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

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• 作者：Fabrizio Gherardi ; Costanzo Panichi ; Roberto Gonfiantini ; Gabriella Magro and Giovanni Scandiffio
• 关键词：Isotopes ; Carbon-bearing gases ; Geothermal fluid ; Partial equilibrium ; Larderello ; Italy
• 刊名：Geothermics
• 出版年：2005
• 出版时间：2005
• 年：2005
• 卷：34
• 期：4
• 页码：442-470
• 全文大小：869 K

文摘

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/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.