Analyses of fluid inclusions in plutonic rocks recovered from the slow-spreading Southwest Indian Ridge (SWIR) record CH4 concentrations of 15-40 times those of hydrothermal vent fluids and of basalt-hosted volcanic gases and provide the first direct sampling of CO2-CH4-H2O-H-2-C-bearing fluids in the oceanic crust. Compositional, thermal, and spatial analyses of these fluids are used to model the evolution of volatiles during the crystallization and cooling of mid-ocean ridge magma chambers and to assess the potential importance of carbon-bearing fluids in geochemical processes in the lower crustal component of hydrothermal systems. Results from these analyses show that the earliest fluids to be exsolved from the melts are dominated by CO2-rich vapors, which with progressive fractionation evolved to more H2O-rich compositions. These later fluids were most likely exsolved under immiscible conditions and involved the development of CO2-H2O-rich vapors and CO2-H2O-NaCl brines that were trapped during mineral growth, as well as during later high-temperature fracturing events. CO2+CH4+/-H2O-rich fluids in olivine and plagioclase minerals that contain up to 30-50 mol % CO2 and 33 mol % CH4 may reflect respeciation of magmatic CO2 during cooling and attendant graphite precipitation. Phase equilibria suggest that these fluids reequilibrated at similar to 500 degrees-600 degrees C and at fO(2)'s similar to -3 log units below, to close to QFM conditions. Alternatively, the inclusions may record respeciation of magmatic fluids attendant with the inward diffusion of H-2 into the inclusions and reduction of entrapped CO2 during degassing and cooling of the gabbros throughout the subsolidus regime. Subsequent seawater reaction, at minimum temperatures of similar to 400 degrees C, with mafic-rich layers within the gabbroic rocks or with ultramafic material that underlies the plutonic sequence resulted in the formation of CH4-H2O fluids that contain up to 40 mol % CH4, molecular H-2, and graphite(?) daughter 2 minerals. These data provide strong evidence that the CH4-H2O-rich fluids produced during serpentinization reactions were trapped under equilibrium conditions in the presence of graphite at very near to QFM conditions. The ubiquitous occurrence of CH4-rich fluids in oceanic crustal layer 3 rocks from the SWIR indicates that these fluids may be a significant and previously unrecognized source for these volatile species in some hydrothermal systems venting on the seafloor and that the deep-seated fluids most likely play an important role in the transfer of carbon from the lithosphere to the hydrosphere.
