THE RELATIONSHIP BETWEEN FLOW AND PERMEABILITY FIELD IN SEA-FLOOR HYDROTHERMAL SYSTEMS

The permeability of oceanic crust is spatially variable and probably a nisotropic as well. Using realistic permeability fields for young ocea nic crust, we have performed numerical simulations of finite amplitude , steady and unsteady convective fluid flow in layered and/or anisotro pic porous media heated from below to investigate particular patterns of fluid flow and temperature in mid-ocean ridge hydrothermal systems. On the flanks of mid-ocean ridges, permeability measurements in deep- sea boreholes suggest that only the top few hundred meters of oceanic crust is permeable. Given this permeability structure (and assuming so me minimum permeability and layer thickness), our models predict that convection occurs in the form of numerous cells with aspect ratios of order unity within this permeable layer. Such convection results in fl uid flux and diagenetic reactions within the permeable layer with a ne gligible effect on heat flow at the seafloor, in agreement with field observations. Estimates of oceanic crust permeability based on hydroth ermal veins in ophiolites and measurements in deep-sea boreholes sugge st that pillow basalts/lava flows are much more permeable than underly ing sheeted dikes. This is particularly true at the ridge crest where voids have not vet collapsed and filled. Given this permeability struc ture, our study suggest that a small percentage of the fluid entering the ridge crest circulates through the sheeted dikes before exiting th e system at high temperatures in very focused discharge zones. In our models, these discharge zones narrow considerably at the interface bet ween the sheeted dikes and the pillow basalts/lava flows. Much of the fluid entering the system, however, never circulates below the pillow basalts/lava flows and exits the seafloor at low temperatures. In gene ral, discharge zones are more focused than recharge zones. These resul ts are consistent with observations of narrow and focused discharge zo nes in ophiolites and localized high-temperature venting amid widespre ad low-temperature flow on the ridge crest. The spacing of upflow zone s at the surface is strongly controlled by convective flow in the bott om permeable layer. In our models, near-field effects dominate over fa r-field effects in systems with lateral variations in permeability, an d no large-scale flow develops between widely spaced areas of contrast ing permeability. The time to steady-state and evolution of convective flow in porous media vary considerably with initial conditions. Our s imulations suggest that the time to steady-state is relatively long an d that it is possible that hydrothermal convection at the ridge axis n ever reaches a steady-state flow pattern, in the sense that the variat ions in the system boundary conditions such as basal heat flux may occ ur on a time scale less than the response time of the hydrothermal sys tem. It is possible, however, that these systems may be quasi-steady f or significant periods of time.