Strong impact of ion filtration on the isotopic composition of chlorine in young clay-rich oceanic sediment pore fluids

A collection of 34 pore fluids from young clay-rich sedimentary piles of the Shikoku basin on the Philippine oceanic plate at the front of the Nankai subduction zone, southeast of Japan, were analyzed for their chloride content and chlorine stable isotope composition (35Cl and 37Cl, reported as δ37Cl). These pore fluids are samples from IODP Sites C0011 and C0012, which penetrate deeper than 500 meters below the seafloor. At both sites, the δ37Cl values display a regular decrease with depth from about 0‰ (seawater value) at the seafloor to −8.5 and −6.5‰ respectively in hydrogeologically isolated, low permeability, sediment levels. At some depths, local deviations of up to 6‰ in δ37Cl interrupt the overall decreasing trends. They may be interpreted as the injection of external fluids into the sedimentary piles as a result of sedimentary levels with higher permeability. At the bottom of the sedimentary piles, the injected fluids, probably originating from the altered oceanic crust, systematically have δ37Cl values closer to seawater. Literature data indicate the display of downward trends of decreasing δ37Cl values of the pore fluids in many other young clay-rich sedimentary piles (Nankai Trough and the Japan Trench accretionary prisms, Black Ridge and Juan de Fuca Ridge flank). Here, we use conservation equations to demonstrate that these δ37Cl depth profiles can be modeled using the effect of the compaction of growing clay–rich sedimentary piles on the water and chlorides (including 35Cl and 37Cl isotopes). This modeling shows that a small isotope fractionation of chlorine isotopes, α37Cl/35Clexpulsed fluid-residual fluid, in the range of 1.006 to 1.001, may explain the observed negative δ37Cl chlorides at the bottom of the sedimentary piles. The complementary positive δ37Cl chlorides must accumulate at the top but are very likely erased by dilution with seawater chlorides across the seafloor interface. This range of chlorine isotope fractionation values is in agreement with the theory of ion filtration of Phillips and Bentley (1987) in which the mobility of chlorides through semi-permeable clay membrane is determined by ion repulsion. Based on this study, we show that young pore fluids in clay-rich sediments of the oceanic crust are 37Cl-depleted chloride reservoirs. They might be the source of 37Cl-depleted chloride fluids advected at mud volcanoes on the seafloor but do not seem to contribute largely to subduction zone magmatic, tectonic, metamorphic products where no such 37Cl-depleted chlorine is documented. This in turn is a strong argument to propose that most of the clay-rich sediment’s pore waters are released back into the ocean rather than subducted.