V11G-07
The bulk isotopic composition of hydrocarbons in subaerial volcanic-hydrothermal emissions from different tectonic settings

Monday, 14 December 2015: 09:40
308 (Moscone South)
Jens Fiebig1, Franco Tassi2, Orlando Vaselli2, M Fatima Viveiros3, Catarina Silva3, Taryn Michelle Lopez4 and Walter D'Alessandro5, (1)Goethe University Frankfurt, Frankfurt, Germany, (2)University of Florence, Florence, Italy, (3)University of Azores, Ponta Delgada, Portugal, (4)University of Alaska Fairbanks, Geophysical Institute, Fairbanks, AK, United States, (5)National Institute of Geophysics and Volcanology, Rome, Italy
Abstract:
Assuming that methane and its higher chain homologues derive from a common source, carbon isotope patterns have been applied as a criterion to identify occurrences of abiogenic hydrocarbons. Based on these, it has been postulated that abiogenic hydrocarbon production occurs within several (ultra)mafic environments.

More evolved volcanic-hydrothermal systems may also provide all the prerequisites necessary for abiogenic hydrocarbon production, such as availability of inorganic CO2, hydrogen and heat. We have investigated the chemical and isotopic composition of n-alkanes contained within subaerial hydrothermal discharges emitted from a range of hot spot, subduction and rift-related volcanoes to determine the origin of hydrocarbons in these systems. Amongst these are Nisyros (Greece), Vesuvio, Campi Flegrei, Ischia, Pantelleria and Vulcano (all Italy), Mt. Mageik and Trident (USA), Copahue (Argentina), Teide (Spain), Furnas and Fogo (Portugal).

The carbon isotopic composition of methane emitted from these sites varies from -65 to -8‰ , whereas δ13C of ethane and propane exhibit a much narrower variation from -17‰ to -31‰. Methane that occurs most enriched in 13C is also characterized by relatively positive δD values ranging up to -80‰. Carbon isotope reversals between methane and ethane are only observed for locations exhibiting δ13C-CH4 values > -20‰, such as Teide, Pantelleria, Trident and Furnas. At Furnas, δ13C-CH4 varies by 50‰ within a relatively short distance of <50m between two vents, whereas δ13C-C2H6 varies by less than 2‰ only. For some of the investigated locations apparent carbon isotopic temperatures between methane and CO2 are in agreement with those derived from gas concentration geothermometers. At these locations methane, however seems to be in disequilibrium with ethane and propane. These findings imply that methane on the one hand and the C2+ hydrocarbons on the other hand often might derive from distinct sources.

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