The deep roots of the Larderello geothermal field (Italy) from heat flux and 3He anomalies.

The surface distribution of the Helium relative abundance and isotopic composition from geothermal wells and gaseous manifestations of the Larderello geothermal field and neighboring areas in Tuscany, were compared with surface heat flux data and deep seismic images of the crustal structures, to study the mantle-crust interactions beneath the Tuscan geothermal area, on the basis of the heat and mass fluxes from below. The apparent misfit between heat flux and He anomalies at local scale was resolved by decomposition of the various components of heat flow anomalies by means of a simple 2-D thermal modeling and inter-comparison with R/Ra variation across the field: short-wavelength thermal perturbations appear likely related to the increase of 4He radiogenic fraction derived from long-term water-rock interaction, whereas long-wavelength thermal perturbations are related to 3He enriched fluid rising from the mantle and reaching the surface along preferential channels, as evidenced by the seismic image of the deep structures.