Hydrocarbons in Icelandic volcanic-hydrothermal discharges: An inventory of concentrations and preliminary bulk carbon, radiocarbon and clumped isotopic data
Final Report Abstract
In this project the origin of n-alkane hydrocarbons in volcanic-hydrothermal emissions was investigated. In addition to Icelandic emissions, discharges were collected from several other volcanic-hydrothermal systems from all over the world, covering all different types of volcanism. Sampled discharges were analyzed for their chemical and stable isotopic composition. Methane in some selected discharges was also analyzed for its radiocarbon content and for its clumped isotopic composition (13CDH3). A global compilation of the carbon and hydrogen isotopic compositions reveals patterns that are characteristic of a biotic origin of the hydrocarbons. They predominantly derive from the thermal decomposition of organic matter. Organic matter is supplied by external waters (meteoric water, seawater) circulating through the rocks hosting the hydrothermal systems at depth and – if present – by sediments. In Reykjanes discharges, the presence of radiocarbon in methane confirms the involvement of surface derived organic matter in hydrocarbon formation. A thermogenic origin of methane is in line with the observation that the carbon isotope fractionation between methane and CO2 in most cases does not correspond to measured reservoir temperatures and to the respective methane clumped isotopic composition, implying that carbon isotope equilibrium between CO2 and CH4 is not attained in these systems. Our results document that criteria previously applied to identify abiotic hydrocarbon occurrences are not reliable, even if applied in combination. In order to evaluate the potential relevance and significance of abiotic hydrocarbon production on Earth the fate of organic matter transported by external waters through the crust needs to be better understood.
Publications
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(2016) The hydrothermal system of the Domuyo volcanic complex (Argentina): A conceptual model based on new geochemical and isotopic evidences. Journal of Volcanology and Geothermal Research 328, 198-209
Tassi, F., Liccioli, C., Agusto, M., Chiodini, G., Vaselli, O., Calabrese, S., Pecoraino, G., Tempesti, L., Caponi, C., Fiebig, J., Caliro, S., Caselli, A.
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(2017) Geochemical constraints on volatile sources and subsurface conditions at Mount Martin, Mount Mageik, and Trident Volcanoes, Katmai Volcanic Cluster, Alaska. Journal of Volcanology and Geothermal Research 347, 64-81
Lopez, T., Tassi, F., Aiuppa, A., Galle, B., Rizzo, A.L., Fiebig, J., Capecchiacci, F., Giudice, G., Caliro, S., Tamburello, G.
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(2017) Isotope systematics of Icelandic thermal fluids. Journal of Volcanology and Geothermal Research 337, 146-164
Stefánsson, A., Hilton, D.R., Sveinsbjörnsdottir, A.E., Torssander, P., Heinemeier, J., Barnes, J.D., Ono, S., Halldorsson, A.A., Fiebig, J., Arnorsson, S.
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(2019) Abiogenesis not required to explain the origin of volcanic-hydrothermal hydrocarbons. Geochemical Perspectives Letters 11, 23-27
Fiebig, J., Stefánsson, A., Ricci, A., Tassi, F., Viveiros, F., Silva, C., Lopez, T.M., Schreiber, C., Hofmann, S., Mountain, B.W.
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(2020) Abiotic synthesis of methane and organic compounds in Earth’s lithosphere. Elements 16, 25-31
Reeves, E.P., and Fiebig, J.
