Helium isotopes in paleofluids and present-day fluids of the Larderello geothermal field: Constraints on the heat source

The He isotope composition of paleofluids entrapped in fluid inclusions of hydrothermal minerals is compared with the present‐day fluid composition of the Larderello geothermal field. Almost constant values of (3He/4He)m/(3He/4He)air (=R/Ra) over time indicate that no important changes have occurred in the deep source of gases, at least during the last 3.8 million years. On a regional scale, a correlation has been found between the R/Ra spatial distribution, heat flow, and Bouguer gravity anomaly. High values of R/Ra and heat flow, and low Bouguer anomaly values indicate that the Larderello field is an area of preferential escape for mantle‐derived fluids. A positive correlation has also been found between the R/Ra spatial distribution and a major seismic reflector named the “K horizon.” A deep magma source, refilled by periodic gas input from the mantle, is the most likely source of 3He‐enriched fluids and the anomalously high heat flow. The nearly constant value of R/Ra clearly indicates that input of fresh mantle material has occurred up to recent times. Clear evidence of mixing between mantle and crustal fluids indicates that the high R/Ra is the lower limit of the actual mantle value, which is suggested to be similar to the subcontinental European mantle. The decrease of R/Ra over time in the peripheral part of the Larderello field indicates that important changes in the feeding fracture system and/or cooling rate have occurred in these areas.