Multi-isotopic tracing (δ7Li, δ11B, 87Sr/86Sr) and chemical geothermometry:: evidence from hydro-geothermal systems in France

In the present work, we report the results of a multi-isotopic study (delta(7) Li, delta(11)B and (87)Sr/(86)Sr) 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 (87)Sr/(86) Sr 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 (87)Sr/(86)Sr ratios (closer to Sr isotopic ratios observed in the sedimentary zones < 0.712). Boron isotopic ratios lay between -8.3 and +21.6 parts per thousand, also reflecting a lithological control over delta(11)B 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. delta(7) Li signatures are comprised between -0.1 and +10.0 parts per thousand for temperatures ranging from 60 to 230 degrees 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 67 Li composition of geothermal waters may be utilised as a geothermometer (delta(7) Li =-0.043 +/- 0.003 T + 11.9 +/- 0.5). (C) 2007 Elsevier B.V. All rights reserved.