Heat flow and helium isotopes in the geothermal areas of Tuscany (Central Italy).

A wide part of Tuscany is affected by a large heat-flow anomaly, with maxima corresponding to the geothermal fields of Larderello-Travale and Mt. Amiata. Anomalous heat flow is very often related to the presence of 3He-enriched fluids. The comparison of R/Ra, He/Ne ratios and heat flow through Tuscany along three deep seismic profiles, shows no straight forward correlation among these parameters. A phased correlation exists at Larderello-Travale, where maxima are almost coincident, while they appear decoupled at Mt. Amiata. Different heat and He transport mechanisms through the crust are the most likely explanation. At Larderello-Travale, the almost constant value of R/Ra in fluids issuing at surface indicates that the contribution of hot 3He-enriched fluids must have occurred through deeply rooted faults. The decoupling of the heat and He anomalies in the Mt. Amiata area is then related not only to a lower degree of fracturing and/or a different depth of the faults roots, but also to infiltration of meteoric water in shallow aquifers.