Aliphatic hydrocarbons in metasomatized gabbroic xenoliths from Hyblean diatremes (Sicily): Genesis in a serpentinite hydrothermal system |
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| Authors: | Enrico Ciliberto Carmelo Crisafulli Fabio Carmelo Manuella Filippo Samperi Salvatore Scirè Vittorio Scribano Marco Viccaro Ezio Viscuso |
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| Institution: | 1. V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia;2. Moscow State University, Leninskie Gory, Moscow, 119234, Russia;3. Kuzbassgiproshakht, ul. Ostrovskogo 34, Kemerovo, 650610, Russia;1. Institut f. Mineralogie, Universität Münster, 48149 Münster, Germany;2. CNRS-Université d''Orléans-BRG, Institut des Sciences de la Terre d''Orléans (ISTO), UMR 7327, 45071 Orléans, France;3. Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, 721302 Kharagpur, India;4. Department of Geological Sciences, University of Bern, CH-3012, Bern, Switzerland |
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| Abstract: | Many tholeiite gabbro xenoliths from the Hyblean tuff-breccia deposits (Sicily, southern Italy) present mineralogical and geochemical evidence for hydrothermal alteration at different temperatures and water/rock ratios. In some cases, the primary mineral assemblage has been entirely replaced by Na-rich alkali feldspar, chlorite/smectite interlayers, zeolites, aegirine–augite, titanite, zircon etc. Hence the chemical composition of such metasomatic rocks displays larger amounts of volatiles, alkalis, Zr, Hf, U, Th and lower Ca, Mg, Fe with respect to the original gabbro. Five hydrothermally altered gabbroic xenoliths were selected for thermal decrepitation and bulk gas analyses by quadrupole mass spectrometry. All the samples analyzed display the same Electron Impact-Direct Pyrolysis Mass spectra (EI-DPMS). These show a series of peaks differing by 14 mass units due to loss of methylene groups (–CH2), by a fragmentation process typical of saturated aliphatic and aliphatic–aromatic hydrocarbons. In addition, Fourier Transform Infrared (FT-IR) spectra of the samples present several bands typical of vibration frequencies of aliphatic hydrocarbons. The high-molecular-weight hydrocarbons observed probably originated from Fischer–Tropsch-type (FT-t) synthesis in the high temperature section of a serpentinite-hosted hydrothermal system. This suggestion may lend support to the recent hypothesis regarding the original oceanic nature of the Hyblean lithospheric basement. |
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