Fluid expulsion related to mud extrusion off Costa Rica - A window to the subducting slab

A large number of mound-shaped structures that originated from mud extrusions is present along the convergent continental margin off Costa Rica and Nicaragua. Active fluid venting is indicated by the existence of CH4- and H2S-rich pore fluids as well as associated benthic fauna and authigenic carbonates. End-member fluid samples from all mounds are significantly depleted in dissolved Cl and other major elements, suggesting a general process of freshwater addition and thus a common source of the fluids. Our data clearly rule out dilution by gas hydrate dissociation as a dominant source of the freshwater. Enrichments of the fluids in B (up to 2 mmol/L) and inversely correlated delta(18)O VS. deltaD values point to clay-mineral dehydration as the cause for these anomalies. Calculations assuming a delta(18)O vs. deltaD equilibrium between the pore fluid and clay minerals at depth of formation indicate temperatures of dehydration between 85 and 130 degreesC. This temperature range is in agreement with the B enrichments and the presence of thermogenically formed CH4. Because temperatures above 50 degreesC are not reached within the sediment cover of the upper plate, the fluids most likely form within the subducted sediments and flow upward along deep-seated faults from greater than or equal to12 km depth. Mound-related fluid expulsion may contribute significantly to the recycling of mineral-bound water.