Large compositional differences in the gases released from the Kizildag ophiolitic body (Turkey): Evidences of prevailingly abiogenic origin

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, CH4 and CO2) and isotopic (He, delta C-13-CH4, delta H-2-CH4, delta H-2-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 degrees C) serpentinization processes and subsequent abiogenic CH4 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 (delta C-13 +15 parts per thousand and delta H-2-68 parts per thousand) 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. (C) 2016 Elsevier Ltd. All rights reserved.