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In this paper we examine geochemical processes that control volatile chemistry at depth in mid-ocean ridge environments by focusing on CO2-CH4-H2O-H2 fluids entrapped in plutonic rocks from the Southwest Indian Ridge (SWIR), Ocean Drilling Program Hole 735B. Compositional and isotopic analyses of CO2-CH4-H2O and CH4-H2O-H2 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 % CO2 and 43 mol % CH4. Isotopic analyses of CO2, CH4, and H2O released at >900°C yields δ13C(CO2) values of −24‰ to −2‰, δ13C(CH4) values of −30‰ to −19‰, δD(CH4) values of −244‰ to −128‰, and average δD(H2O) values of −43±6‰. Phase equilibria and isotopic data strongly indicate that the CO2-CH4-H2O fluids reflect Rayleigh distillation of evolved magmatic CO2, subsequent closed-system respeciation, and attendant graphite precipitation at temperatures of ∼500–800°C, and at fO2 from −3 log units below, to close to quartz-fayalite-magnetite oxygen fugacity (QFM) conditions. The second phase of CH4 production involves CH4-H2O±H2±C-fluids that contain >40 mol % CH4. Phase equilibria indicate that the CH4-H2O fluids were trapped under equilibrium conditions at 400°C, very near to QFM conditions. Our study suggests that in the absence of CO2 as a stable fluid component, extensive distillation fractionation or alteration processes are required to form this later generation of methane. The mean δ13C 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 (∼1019 gCH4) 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.