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

Abstract

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 kext 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 Tv is impacted more by the ratio kext/kd, where kd is the permeability of the deep discharge channel, than by h. Generally, high‐temperature discharge (Tv > 250°C) occurs provided kd > kext. In this case, the presence of a low‐permeability barrier further enhances Tv. Low‐temperature discharge (Tv < 150°C) can occur provided kext > 10kd. For systems such as the Galapagos Spreading Center, where vent temperatures are ∼20°C, kext/kd > 104, 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°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.