Large heat and fluid fluxes driven through mid-plate outcrops on ocean crust

Abstract

Geophysical data for the Cocos Plate sea floor suggest that basement outcrops along mid-ocean ridge flanks can discharge very large quantities of heat and fluid. This is indicative of high crustal permeability at the regional scale. Hydrothermal circulation on the sea floor at mid-ocean ridge flanks extracts ∼30% of heat from the oceanic lithosphere on a global basis1 and affects numerous tectonic, magmatic and biogeochemical processes2,3,4. However, the magnitude, mechanisms and implications of regional-scale fluid and heat flow on mid-ocean ridge flanks are poorly understood. Here we analyse swath-map, seismic and sea-floor heat-flux data to quantify the heat and fluid discharge through a few widely spaced basement outcrops on the Cocos Plate. Heat removed by conduction from a 14,500 square kilometre region of the sea floor is 60–90% lower than that predicted by lithospheric cooling models. This implies that a substantial portion of the heat is extracted by advection, which requires fluid discharge of 4–80×103 litres per second. The heat output of individual discharging outcrops is inferred to be comparable to that from black-smoker vent fields seen on mid-ocean ridges. Our analysis shows that hydrothermal circulation on mid-ocean ridge flanks through widely spaced outcrops can extract a large fraction of lithospheric heat. This circulation requires a very high crustal permeability at a regional scale. Focused flows of warm, nutrient-rich hydrothermal fluid may enhance sub-seafloor microbial habitats5,6 and enable direct sampling of these systems.