Long-term transport processes across an argillaceous aquitard sequence evidenced by isotope geochemistry of veins

Argillaceous rocks constitute effective natural barriers to fluid flow, making them potential hosts for radioactive waste disposal or seals for gas storage. However, the presence of veins may indicate that in the past, such rocks could have been episodically permeable. This study examines calcite and celestite veins from the clay-rich Jurassic aquitards – particularly the Opalinus Clay – in northern Switzerland. The veins were analysed for δ 18 O, δ 13 C, δ 34 S, 87 Sr/ 86 Sr and clumped isotope thermometry (Δ 47 ) in combination with geochronology (U-Pb and U-Th on calcite), augmented by 1D transport simulations for δ 18 O of porewaters. The main study objectives included the identification of the source and evolution of the mineral-forming fluids, as well as the dominant transport processes. We show that the fluid evolution in these aquitards was mainly determined by changing conditions at the surface with alternating marine and subaerial exposure but also by contributions from underlying units. Transport simulations for δ 18 O show that the observed vein geochemistry can be explained by mineral precipitation from local porewaters that evolved by diffusive exchange with waters in the over- and underlying units. This diffusion-dominated base scenario is consistent with the other isotope tracers, although some ambiguity in the interpretation of C and Sr isotopes remains due to rock-buffering and possible fluid mixing. One locality situated in proximity of the southern Upper Rhine Graben requires a more complex explanation that involves either early recharge of bounding aquifers and/or an impact of nearby Graben faults.