Chemical and isotopic characterization of water–rock interactions in shales induced by the intrusion of a basaltic dike: A natural analogue for radioactive waste disposal

Disposal of nuclear waste in deep geological formations is expected to induce thermal fluxes for hundreds of years with maximum temperature reaching about 100–150 °C in the nearfield argillaceous environment. The long-term behavior of clays subjected to such thermal gradients needs to be perfectly understood in safety assessment considerations. In this respect, a Toarcian argillaceous unit thermally disturbed by the intrusion of a 1.1-m wide basaltic dike at the Perthus pass (Herault, France), was studied in detail as a natural analogue. The thermal imprint induced by the dike was evaluated by a mineralogical, chemical and K–Ar study of the <2 μm clay fraction of shale samples collected at increasing distance from the basalt. The data suggest that the mineral composition of the shales was not significantly disturbed when the temperature was below 100–150 °C. Closer to the dike at 150–300 °C, changes such as progressive dissolution of chlorite and kaolinite, increased content of the mixed layers illite–smectite with more illite layers, complete decalcification and subsequent increased content of quartz, were found.