Clumped isotopologue constraints on the origin of methane at seafloor hot springs

Hot-spring fluids emanating from deep-sea vents hosted in unsedimented ultramafic and mafic rock commonly contain high concentrations of methane. Multiple hypotheses have been proposed for the origin(s) of this methane, ranging from synthesis via reduction of aqueous inorganic carbon (Sigma CO2) during active fluid circulation to leaching of methane-rich fluid inclusions from plutonic rocks of the oceanic crust. To further resolve the process(es) responsible for methane generation in these systems, we determined the relative abundances of several methane isotopologues (including (CH3D)-C-13, a clumped isotopologue containing two rare isotope substitutions) in hot-spring source fluids sampled from four geochemically-distinct hydrothermal vent fields (Rainbow, Von Damm, Lost City, and Lucky Strike). Apparent equilibrium temperatures retrieved from methane clumped isotopologue analyses average 310(-42)(+53) degrees C, with no apparent relation to the wide range of fluid temperatures (96-370 degrees C) and chemical compositions (pH, [H-2], [Sigma CO2], [CH4]) represented. Combined with very similar bulk stable isotope ratios (C-13/C-12 and D/H) of methane across the suite of hydrothermal fluids, all available geochemical and isotopic data suggest a common mechanism of methane generation at depth that is disconnected from active fluid circulation. Attainment of equilibrium amongst methane isotopologues at temperatures of ca. 270-360 degrees C is compatible with the thermodynamically-favorable reduction of CO2 to CH4 at temperatures at or below ca. 400 degrees C under redox conditions characterizing intrusive rocks derived from sub-ridge melts. Collectively, the observations support a model where methane-rich aqueous fluids, known to be trapped in rocks of the oceanic lithosphere, are liberated from host rocks during hydrothermal circulation and perhaps represent the major source of methane venting with thermal waters at unsedimented hydrothermal fields. The results also provide further evidence that water-rock reactions occurring at temperatures lower than 200 degrees C do not contribute significantly to the quantities of methane venting at mid-ocean ridge hot springs. (C) 2017 Elsevier Ltd. All rights reserved.