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

¹ Sonderforschungsbereich 574, Kiel University, Wischhofstrasse 1-3, 24148 Kiel, Germany, and GEOMAR, Wischhofstrasse 1-3, 24148 Kiel, Germany
² GEOMAR, Wischhofstrasse 1-3, 24148 Kiel, Germany, and Sonderforschungsbereich 574, Kiel University, Wischhofstrasse 1-3, 24148 Kiel, Germany
³ Institute of Geosciences, Kiel University, Ludewig-Meyn-Strasse 10, 24118 Kiel, Germany, and Sonderforschungsbereich 574, Kiel University, Wischhofstrasse 1-3, 24148 Kiel, Germany
⁴ GEOMAR, Wischhofstrasse 1-3, 24148 Kiel, Germany, and Sonderforschungsbereich 574, Kiel University, Wischhofstrasse 1-3, 24148 Kiel, Germany

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 ¹⁸O vs. D values point to clay-mineral dehydration as the cause for these anomalies. Calculations assuming a ¹⁸O vs. D equilibrium between the pore fluid and clay minerals at depth of formation indicate temperatures of dehydration between 85 and 130 °C. This temperature range is in agreement with the B enrichments and the presence of thermogenically formed CH₄. Because temperatures above 50 °C 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 12 km depth. Mound-related fluid expulsion may contribute significantly to the recycling of mineral-bound water.

Key Words: fluid dynamics • isotope geochemistry • pore water • chlorinity • active margins • Costa Rica

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