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

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Deschamps, P. ; Hillaire-Marcel, C. ; Michelot, J.-L. ; Doucelance, R. ; Ghaleb, B. ; Buschaert, S. (2004)

The (<sup>234</sup>U/<sup>238</sup>) 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 (<sup>234</sup>U/<sup>238</sup>U) activity ratio show that limestone samples characterised by pressure dissolution structures (stylolites or dissolution seams) display systematic (<sup>234</sup>U/<sup>238</sup>U) 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 <sup>234</sup>U over <sup>238</sup>U ((<sup>234</sup>U/<sup>238</sup>U) 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.</p> <p style="line-height: 20px;"><b>Keywords: </b>uranium isotopes, multiple-collector ICP-MS, waste management, remobilisation, migration
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