Pore waters from serpentinite seamounts in the Mariana and Izu-Bonin Forearcs, Leg 125: evidence for volatiles from the subducting slab

Abstract

Sediment pore fluids sampled by drilling in subduction zones are commonly depeleted in chloride relative to seawater and highly enriched in methane. These features have been attributed to processes within the accretionary prism of sediment that is typical of most subduction zones. Sites 778 through 780 in the Mariana forearc yielded low-chlorinity pore fluids rich in methane, ethane, and propane from a subduction zone setting that lacks an accretionary prism. The absence of an accretionary prism in the Mariana forearc severely constrains the origin of the fluids upwelling through Conical Seamount. Their freshening relative to seawater does not result from uptake of chloride into solids during serpentinization, but it requires instead a source of H2O deeper than was drilled. The fluids at Conical Seamount probably originate at the top of the downgoing slab, 30 km below the seafloor, by heating of the sediments and basalt of the subducted oceanic crust. -from Author