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Signature géochimique de paléocirculations aqueuses dans la calcite de remplissage de fractures de massifs argileux peu perméables et de leurs encaissants : exemples pris sur les sites de Bure, Tournemire et Suisse du Nord

Abstract : Fractures in rock masses represent preferential path for fluid transfer and, as such, are the most efficient way for migration of radionuclides at the regional scale. The impact of fracturation on hydrogeological system is a major challenge for underground radioactive waste storage projects. In this context, geochemistry of fracture-filling calcite is used to better understand physical and chemical properties of paleofluids. A new methodology has been developped to analyse Mg, Mn, Fe, Sr and Rare Earth Elements REE (La, Ce, Nd, Sm, Eu, Dy and Yb) in calcite by Secondary Ion Mass Spectrometry. Analyses of calcite crystals have been performed in fractures from Jurassic clays and limestones in Franc (Bure and Tournemire sites) and northern Switzerland (Mt Terri's tunnel and deep borehole). On each case, several geochemical signatures are observed, according to REE partitionning and Mn and Fe concentrations. In the Bure site, a dependance of calcite geochemistry from fracture host rock has been evidenced. On the other hand, speciation of REE in solution equilibrated with clayey or calcareous rocks at circumneutral pH (7 to 8) is not significantly influenced by the media: speciation is dominated by carbonate species in both cases and phosphate complexes can modify heavy REE disponibility in relatively to light REE. These results point out that in fractures in clays, calcite crystallizes at equilibrium with a fluid expulsed during diagenesis from clay minerals, recording the effect of clays and accessory phases. In limestone fractures, calcite records a later event related to the past functionning of the present aquifer, and the fluid has reached equilibrium with the rock minerals. In secondary filling calcite from Toarcian Argilites faults close to Tournemire' s tunnel, three successive generations of calcite are observed in an extensive fault, and a fourth in a compressive one. In Aalenian Opalinus Clays veins, comparison between existing isotopic data and Mn, Fe and REE signal in calcite show that calcite from Mt Terri is mainly influenced by argilite host rocks, whereas calcite from deep boreholes did record other processes, possibly during a later event. To conclude, geochemistry of Mn, Fe and the Rare Earth in fracture filling calcite, completed with geochemical modelling, allow us a better understanding of paleofluid transfers that hapenned in a sedimentary basin.
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https://pastel.archives-ouvertes.fr/pastel-00000638
Contributor : Ecole Mines Paristech <>
Submitted on : Tuesday, September 14, 2004 - 8:00:00 AM
Last modification on : Wednesday, October 14, 2020 - 3:52:21 AM
Long-term archiving on: : Thursday, September 30, 2010 - 6:03:38 PM

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  • HAL Id : pastel-00000638, version 1

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Delphine Lecocq. Signature géochimique de paléocirculations aqueuses dans la calcite de remplissage de fractures de massifs argileux peu perméables et de leurs encaissants : exemples pris sur les sites de Bure, Tournemire et Suisse du Nord. Géochimie. École Nationale Supérieure des Mines de Paris, 2002. Français. ⟨pastel-00000638⟩

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