Methane‐rich fluids in the oceanic crust

Analyses of fluid inclusions in plutonic rocks recovered from the slow‐spreading Southwest Indian Ridge (SWIR) record CH 4 concentrations of 15–40 times those of hydrothermal vent fluids and of basalt‐hosted volcanic gases and provide the first direct sampling of CO 2 ‐CH 4 ‐H 2 O‐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 CO 2 ‐rich vapors, which with progressive fractionation evolved to more H 2 O‐rich compositions. These later fluids were most likely exsolved under immiscible conditions and involved the development of CO 2 ‐H 2 O‐rich vapors and CO 2 ‐H 2 O‐NaCl brines that were trapped during mineral growth, as well as during later high‐temperature fracturing events. CO 2 + CH 4 ± H 2 O‐rich fluids in olivine and plagioclase minerals that contain up to 30–50 mol % CO 2 and 33 mol % CH 4 may reflect respeciation of magmatic CO 2 during cooling and attendant graphite precipitation. Phase equilibria suggest that these fluids reequilibrated at ∼500°–600°C and at f O 2 's ∼ −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 CO 2 during degassing and cooling of the gabbros throughout the subsolidus regime. Subsequent seawater reaction, at minimum temperatures of ∼400°C, with mafic‐rich layers within the gabbroic rocks or with ultramafic material that underlies the plutonic sequence resulted in the formation of CH 4 ‐H 2 O fluids that contain up to 40 mol % CH 4 , molecular H 2 , and graphite(?) daughter minerals. These data provide strong evidence that the CH 4 ‐H 2 O‐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 CH 4 ‐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.