234U/238U Disequilibrium along stylolitic discontinuities in deep Mesozoic limestone formations of the Eastern Paris basin: evidence for discrete uranium mobility over the last 1–2 million years
- 1GEOTOP-UQAM-McGILL, P.O. Box 8888, Succ. Centre-Ville, Montréal, QC, Canada H3C 3P8
- 2CEREGE, Europôle méditerranéen de l'Arbois, BP 80 13 545 Aix-en-Provence, France
- 3Université Blaise Pascal, CNRS (UMR 6524), Observatoire de Physique du Globe de Clermont-Ferrand, 5 Rue Kessler, 63038 Clermont-Ferrand Cedex, France
- 4ANDRA, 1-7 rue Jean Monnet, 92298 Châtenay Malabry, France
- Email for corresponding author: email@example.com
Abstract. The (234U/238) equilibrium state of borehole core samples from the deep, low-permeability limestone formations surrounding the target argilite layer of the Meuse/Haute-Marne experimental site of the French agency for nuclear waste management -ANDRA- (Agence nationale pour la gestion des déchets radioactifs) was examined to improve understanding of naturally occurring radionuclide behaviour in such geological settings. Highly precise, accurate MC-ICP-MS measurements of the (234U/238U) activity ratio show that limestone samples characterised by pressure dissolution structures (stylolites or dissolution seams) display systematic (234U/238U) disequilibria, while the pristine carbonate samples remain in the secular equilibrium state. The systematic feature is observed throughout the zones marked by pressure dissolution structures: (i) the material within the seams shows a deficit of 234U over 238U ((234U/238U) down to 0.80) and (ii) the surrounding carbonate matrix is characterised by an activity ratio greater than unity (up to 1.05). These results highlight a centimetric-scale uranium remobilisation in the limestone formations along these sub-horizontal seams. Although their nature and modalities are not fully understood, the driving processes responsible for these disequilibria were active during the last 1–2 Ma.
Keywords: uranium isotopes, multiple-collector ICP-MS, waste management, remobilisation, migration