Origin of 87Sr enrichment in calcite cements in Jurassic limestones (Eastern Paris Basin, France). (January 2022) - Blaise, Thomas
Abstract: In this contribution, the origin of fluids having cemented the Jurassic limestones in the eastern part of the Paris Basin is discussed through the isotopic composition (δ 18 O, δ 13 C, 87 Sr/ 86 Sr) of authigenic calcite crystals filling vugs and fractures. The Upper and Middle Jurassic limestones, overlying and underlying the Callovian-Oxfordian claystones, experienced dissolution and crystallization leading to a succession of calcite cements depleted in 18 O and enriched in 87 Sr compared to the bulk host limestone isotope composition. These isotopic shifts originate from the flow of allochthonous basinal fluids through the fracture network that developed during the main tectonic deformation events. Mixing between the allochthonous fluids and the autochthonous local porewaters is attested by variable 87 Sr/ 86 Sr in diagenetic calcite. 87 Sr-enriched fluids that precipitated calcite cements strongly impact the 87 Sr/ 86 Sr ratio of present-day groundwaters in Jurassic limestones and may have modified the Callovian-Oxfordian porewater isotope composition. Highlights: 87 Sr/ 86 Sr records the input of basinal allochthonous fluids through the fracture network in the Jurassic limestones. The Sr isotope shift between calcite cements and the host limestones depends on the dilution rate of allochthonous fluids. Paleo-water flows strongly modified the primary δ 18 O and 87 Sr/ 86 Sr composition in bulk limestones.