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[1] 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 ( 3 He/ 4 He) m /( 3 He/ 4 He) air (=R/R a ) 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/R a spatial distribution, heat flow, and Bouguer gravity anomaly. High values of R/R a 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/R a 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 3 He-enriched fluids and the anomalously high heat flow. The nearly constant value of R/R a 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/R a is the lower limit of the actual mantle value, which is suggested to be similar to the subcontinental European mantle. The decrease of R/R a 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.

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