Numerical simulations of single-pass hydrothermal convection at mid-ocean ridges: Effects of the extrusive layer and temperature-dependent permeability

[1] We develop numerical models of hydrothermal convection at oceanic spreading centers to understand the interplay between deeply circulating high-temperature hydrothermal fluid and cooler seawater circulating in basalts of the upper crust. We assume the deep circulation follows an idealized single-pass geometry and consider the effects of the thickness h and permeability of the extrusive layer k(ext) both on the shallow circulation and on the temperature and heat output of the high-temperature discharge. We also attempt to model the effect of mineral precipitation on mixing in the shallow crust by emplacing a low-permeability vertical barrier in the extrusives to separate the high-temperature discharge from the circulation in the extrusives. Finally, we investigate the effects of temperature-dependent permeability on the mixing scenarios. The results show that maximum discharge temperature T-v is impacted more by the ratio k(ext)/k(d), where k(d) is the permeability of the deep discharge channel, than by h. Generally, high-temperature discharge ( T-v < 250 degrees C) occurs provided kd > kext. In this case, the presence of a low-permeability barrier further enhances Tv. Low-temperature discharge ( T-v < 150 degrees C) can occur provided kext > 10k(d). For systems such as the Galapagos Spreading Center, where vent temperatures are -20 degrees C, k(ext)/k(d) > 10(4), and the extrusive layer is likely to be thick. The results also suggest that sites of diffuse flow will occur either between high-temperature vents along the ridge axis or off axis. The chemical composition of the fluid at these distal sites would be seawater, perhaps modified by low-temperature water-rock reactions. In contrast, the diffuse flow fluids near high-temperature vents are mixture of seawater with high-temperature hydrothermal fluid. Finally, the results show that the 150 degrees C isotherm, which lies nearly horizontally at some distance from the discharge channel, may be within the extrusive layer, near the extrusive-dike interface, or within the low-permeability dike layer. This result supports the idea that the seismically defined layer 2A-2B boundary within the oceanic crust may represent a mineral precipitation front rather than a lithologic boundary.