Insights into the Larderello geothermal field: Structural setting and distribution of thermal and 3He anomalies.

In the Larderello geothermal field, similarly to the other geothermal areas in the world (i.e. Yellowstone, The Geysers, Rhinegraben), heat flow at a regional scale anomaly corresponds to the presence of 3He enriched fluids. At least in the last one million years in a typical crustal melting setting, high R/Ra values (ranging from 0.5 to 3.2) in present and inclusion fluids indicate mantle as the main source of the thermal and 3He anomaly. The combination of thermal and He data provide information on transport mechanism in upper and lower crust, which is strongly controlled by fault systems. A targeted geological section of the Larderello field, integrating field, borehole and reflection seismic surveys data correspond to shear zones, where upward displacement of isotherms correspond to the NE-dipping normal fault system. At surface, higher R/Ra values correspond to heat flow maxima, though slightly biased in space in the order of 1 - 2 km, and indicate that fault systems act as preferential pathways for mantle-derived fluids. The similar evolution in time and space of heat and 3He anomalies, even though they differ by at least 1-3 orders of magnitude in diffusion time constants, indicate that they share sources and transfer mechanisms. Mantle-He transfer through the crust requires fluid advection or diffusion via fluid filled conduits while heat is more efficiently transferred by conductivity through the bulk rocks, enhanced by fluid circulation in the upper brittle crust. The local addition of an advective component to the regional background thermal anomaly caused by increased permeability along faults could explain the presence of maxima of thermal and 3He anomaly at Larderello.