Chlorine isotopes to alter our view of pore fluids in the sediments of the oceanic crust, using the archives of ODP-IODP

Pore fluids are stored in oceanic sediments. They are ancient seawater more or less geochemically modified by diagenetic and fluid transport processes. In this regard, they are of great help for reading the history of the host sedimentary pile. Within the geochemical tools available for this, chlorides, although a major constituent of pore fluids, received little attention because it is believed to behave conservatively in pore fluids. Indeed, low-T minerals and organic matter may contain Cl^-, but only in small amounts (≤ 1000 ppm) in comparison to pore fluids (>> 6000 ppm). Measurements of the Cl isotopes (δ³⁷Cl ie. ³⁷Cl/³⁵Cl) of chlorides of pore fluids with CH₃Cl (Eggenkamp, 1994 & Godon et al., 2004) can now be performed with a very high level of precision (s = ± 0.03 ‰). This allowed us to reveal very large range of variations of δ³⁷Cl in pore fluids (from +0.2 down to -8.5 ‰), at odds with the idea that chlorides behave conservatively. In young clay-rich sedimentary piles from various sedimentary and tectonic contexts: rifted continental margins (ODP Leg 105); rifted oceanic platform (ODP Leg 121); Ridge flank sediments (ODP Leg 168) and Nankai subduction zone (IODP Legs 322 & 333), chlorides are systematically ³⁷Cl-depleted. This depletion is linked to the degree of compaction of the sediments and may be to smectites with large cation-exchange capacity (CEC) where ion filtration occurs (Agrinier et al., 2019). In contrast, the serpentine-dominated sites, with weak CEC, (Serpentine mud volcano basements, IODP Leg 366) have pore fluid chlorides unchanged from seawater d³⁷Cl or slightly ³⁷Cl-enriched. These results may point to a first order lithologic control on the efficiency of ion filtration in sedimentary piles.

Refs Eggenkamp H., 1994. δ³⁷Cl: The Geochemistry of Chlorine Isotopes Ph.D. Thesis. Utrecht University, Utrecht. Godon A., et al., 2004, Chem. Geol. 207, 1-12. Agrinier et al., 2019, Geochim. Cosmo. Acta. 245, 525-541.