Multi-isotopic tracing (δ7Li, δ11B, 87Sr/86Sr) and chemical geothermometry: evidence from hydro-geothermal systems in France
Résumé
In the present work, we report the results of a multi-isotopic study (δ7Li, δ11B and 87Sr/86Sr) on waters collected from different hydro-and geothermal sites in France. The aim of this study was to characterize hydro-and geothermal reservoirs by coupling a multi-isotopic approach with in-depth temperature of the reservoir calculated using chemical geothermometers. A database constituted by 36 water samples (thermal, thermo-mineral and geothermal waters) was selected in order to cover a wide range of temperatures and to be representative of water/rock interactions with various types of lithologies. We clearly observed that the wide range of 87Sr/86Sr isotopic ratios in the waters (0.70755-0.72187) appear to be largely controlled by the dominant host lithologies with Sr isotopic signatures ranging from high values (resulting from interaction from granite and/or gneiss type > 0.714) to lower 87Sr/86Sr ratios (closer to Sr isotopic ratios observed in the sedimentary zones < 0.712). Boron isotopic ratios lay between - 8.3 and + 21.6‰, also reflecting a lithological control over δ11B signatures. In addition, no simple or direct relationship was observed for either boron or strontium isotopic signatures with the temperature of the reservoir. In contrast, temperature plays an important role in controlling the lithium isotope composition of the hydro-and geothermal fluids. δ7Li signatures are comprised between - 0.1 and + 10.0‰ for temperatures ranging from 60 to 230 °C. This result seems to prove that lithium isotopic systematics represents a useful tool suitable for the characterization of hydrothermal and geothermal waters and suggests that the δ7Li composition of geothermal waters may be utilised as a geothermometer (δ7Li = - 0.043 ± 0.003 T + 11.9 ± 0.5).