Soil gas measurements around the most recent volcanic system of metropolitan France (Lake Pavin, Massif Central)

Soil gas monitoring techniques (CO2, O2, 222Rn, 4He) are used in the geographical context of the recent volcanic system of Lake Pavin (Puy-de-Dôme), to get a better knowledge of local gaseous emissions, in order to establish whether or not this system can present evidence of reactivation. Concentrations up to 100% CO2 and 50 ppm of helium are measured in a narrow geographical area (Escarot mofette), together with a magmatic origin for these gases. Radon activity in the mofette area is quite high, but does not show, compared to surrounding areas, enrichments as high as those measured for CO2 or helium. Hourly records of these radon activities, performed during several weeks, suggest the existence of pulsed radon exhalation in the mofette area. The period of this pulsation is around 40 days but its origin remains poorly understood. Apart from this mofette, no evidence of gas originating from depth is highlighted.