Origin of 87Sr enrichment in calcite cements in Jurassic limestones (Eastern Paris Basin, France)

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 (δ18O, δ13C, 87Sr/86Sr) 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 18O and enriched in 87Sr 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 87Sr/86Sr in diagenetic calcite. 87Sr-enriched fluids that precipitated calcite cements strongly impact the 87Sr/86Sr ratio of present-day groundwaters in Jurassic limestones and may have modified the Callovian-Oxfordian porewater isotope composition.