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 (delta13C from -1.4 to -7.1 versus V-PDB) is mainly of crustal origin. The carbon isotopes of methane (delta13C 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.