Isotopic anomalies observed at the vicinity of fractures in porewater of Tournemire shales: Experimental artefacts or local palaeo-circulations?

Titre du congrès :Clays in natural and engineered barriers for radioactive waste confinement Ville du congrès :Lille Date du congrès :17/09/2007

Patriarche et al. (2004) acquired in the Toarcian argillaceous formation of Tournemire a vertical profile of stable isotope content of interstitial water. They found a good agreement between these experimental data and calculated values obtained from a pure diffusion model except for samples collected less than one meter from fractures. Samples closely located to fractures displayed a systematic increase of their water stable isotope contents. Two types of hypotheses were proposed for accounting for these discrepancies: (i) some local circulations could have occurred around the fractures and may have affected stable isotope concentrations and (ii) the vacuum distillation technique used for determining the isotope contents may have induced experimental artefacts on these particular samples. In the present study, the influence of fractures on the stable isotope contents was addressed by acquiring an isotopic profile along a fracture by means of several techniques (extraction and equilibration approaches) coupled with mineralogical and petrophysical characterisations of rock samples.