Research paper Large compositional differences in the gases released from the Kizildag ophiolitic body (Turkey): Evidences of prevailingly abiogenic origin Walter D'Alessandro a, * , Galip Yüce b , Francesco Italiano a , Sergio Bellomo a , Ahmet H. Gülbay c , Didem U. Yasin c , Antonina Lisa Gagliano a a Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Via Ugo La Marfa 153, 90146 Palermo, Italy b Hacettepe University, Department of Geological Engineering, Hydrogeology Division, Beytepe, 06800 Ankara, Turkey c Eskisehir Osmangazi University, Department of Geological Engineering, Meselik, 26480 Eskisehir, Turkey article info Article history: Received 16 August 2016 Received in revised form 7 December 2016 Accepted 19 December 2016 Available online 21 December 2016 Keywords: Gas geochemistry Serpentinization Hydrogen Abiogenic methane Stable isotopes abstract We investigated the geochemical features of the gases released from the Kizildag ophiolitic complex (Hatay, Turkey). Twenty-three samples both dissolved in hyperalkaline waters and free gases (bubbling gases and dry seeps) were collected. Samples were analysed for their chemical (He, H 2 ,O 2 ,N 2 , CH 4 and CO 2 ) and isotopic (He, d 13 C-CH 4 , d 2 H-CH 4 , d 2 H-H 2 ) composition including the content and C-isotopic composition of C 2 to C 5 alkanes in free gases. Analytical results evidence H 2 production through low- temperature (<80  C) serpentinization processes and subsequent abiogenic CH 4 production through Fischer-Tropsch-type reactions. In some sample small additions of methane either of microbial or of thermogenic origin can be hypothesized. At one of the sites (Kisecik) a clear fractionation pattern due to microbial methane oxidation leading to strongly enriched isotopic values (d 13 C þ15‰ and d 2 H 68‰) and depletion in methane concentrations has been evidenced. At the dry gas seep of Kurtbagi methane flux measurements have been made and a preliminary output estimation of about 1000 kg per year has been obtained. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction The Kizildag ophiolitic body crops out in the Hatay region (southern Turkey) (Dilek and Thy, 2009). It belongs to the peri- Arabian ophiolite belt that includes the Troodos (Cyprus), Ba€ er- Bassit (Syria) and Semail (Oman) ophiolites in the eastern Medi- terranean region which are the remnants of the Southern Tethys oceanic lithosphere (Seng€ or and Yilmaz, 1981). The area, seismi- cally very active, is close to the boundary of three tectonic plates (Anatolian, Arabian and African plates) and is characterised by important tectonic lineaments such as the Dead Sea Transform fault and the Karasu Fault which connects the former to the East Anatolian fault system (Mahmoud et al., 2013). The Cretaceous Kizildag ophiolitic body belongs to the Paleo- tectonic units of the area (pre-Pliocene basement rocks) over- thrusted onto the autochthonous pre-Cambrian to Campanian units and covered by the Campanian-Maastrichtian to Miocene units (Tekeli et al., 1983). The cretaceous ophiolite and ophiolitic complex have an extensive spatial distribution in the region (~1000 km 2 ) and are made up of ultramafic tectonites, mafic and ultramafic cumulates, gabbros, sheeted dyke complexes, plagiogranites, pillow lavas, bedded cherts and pelagic limestones (Dilek and Thy, 2009). Ultramafic rocks and their hydration products (serpentinites) represent mantle pieces that have been displaced by geodynamic processes close to the surface where they are exposed to circulating ground- or seawater. Their study received great impulse in recent years, in particular serpentines, because they play an important role in many geological situations. For example, their formation has a strong impact on the rheology of the lithosphere (Hirth and Guillot, 2013), they could play an important role in the C cycle and geological CO 2 sequestration (Power et al., 2013), they often host important ore deposits (Butt and Cluzel, 2013), and are considered as a possible candidate for the origin of life on the Earth or other planetary systems (McCollom and Seewald, 2013). A distinguishing characteristic of low temperature (<200  C) * Corresponding author. E-mail address: walter.dalessandro@ingv.it (W. D'Alessandro). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo http://dx.doi.org/10.1016/j.marpetgeo.2016.12.017 0264-8172/© 2016 Elsevier Ltd. All rights reserved. Marine and Petroleum Geology 89 (2018) 174e184