Paleo water-rock interaction determined through isotopic tracing (Sr, O, C, U) in fracture-fill minerals: Evidence from the Vienne granitoids (France)
Résumé
Based on the concepts (a) that the stable C and O isotopes combined with the Sr isotope ratios of fracture fills should reflect the source groundwater from which the solid phases precipitated and (b) that U-series disequilibria (USD) enable the calculation of residence time for the U by using Fe oxides as the best candidate, an "isotopic toolbox'' was applied to fracture fill from the crystalline basement of the Vienne district. The fracture fills are formed mainly of carbonates, clays and Fe oxides. The isotope data indicate two main generations of carbonate that originated from hydrothermal circulation and equilibrium with present-day groundwaters but the Sr isotope ratios highlight another component with a higher Sr-87/Sr-86 ratio reflecting the complexity of the water-rock interactions. For the USD, the Fe-hydroxides located at 207 m depth yield an age of 102 +/- 5 ka (St. Germain I interglacial stage), whereas those located at 277 m and 300 m yield respective ages of 173 +/- 15 ka and 181 +/- 10 ka. These corresponding to the transition between the penultimate glacial period (isotopic stage 6) and the end of the preceding interglacial stage (isotopic sub-stage 7a). Investigating water-rock interaction (Sr-87/Sr-86, O-18, C-13, USD) in the fracture-fill minerals from the crystalline basement has shown that such an approach is relevant to developing an understanding of how the groundwater system has changed over time.