Carbon dioxide degassing and thermal energy release in the Monte Amiata volcanic-geothermal area (Italy)

The quaternary volcanic complex of Mount Amiata is located in southern Tuscany (Italy) and represents the most recent manifestation of the Tuscan Magmatic Province. The region is characterised by a large thermal anomaly and by the presence of numerous CO 2-rich gas emissions and geothermal features, mainly located at the periphery of the volcanic complex. Two geothermal systems are located, at increasing depths, in the carbonate and metamorphic formations beneath the volcanic complex. The shallow volcanic aquifer is separated from the deep geothermal systems by a low permeability unit (Ligurian Unit). A measured CO 2 discharge through soils of 1.8 × 10 9 mol a −1 shows that large amounts of CO 2 move from the deep reservoir to the surface. A large range in δ 13C TDIC (−21.07 to +3.65) characterises the waters circulating in the aquifers of the region and the mass and isotopic balance of TDIC allows distinguishing a discharge of 0.3 × 10 9 mol a −1 of deeply sourced CO 2 in spring waters. The total natural CO 2 discharge (2.1 × 10 9 mol a −1) is slightly less than minimum CO 2 output estimated by an indirect method (2.8 × 10 9 mol a −1), but present-day release of 5.8 × 10 9 mol a −1 CO 2 from deep geothermal wells may have reduced natural CO 2 discharge. The heat transported by groundwater, computed considering the increase in temperature from the infiltration area to the discharge from springs, is of the same order of magnitude, or higher, than the regional conductive heat flow (>200 mW m −2) and reaches extremely high values (up to 2700 mW m −2) in the north-eastern part of the study area. Heat transfer occurs mainly by conductive heating in the volcanic aquifer and by uprising gas and vapor along fault zones and in those areas where low permeability cover is lacking. The comparison of CO 2 flux, heat flow and geological setting shows that near surface geology and hydrogeological setting play a central role in determining CO 2 degassing and heat transfer patterns.