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Federal Institute for Geosciences and Natural Resources, Stilleweg 2, Hannover, Germany
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- PublicationRestrictedMolecular and isotopic composition of free hydrocarbon gases from Sicily, Italy(2004)
; ; ; ; ; ; ;Grassa, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Palermo, Italia ;Capasso, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Palermo, Italia ;Favara, R.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Palermo, Italia ;Inguaggiato, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Palermo, Italia ;Faber, E.; Federal Institute for Geosciences and Natural Resources, Hannover, Germany ;Valenza, M.; Dipartimento CFTA, Universita` di Palermo, Palermo, Italy; ; ; ; ; 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.173 87 - PublicationRestrictedNitrogen isotopes in thermal fluids of a forearc region (Jalisco Block, Mexico): evidence for heavy nitrogen from continental crust(2004)
; ; ; ; ; ; ; ;Inguaggiato, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Palermo, Italia ;Taran, Y.; Instituto de Geofisica, UNAM, Coyoacan, Mexico, D.F. 04510 ;Grassa, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Palermo, Italia ;Capasso, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Palermo, Italia ;Favara, R.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Palermo, Italia ;Varley, N.; Facultad de Ciencias, Universidad de Colima, Colima, Mexico ;Faber, E.; Federal Institute for Geosciences and Natural Resources, Stilleweg 2, Hannover, Germany; ; ; ; ; ;The Jalisco Block (JB) is a geologically and tectonically complex part of northwestern Mexico characterized by active subduction-type volcanism, rifting, and old stable structures. Thermal springs and groups of springs are widely distributed over JB. Bubbling gas from seven thermal springs located within different tectonic environments of the JB was analyzed for He, 20Ne, and N2 concentrations and d15N ratios. All gases are N2-dominant (>84%) with the exception of one sample (Rio´ Purificacio´n), which has a significant CH4 content (about 50%). All collected gas samples are relatively high in He, up to 1500 ppm vol and with 3He/4He values ranging from 0.6 to 4.5 Ra. All measured nitrogen isotope ratios are heavier than air with d15N values ranging from 0.5 to 5.0%. The relative N2 excess with respect to air-saturated water computed on the basis of N2 and 20Ne contents indicates the contribution of a nonatmospheric N2 source. All the samples show a good correlation between d15N and the relative excess of N2 with d15N +5.3% for the maximum N2 excess of 100%. Due to a presumed lack of seafloor sediment involved in the subduction process, such a d15N positive value seems to reflect the addition to the fluids of a heavy nitrogen originating from metamorphism processes of rocks occurring within the overlying continental crust.176 75