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

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, δ 13 C-CH 4 , δ 2 H-CH 4 , δ 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 ( 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 (δ 13 C +15‰ and δ 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.