Isotope systematics of C-bearing gas compounds in the geothermal fluids of Larderello, Italy

The origin of carbon-bearing compounds (CO2, CH4, C2-C4 saturated hydrocarbons) and helium in the geothermal fluid of Larderello is investigated by means of the variations in concentration and isotopic composition. The CO2 (δ13C from −1.4 to −7.1‰ versus V-PDB) is mainly of crustal origin. The carbon isotopes of methane (δ13C from −20.9 to −31.7‰) and other hydrocarbons indicate a complex thermogenic origin. The temperatures obtained with the CH4-CO2 isotope geothermometer are in rough agreement with those observed in deeper geothermal wells. The CH4/C2H6 ratios show a tendency towards partial equilibrium with increasing temperature. He isotopes (R/RA from 0.5 to 3) indicate that although the major part of helium derives from crustal sources, a significant fraction of mantle helium is also present. Helium contamination by air, deducted from He/Ne ratios, is generally negligible.