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

Abstract The origin of carbon-bearing compounds (CO 2 , CH 4 , C 2 –C 4 saturated hydrocarbons) and helium in the geothermal fluid of Larderello is investigated by means of the variations in concentration and isotopic composition. The CO 2 ( δ 13 C from −1.4 to −7.1‰ versus V-PDB) is mainly of crustal origin. The carbon isotopes of methane ( δ 13 C from −20.9 to −31.7‰) and other hydrocarbons indicate a complex thermogenic origin. The temperatures obtained with the CH 4 –CO 2 isotope geothermometer are in rough agreement with those observed in deeper geothermal wells. The CH 4 /C 2 H 6 ratios show a tendency towards partial equilibrium with increasing temperature. He isotopes ( R / R A 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.