CO2 and heat content of the French Massif Central thermal and mineral waters.

The Massif Central, located in central-southern France, is characterized by the presence of deep CO₂-rich hydrothermal systems generally hosted by the rocks of the crystalline basement. The surficial manifestation consists in bubbling pools, mofettes and a huge number of CO₂ rich springs, both thermal and fresh. Since the ’70, the area has been extensively investigated in order to evaluate its geothermal potential, becoming a “natural laboratory” for the development of increasingly responsive geothermometers. Here, using both new data and data from previous studies, we estimate the T/pCO₂ conditions of the reservoir and the heat content (Q_H, MW) and the CO₂ mass flow rate (QCO₂, Kg s^-1). of each spring. Results show that circulating waters are characterized by partial equilibrium with respect to silicates oversaturation with respect to calcite and pCO₂ up to 2 bar. Temperatures of the hydrothermal reservoirs, estimated with Na/K, Na/Li, Mg-Na-K and silica geothermometers range from 120 °C to 200 °C, in agreement with previous studies. The CO₂/heat content ranges from 0.001 and 0.006 kg MJ^-1, of the same order of magnitude than Taupo and Salton Trough geothermal systems (0.003 kg MJ^-1), slightly higher than Iceland geothermal systems (e.g. Reykjanes, <0.001 kg MJ^-1) but much lower than geothermal systems of Southern Europe, e.g. Kizildere (Turkey), Nisyros (Greece), Latera and Torre Alfina (Italy), characterized by CO₂/heat ratio one order of magnitude higher.