Abiogenic methane in deep‐seated mid‐ocean ridge environments: Insights from stable isotope analyses

In this paper we examine geochemical processes that control volatile chemistry at depth in mid‐ocean ridge environments by focusing on CO 2 ‐CH 4 ‐H 2 O‐H 2 fluids entrapped in plutonic rocks from the Southwest Indian Ridge (SWIR), Ocean Drilling Program Hole 735B. Compositional and isotopic analyses of CO 2 ‐CH 4 ‐H 2 O and CH 4 ‐H 2 O‐H 2 fluids show that methane production involved two phases of magma‐hydrothermal activity, which spanned supersolidus to greenschist facies metamorphic conditions. The first phase of methane generation is characterized by fluid inclusions that contain up to 30–50 mol % CO 2 and 43 mol % CH 4 . Isotopic analyses of CO 2 , CH 4 , and H 2 O released at >900°C yields δ 13 C(CO 2 ) values of −24‰ to −2‰, δ 13 C(CH 4 ) values of −30‰ to −19‰, δD(CH 4 ) values of −244‰ to −128‰, and average δD(H 2 O) values of −43±6‰. Phase equilibria and isotopic data strongly indicate that the CO 2 ‐CH 4 ‐H 2 O fluids reflect Rayleigh distillation of evolved magmatic CO 2 , subsequent closed‐system respeciation, and attendant graphite precipitation at temperatures of ∼500–800°C, and at f O2 from −3 log units below, to close to quartz‐fayalite‐magnetite oxygen fugacity (QFM) conditions. The second phase of CH 4 production involves CH 4 ‐H 2 O±H 2 ±C‐fluids that contain >40 mol % CH 4 . Phase equilibria indicate that the CH 4 ‐H 2 O fluids were trapped under equilibrium conditions at 400°C, very near to QFM conditions. Our study suggests that in the absence of CO 2 as a stable fluid component, extensive distillation fractionation or alteration processes are required to form this later generation of methane. The mean δ 13 C values of methane extracted at 500°C from the gabbros (−25±4.4‰) are remarkably similar to the range of light carbon observed in studies of mantle rocks. We conclude that the presence of reduced carbon species in oceanic gabbros and mantle peridotites is a potential source of carbon in hydrothermal fluids and that serpentinization processes play a key role in the production of methane at greenschist facies conditions. Although total methane concentrations are low (0.3–0.6 mmol/kg) in the SWIR samples, on a global scale, plutonic layer 3 comprises ∼60% of the oceanic crust and thus represents a potentially immense reservoir (∼10 19 gCH 4 ) for abiogenic methane in mid‐ocean ridge hydrothermal systems. Production of methane and hydrocarbon species should be a common process in mid‐ocean ridge systems where high‐temperature fluids interact with mafic mineral phases. This is particularly significant because carbon‐bearing fluids may provide sustenance to subsurface‐and vent‐associated microbial communities and therefore represent an important link between deep‐seated hydrothermal systems and more shallow crustal environments.