Surface Heat Flow vs. Helium Isotopes in the Thermal Anomaly Areas of Tuscany (Italy)

A wide part of Tuscany (North-Central Italy) is affected by a large heat flow anomaly: the highest values of heat flow correspond to the two geothermal fields of Larderello and Mt. Amiata, where values up to 1W/m² and 0.6 W/m² are reached, respectively. Several other thermal manifestations are scattered in the region. These geothermal fields are located in the inner Northern Apennines, affected by post-collisional extensional tectonics and widespread Late Miocene-Quaternary magmatism. The geothermal fields of Larderello and Mt. Amiata display considerable similarities from the geological-structural and thermal point of view, with some important difference. They differ with regard to fluids characteristics: the Larderello field is steam-dominated while the Mt. Amiata field is water-dominated. We compare the distribution at surface of ³He/⁴He ratio (R/Ra), a sensitive geochemical tracer of source, with heat flow and other geophysical and structural parameters like Bouguer anomaly and normal fault geometry, to improve knowledge on the two geothermal systems. To investigate the relationship among normal faults, geothermal fluids pathways, He surface distribution and heat flow, we constructed targeted geological sections through the geothermal areas of Tuscany. The geological sections were drawn down to the K-horizon (a regional seismic reflector discontinuously underlying the whole geothermal areas at a depth ranging between 3 and 7 km), integrating field data with borehole stratigraphies and reflection seismic surveys. Though slightly biased in space at surface, due to the normal fault geometry governing the fluids pathways, the highest R/Ra values show a good correspondence to the heat flow maxima. These results account for the role of the extensional shear zones as preferential ways of mantle-derived fluids uprise in the Larderello field. We stress the importance of mapping simple primary geophysical (temperature gradient, heat flow) and geochemical (He isotopic composition) data, compared with the structural geology, to define the principal fluid-flow paths in geothermal areas.