Stable isotope composition of CO2 outgassed from cores of argillites : A simple method to constrain δO of porewater and δC of dissolved carbon in mudrocks

Stable isotope characterization of porewater, and dissolved species, in mudrocks and argillaceous rocks is notoriously difficult. Techniques based on physical or chemical extraction of porewater may generate significant analytical artefacts. We report a novel, simple approach to determine the δ 18 O of porewater and δ 13 C of dissolved carbon in argillites. The method uses core samples placed in specifically-designed outgassing cells, sealed shortly after drilling and stored in well-controlled conditions. After 1-2 months, CO 2 naturally outgassed by argillite porewater is collected, purified and analyzed for C and O isotopes. Porewater δ 18 O and dissolved carbon δ 13 C are calculated from CO 2 isotope data using appropriate fractionation factors. This methodology was successfully applied to the Callovo-Mont Terri, Switzerland. The main advantage of the approach is that it does not induce any major physical or chemical disturbance to the clay-water system investigated.