© 2008 Macmillan Publishers Limited. All rights reserved. © 2008 Macmillan Publishers Limited. All rights reserved. LETTERS Large heat and fluid fluxes driven through mid-plate outcrops on ocean crust M. HUTNAK 1 *, A. T. FISHER 1 , R. HARRIS 2 , C. STEIN 3 , K. WANG 4 , G. SPINELLI 5 , M. SCHINDLER 6 , H. VILLINGER 7 AND E. SILVER 1 1 Department of Earth and Planetary Sciences and Institute for Geophysics and Planetary Physics, University of California Santa Cruz, Santa Cruz, California 95064, USA 2 College of Ocean and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA 3 Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois 60607, USA 4 Pacific Geoscience Centre, Geological Survey of Canada, Sydney, BC, V8L 4B2, Canada 5 Department of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico 87801, USA 6 FB Federal Institute for Geosciences and Natural Resources (BGR), 30655 Hannover, Germany 7 FB Geowissenschaften, Universit ¨ at Bremen, Klagenfurter Strasse, 28359 Bremen, Germany *e-mail: mhutnak@pmc.ucsc.edu Published online: 1 August 2008; doi:10.1038/ngeo264 Hydrothermal circulation on the sea floor at mid-ocean ridge flanks extracts ∼30% of heat from the oceanic lithosphere on a global basis 1 and affects 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 × 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. Global estimates of heat, fluid and solute fluxes through mid-ocean ridge flanks have been made using thermal and chemical constraints 7 , but regional fluxes and the properties and processes that control them are poorly understood because of a lack of collocated, high-resolution data sets. Basement outcrops can facilitate advective extraction of lithospheric heat by providing highly permeable conduits that enable fluids to bypass low-permeability sediments 8–11 . The driving forces that sustain outcrop-to-outcrop discharge are limited to tens to a few hundreds of kilopascals, based largely on the difference between fluid pressures at recharging (cool) and discharging (warm) zones in the crust. Modest driving forces and the long distances between recharge and discharge sites require high crustal permeability 9,11,12 , consistent with borehole hydrogeological, tidal, and seismic analyses 13,14 . Previous studies of mid-ocean ridge-flank hydrothermal fluxes focused on individual features (local scale) 10,11,15 or composite data sets from many areas (global scale) 16 , but no earlier studies have shown that widely spaced basement outcrops can mine a large fraction of lithospheric heat on a regional scale. Collocated seafloor bathymetric, seismic-reflection and heat-flux data from a large area of 18–24 million-year-old (Myr) sea floor of the eastern Pacific Ocean, on the Cocos Plate seaward of the Middle America Trench (Fig. 1), provide the foundation for a quantitative assessment of advective heat and fluid fluxes on a regional basis. The methods used to collect and process swath-map, seismic and seafloor heat-flux data from this area are described in detail elsewhere 17 . Swath-mapping across a 50,000 km 2 region achieved 40% spatial coverage, and is overlain on complete bathymetric data coverage from satellite gravimetry 18 (Fig. 1a). Multichannel seismic reflection data were acquired along 3,000 km of profiles. Seafloor heat-flux data were acquired with a 3.5 m, 11-sensor, violin-bow multipenetration probe with in situ thermal conductivity and real- time data telemetry, and with three to five autonomous outrigger probes mounted on core barrels 8,17 . Heat-flux, seismic and nearby drill-core data 19 were combined to extrapolate surface thermal conditions to the sediment–basement interface to map spatial variations in upper basement temperatures (these interpretations and the complete heat-flux data set are provided as Supplementary Information, Table S1). The Cocos Plate has a complex tectonic history in the survey area, where it comprises lithosphere generated at the fast-spreading East Pacific Rise (EPR) and the medium-spreading Cocos-Nazca Spreading Centre (CNS), separated by a plate suture 20 (Fig. 1b). Drilling and seismic data from this area show that sediment is typically 400–500 m thick, except where disrupted by seamounts and other basement outcrops, and comprises mainly pelagic and hemipelagic material 8,17,21 . Basement outcrops are unevenly distributed regionally. Outcrops are relatively common on EPR-generated sea floor northwest of the plate suture (Fig. 1), ranging in diameter from hundreds to thousands of metres (Table 1), and are typically separated by 20–50 km. In contrast, nature geoscience VOL 1 SEPTEMBER 2008 www.nature.com/naturegeoscience 611