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

Hydrothermal circulation on the sea floor at mid-ocean ridge flanks extracts similar to 30% of heat from the oceanic lithosphere on a global basis(1) and a V ects numerous tectonic, magmatic and biogeochemical processes(2-4). 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 x 10(3) 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 habitats(5,6) and enable direct sampling of these systems.