Aliphatic hydrocarbons in metasomatized gabbroic xenoliths from Hyblean diatremes (Sicily): Genesis in a serpentinite hydrothermal system Enrico Ciliberto a , Carmelo Crisafulli a , Fabio Carmelo Manuella b , Filippo Samperi a,c , Salvatore Scirè a , Vittorio Scribano b, ⁎, Marco Viccaro b , Ezio Viscuso a a Dipartimento di Scienze Chimiche, Università degli Studi di Catania, Viale Andrea Doria 6, I-95125 Catania, Italy b Dipartimento di Scienze Geologiche, Università degli Studi di Catania, Corso Italia 55, I-95129 Catania, Italy c Istituto di Chimica e Tecnologia dei Polimeri (ICTP) — CNR Catania, Viale Andrea Doria 6, I-95125 Catania, Italy abstract article info Article history: Received 15 July 2008 Received in revised form 15 October 2008 Accepted 19 October 2008 Editor: R.L. Rudnick Keywords: Sicily Xenoliths Gabbro Hydrothermal system Hydrocarbons Fischer–Tropsch-type synthesis 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 (–CH 2 ), 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. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Long-running petrological and geochemical research on deep- seated xenoliths from Hyblean diatremes (Sicily, Southern Italy) has provided the following fundamental evidence: 1) the dominant peridotites exhibit chemical composition and isotopic signatures consistent with a depleted mantle origin (Tonarini et al., 1996; Punturo et al., 2000; Sapienza et al., 2005); 2) the average chemical composition of the entire crustal suite is akin to that of basalts; 3) typical rocks of the continental crust are absent (Sapienza and Scribano, 2000). In addition, Scribano et al. (2006a) reported evidence of sheared oxide-rich gabbros resembling those from modern and fossil oceanic fracture-zones associated with slow-spreading ridges. The same authors also introduced the unconventional hypothesis that the Hyblean Meso-Cenozoic carbonatic and volcanic succession lies upon an oceanic core-complex, tectonically exposed at the seafloor of the Permo-Triassic Paleo-Tethys ocean (i.e. the present-day Ionian lithospheric domain: Vai, 2003). According to the above hypothesis, several Hyblean deep-seated xenoliths show abundant evidence of hydrothermal alteration related to seawater, which probably penetrated the mafic/ultramafic core- complex through deep fracture systems (Scribano et al., 2006a). Further, Scribano and Ioppolo (2006) briefly noted that such an ultramafite-hosted hydrothermal system may account for the pro- duction of methane and heavier hydrocarbons, as reported for modern deep-sea hot springs, particularly those located at the intersection of slow-spreading ridges and fracture zones or off-axis on inside-corner-highs (e.g., at Mid Atlantic Ridge: Rainbow vent field at 36°14′ N, Charlou et al., 2002; Lost City, at 30°N: Proskurowski et al., 2008; Logatchev hydrothermal field at 15°N, Schmidt et al., 2007; Saldanha hydrothermal field, 36°34′N, Dias and Barriga, 2006; Nibelungen field, 8° S, Melchert et al., 2008). In addition, diffuse CH 4 positive anomalies have been reported from the deepest parts of the seawater columns above exposed sea-bottom serpentinite diapirs (e.g. Charlou et al., 1998). Methane has been also detected in fluid inclusions from hydrothermally altered oceanic gabbros (e.g. Vanko and Stakes, 1991). Methane fluid inclusions generated in the oceanic crust persist during dramatic tectonic events, as well as Chemical Geology 258 (2009) 258–268 ⁎ Corresponding author. Tel.: +39 095 7195743; fax: +39 095 7195760. E-mail addresses: cilibert@unict.it (E. Ciliberto), ccrisafulli@unict.it (C. Crisafulli), manuella@unict.it (F.C. Manuella), fsamperi@unict.it (F. Samperi), sscire@unict.it (S. Scirè), scribano@unict.it (V. Scribano), m.viccaro@unict.it (M. Viccaro), ezio.viscuso@libero.it (E. Viscuso). 0009-2541/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2008.10.029 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo