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Authors: Grassa, F.* 
Capasso, G.* 
Favara, R.* 
Inguaggiato, S.* 
Faber, E.* 
Valenza, M.* 
Title: Molecular and isotopic composition of free hydrocarbon gases from Sicily, Italy
Issue Date: 2004
Series/Report no.: 31 (2004)
DOI: 10.1029/2003GL019362
Keywords: Isotopic composition/chemistry
Organic geochemistry
Subject Classification03. Hydrosphere::03.04. Chemical and biological::03.04.06. Hydrothermal systems 
04. Solid Earth::04.08. Volcanology::04.08.01. Gases 
05. General::05.02. Data dissemination::05.02.01. Geochemical data 
Abstract: Chemical and isotopic data have been used as geochemical tracers for a genetic characterization of hydrocarbon gases from a total of eleven manifestations located in Eastern and Central-Southern Sicily (Italy). The molecular analysis shows that almost all the samples are enriched in methane (up to 93.2% Vol.), with the exception of four gas samples collected around Mt. Etna showing high mantle-derived CO2 content. Methane isotope signatures suggest that these are thermogenic gases or a mixture between thermogenic gases and microbial gases. Although samples from some mud volcanoes in Southern Sicily (Macalube di Aragona) show isotope signatures consistent with a mixing model between thermogenic and microbial, by combining the molecular compositions (C1/(C2 + C3))and the methane isotope ratios (d13C1), such a process seems to be excluded. Therefore, the occurrence of secondary post-genetic processes should be invoked. Two main hypotheses have been considered: the first hypothesis includes that the gas is produced by microbial activity and altered post-genetically by microbial oxidation of methane, while according to the second hypothesis thermogenic gas have modified their molecular ratios due to vertical migration.
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