Résumé
Geothermal exploration can be challenging in the absence of obvious surface indicators of a high enthalpy geothermal resource such as geysers, fumaroles or thermal springs with temperatures near 100 °C. This is the case at the Piton des Neiges volcano (Réunion Island), which displays evidence of hydrothermal activity mostly as thermal springs with temperatures not exceeding 50 °C, affected by mixing and reequilibration, and emerging in a complex geological framework, resulting from the polygenic history of this dormant volcano. Yet, these thermal springs keep a partial record of the reservoir geochemistry. Here, we propose an exploration method to set this thermal signature apart from the contribution of other processes and trace back the equilibrium conditions, enabling us to locate thermal fluid circulations, and evaluate their temperature and availability. We report its application at Piton des Neiges volcano. We led an extensive geochemical survey which nearly doubled the number of thermal springs reported on the volcano and dramatically extended the existing geochemical database in space and time. In tandem, we also monitored δ18O and δD values in rainwaters and thermal springs over two years to compare their temporal variations.Our surveys show that the repartition of thermal springs is controlled by only one of the volcanic rift zones (the N30°E axis), which is thus identified as the heat source of hydrothermal circulations. This N30°E rift zone is also a preferential pathway for the degassing of mantellic CO2. In addition, the composition of thermal waters is influenced by syenite intrusions along the rim of the caldera, acting as drains for the uprise of hydrothermal waters. O, H and Sr isotopic signatures of springs show that the geothermal resource of Piton des Neiges is in fact made of two independent and unconnected hydrothermal systems on the northern and southern circular depressions of the volcano (the cirques of Salazie and Cilaos, respectively). Their recharge is meteoric, multi-year, and mostly achieved by cyclonic events. Thermal waters experience reequilibration during their ascent and various degrees of CO2 input or degassing. Last equilibrium temperatures of water/rock interaction, estimated by multicomponent geothermometry, reach up to 125 ± 6 °C in the cirque of Salazie, and up to 141 ± 15 °C in the cirque of Cilaos, with possibility for hotter waters before reequilibration. Taken together, our results suggest that the most promising target for future exploration is located in the cirque of Cilaos, along the caldera rim, and in proximity of syenite intrusions.With a thorough prospecting of thermal springs, a comprehensive geochemical study and a geochemical monitoring of the different water reservoirs, we improved our understanding of the geothermal resource and identified future exploration targets at Piton des Neiges volcano.A separation scheme for strontium and light rare earth elements and its application to the isotopic analysis of strontium and neodymium in silicate rocks are presented. This method benefits from the selectivity and high capacity of two newly introduced extraction Chromatographic materials, referred to as Sr.Spec and TRU.Spec, respectively. These afford a straightforward separation of Sr and Sm + Nd with high yield, good purity and satisfactory blank levels, on very small (0.25 ml) columns using small volumes of solutions of a single mineral acid, HNO3. The validity of the method is illustrated by the measurement of 87Sr/ 86Sr and 143Nd/144Nd ratios in twelve international standard reference materials.