wide, where seawater is free to enter and exit at its top while the bottom is held at a constant temperature of 420 degreesC. Since the fluid c irculation is slow and the fissures in which seawater circulates are narrow , the reactions between seawater and the crust achieve local equilibrium. T he rate of mineral precipitation or dissolution is proportional to the tota l derivative of the temperature with respect to time, Precipitation of mine rals reduces the width of the fissures and thus percolation. Using conventi onal permeability versus porosity laws, we evaluate the evolution of the pe rmeability field during the hydrothermal circulation. Our computations begi n with a uniform permeability and a conductive thermal profile. After impos ing a small random perturbation on the initial thermal field, the circulati on adopts a finger-like structure, typical of convection in vertical porous slots thermally influenced by surrounding walls. Due to the strong tempera ture dependence of the fluid viscosity and thermal expansion, the hot risin g fingers are strongly buoyant and collide with the top cold stagnant water layer. At the interface of the cold and hot layers, a horizontal boundary layer develops causing massive precipitation. This precipitation front prod uces a barrier to the hydrothermal flow. Consequently, the flow becomes lay ered on both sides of the front. The fluid temperature at the top of the la yer remains quite low: it never exceeds a temperature of 80 degreesC, well below the exit temperature of hot vent sites observed at black or white 'sm okers'. We show that the development of this front is independent of the Ra yleigh number of the hydrothermal flow, indicating that the mineral precipi tation causes cold, diffusive vents. Finally, we present a model suggesting that the development of smokers is possible when successive tectonic/volca nic events produce a network of new permeable fissures that can overcome th e permeability decrease caused by mineral precipitation. Such a model is co nsistent with recent seismic data showing hydrothermal vents located at sei smologically active ridge segments. (C) 2001 Elsevier Science B.V. All righ ts reserved.