Stable isotope and noble gas isotope compositions of inclusion fluids from Larderello geothermal field (Italy): Constraints to fluid origin and mixing processes

Noble gas and N2 compositions have been determined in paleofluids of core samples from geothermal wells in the Larderello area, Italy. The results were coupled with hydrogen and carbon isotope compositions of similar fluids from an independent batch of samples to provide constraints on the origin of the fluids. The fluids trapped during early-stage hydrothermal circulation are characterized by relatively high 40Ar / 36Ar and N2 / Ar ratios (332 ≤ 40Ar / 36Ar ≤ 564 and 248 ≤ N2 / Ar ≤ 4197, respectively) and indicate the presence of radiogenic 40Ar and excess N2 when compared to air/asw (air saturated water). The fluid inclusions related to late-stage hydrothermal activity show low N2 / Ar and 40Ar / 36Ar ratios (309 ≤ 40Ar / 36Ar ≤ 354 and 47 < N2 / Ar < 66, respectively), similar to present-day geothermal fluids. The δD values of the inclusion fluids range from − 53‰ to − 85‰, indicating that the fluids were derived from mixing of water-rich components, probably from a shallow reservoir similar in composition to present-day meteoric waters, and fluids from a deeper reservoir. The R / Ra ratios (up to 3.2) of the paleofluids and present-day fluids suggest a deep He contribution at Larderello. The δ13C values for CO2 trapped during early-stage activity (− 3.8‰ to 0.5‰) were produced by carbonate country rock volatilisation; δ13C values as low as − 17‰ were measured in inclusion fluids related to late-stage circulation, and were likely produced by oxidation of organic matter at shallow depths.