Natural fluids with a pH (25°C) up to 12.3 were collected from a sub-seafloor borehole observatory (Ocean Drilling Program (ODP) Hole 1200C) on South Chamorro Seamount, a serpentinite mud volcano in the Mariana forearc. We used systematic differences in the chemical compositions of pore waters from drilling operations during ODP Leg 195 and borehole fluids collected subsequently from Hole 1200C to define two endmember solutions, one of which was a sulfate-rich fluid with a methane concentration of 50 mM that ascends from the subduction channel and the other was a low-sulfate fluid. The sequence of sample collection and fluid compositions constrain subsurface hydrologic conditions. Deep-sourced, sulfate- and methane-rich, sterile fluids from the subduction channel can reach the seafloor unchanged within the central conduit, whereas other fluid pathways likely intersect the pelagic sediment that underlies the serpentinite mud volcano, providing potentially suitable conditions and inoculum for microbial anaerobic oxidation of methane (AOM). These AOM-affected, low-sulfate fluids also make it to the seafloor where they discharge. The source of the sulfate- and methane-rich fluid in the subduction channel is attributed to abiotic methane production fueled by hydrogen production from serpentinization and carbonate dissolution. This methane production includes a mechanism to raise the pH above values from serpentinization alone. Results from South Chamorro Seamount represent an end member along a transect defined by the distance from the trench. Results from this site are applied to other serpentinite mud volcanoes along this transect to speculate on likely chemical conditions within shallower and cooler portions of the subduction channel.