The relationship between flow and permeability field in seafloor hydrothermal systems

Abstract

The permeability of oceanic crust is spatially variable and probably anisotropic as well. Using realistic permeability fields for young oceanic crust, we have performed numerical simulations of finite amplitude, steady and unsteady convective fluid flow in layered and/or anisotropic 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 some 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 fluid flux and diagenetic reactions within the permeable layer with a negligible effect on heat flow at the seafloor, in agreement with field observations. Estimates of oceanic crust permeability based on hydrothermal veins in ophiolites and measurements in deep-sea boreholes suggest that pillow basalts/lava flows are much more permeable than underlying sheeted dikes. This is particularly true at the ridge crest where voids have not yet collapsed and filled. Given this permeability structure, our study suggest that a small percentage of the fluid entering the ridge crest circulates through the sheeted dikes before exiting the system at high temperatures in very focused discharge zones. In our models, these discharge zones narrow considerably at the interface between 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 general, discharge zones are more focused than recharge zones. These results are consistent with observations of narrow and focused discharge zones in ophiolites and localized high-temperature venting amid widespread low-temperature flow on the ridge crest. The spacing of upflow zones at the surface is strongly controlled by convective flow in the bottom permeable layer. In our models, near-field effects dominate over far-field effects in systems with lateral variations in permeability, and no large-scale flow develops between widely spaced areas of contrasting permeability. The time to steady-state and evolution of convective flow in porous media vary considerably with initial conditions. Our simulations suggest that the time to steady-state is relatively long and that it is possible that hydrothermal convection at the ridge axis never reaches a steady-state flow pattern, in the sense that the variations in the system boundary conditions such as basal heat flux may occur on a time scale less than the response time of the hydrothermal system. It is possible, however, that these systems may be quasi-steady for significant periods of time.