[1] We use one-and two-limb single-pass models to de termine vent field characteristics such as mass flow rate Q, bulk permeability in the discharge zone k d , thickness of the conductive boundary layer at the base of the system d, magma replenishment rate, and residence time in the discharge zone. Data on vent temperature, vent field area, heat output, and the surface area and depth of the subaxial magma chamber (AMC) constrain the models. The results give Q~100 kg/s, k d~1 0 À13 m 2 , and d~10 m, essentially independent of spreading rate, and detailed characteristics of the AMC. In addition, we find no correlation between heat output at individual vent fields and spreading rate or depth to the AMC. We conclude that high-temperature hydrothermal systems are driven by local magma supply rates in excess of that needed for steady state crustal production and that crustal permeability enables hydrothermal circulation to tap magmatic heat regardless of AMC depth. Using data on partitioning of heat flow between focused and diffuse flow, we find that 80-90% of the hydrothermal heat output is derived from high-temperature fluid, even though much of the heat output discharges as low-temperature fluid. In some cases, diffuse flow fluids may exhibit considerable conductive cooling or heating. By assuming conservative mixing of diffuse flow fluids at East Pacific Rise 9 50 0 N, we find that most transport of metals such as Fe and Mn occurs in diffuse flow and that CO 2 , H 2 , and CH 4 are taken up by microbial activity.

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We performed flow‐through laboratory experiments on five cylindrically cored samples of ultramafic rocks, in which we generated a well‐mated through‐going tensile fracture, to investigate evolution of fracture permeability during serpentinization. The samples were tested in a triaxial loading machine at a confining pressure of 50 MPa, pore pressure of 20 MPa, and temperature of 260°C, simulating a depth of 2 km under hydrostatic conditions. A pore pressure difference of up to 2 MPa was imposed across the ends of the sample. Fracture permeability decreased by 1–2 orders of magnitude during the 200–330 h experiments. Electron microprobe and SEM data indicated the formation of needle‐shaped crystals of serpentine composition along the walls of the fracture, and chemical analyses of sampled pore fluids were consistent with dissolution of ferro‐magnesian minerals. By comparing the difference between fracture permeability and matrix permeability measured on intact samples of the same rock types, we concluded that the contribution of the low matrix permeability to flow is negligible and essentially all of the flow is focused in the tensile fracture. The experimental results suggest that the fracture network in long‐lived hydrothermal circulation systems can be sealed rapidly as a result of mineral precipitation, and generation of new permeability resulting from a combination of tectonic and crystallization‐induced stresses is required to maintain fluid circulation.

A vent-field-scale model of the East Pacific Rise 9°50'N magma-hydrothermal system.

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