Origin and fate of hydrothermal fluids at Piton des Neiges volcano (Reunion Island) : a geochemical and isotopic (O, H, C, Sr, Li, Cl) study of thermal springs

As many ocean basaltic volcanoes worldwide, Piton des Neiges (Reunion Island) hosts a hydrothermal system that represents a potential geothermal resource. Despite several prospection campaigns over the past decades, this potential has never been confirmed because many aspects of fluid supply and circulation remain unclear. To track the origin and fate of hydrothermal fluids, we analyzed the geochemistry of thermal springs (water and gas), by combining conventional tracers (major-trace elements and O, H, C, Sr, Li isotopes) with Cl isotopes, a geochemical tool under development. Added to literature data, our new results allow us to compare the composition of thermal springs with cold waters, rocks, gas, and fumarolic deposits from La Reunion and elsewhere, and to untwine the complex history of fluid mixing that constitutes the geothermal system of Piton des Neiges. delta O-18(H2O) and delta D-H2O values of springs (-8.67 to -4.9 and -56.3 to -23.5 parts per thousand VSMOW, respectively) are positioned close to the local meteoric water line, showing that thermal waters are of meteoric origin. However, their depletion in heavy isotopes relative to cold waters suggests a recharge strongly influenced by cyclones, with a possible contribution from high altitude (>= 2000m) rainfalls. Their absence of O-18 enrichment relative to the local meteoric water line indicates that the isotopic exchange between rocks and water is very limited, and therefore suggests that their deep temperatures are relatively low and/or the water/rock ratios are high. delta C-13 values in thermal waters (-8.0 to +3.2 parts per thousand PDB) and gases (-6.7 to -5.3 parts per thousand PDB) confirm that carbon is essentially of magmatic origin, which we interpret as supplied by regional degassing of La Reunion hotspot. Major-trace elements and Sr-87/Sr-86 (0.704142 to 0.704336), delta Li-7 (+2 parts per thousand and +34.8 parts per thousand LSVEC), and delta Cl-37(-0.35 parts per thousand and +0.40 parts per thousand SMOC) compositions show that three additional processes contribute to the mineralization of thermal waters: 1) interaction with basalt at temperatures >= 50 degrees C, 2) very limited seawater contamination (< 3 mol parts per thousand), and/or 3) leaching of trachyte or scrubbing of fumarolic gas or condensates emitted by a shallow trachytic magma in the volcanic edifice. The geothermal system of Piton des Neiges is thus the result of meteoric water interaction with a trachytic heat source (as a solidified intrusion or a magma pocket), unrelated to the CO2 flux. We also emphasize that thermal waters at Piton des Neiges are exceptionally isolated from seawater compared to other geothermal systems in ocean island volcanoes worldwide, which implies a heat source located above the seawater/freshwater interface. These inferences are of critical importance for the future geothermal exploration on the island, as they suggest that the geothermal resource is within reach of relatively shallow drilling depths (<= 1000 m)