Yalcin, T. H.

We investigated the geochemical features of the fluids circulating over the Amik Basin (SE Turkey–Syria border), which is crossed by the Northern extension of theDSF (Dead Sea Fault) and represents the boundary area of three tectonic plates (Anatolian, Arabian and African plates). We collected 34 water samples (thermal and cold from natural springs and boreholes) as well as 8 gas samples (bubbling and gas seepage) besides the gases dissolved in the sampled waters. The results show that the dissolved gas phase is a mixture of shallow (atmospheric) and deep components either of mantle and crustal origin. Coherently the sampled waters are variable mixtures of shallow and deep ground waters, the latter being characterised by higher salinity and longer residence times. The deep groundwaters (fromboreholes deeper than 1000 m)have a CH4-dominated dissolved gas phase related to the presence of hydrocarbon reservoirs. The very unique tectonic setting of the area includes the presence of an ophiolitic block outcropping in the westernmost area on the African Plate, as well as basalts located to the North and East on the Arabic Plate. The diffuse presence of CO2-enriched gases, although diluted by the huge groundwater circulation, testifies a regional degassing activity. Fluids circulating over the ophiolitic block are marked by H2-dominated gases with abiogenic methane and high-pH waters. The measured 3He/4He isotopic ratios display contributions from both crustal and mantle-derived sources over both sides of the DSF. Although the serpentinization process is generally independent from mantle-type contribution, the recorded helium isotopic ratios highlight variable contents of mantle-derived fluids. Due to the absence of recent volcanism over the western side of the basin (African Plate), we argue that CO2-rich volatiles carrying mantle-type helium and enriched in heavy carbon, are degassed by deep-rooted regional faults rather than from volcanic sources.

The Amik Basin is an asymmetrical composite transtensional basin developed between the seismically active left-lateral Dead Sea Fault (DSF) splays and the left-lateral oblique-slip Karasu Fault segment during neotectonic period. The relationship between the DSF and the East Anatolian Fault Zone is important as it represents a triple junction between Arabian Plate, African Plate and Anatolian Block in which the Amik Basin developed. The basin was formed on a pre-Miocene basement consisting of two rock series: Paleozoic crustal units with a Mesozoic allochthonous ophiolitic complex and ~1300 m thick Upper Miocene-Lower Pliocene sedimentary sequence. Plio-Quaternary sediments and Quaternary volcanics unconformably overlie the deformed and folded Miocene beds. Quaternary alkali-basaltic volcanism, derived from a metasomatized asthenospheric or lithospheric mantle, is most probably related to the syn-collisional transtensional strike-slip deformation in the area. Active faults in the region have the potential to generate catastrophic earthquakes (M>7). Nineteen samples of cold and thermal groundwaters have been collected over the Amik Basin area for dissolved gas analyses as well as two samples from the gas seeps, and one bubbling gas from a thermal spring Samples were analysed for their chemical and isotopic (He, C) composition. On the basis of their chemical composition, three main groups can be recognized. Most of the dissolved gases (16; Group I) collected from springs or shallow wells (< 150 m depth), contain mainly atmospheric gasses with very limited H2 (< 80 ppm) and CH4 (1– 2700 ppm) contents and minor concentrations of CO2 (0.5–11.2 %). The isotopic composition of Total Dissolved Carbon evidences a prevailing organic contribution with possible dissolution of carbonate rocks. However the CO2-richest sample shows a small but significant deep (probably mantle) contribution which is also evidenced by its He isotopic composition. Further three samples, taken from the northern part of the basin close to Quaternary volcanic outcrops and main tectonic structures, also exhibit a small mantle He contribution (Fig. 1). The two dissolved gases (Group II) collected from deep boreholes (> 1200 m depth) are typical of hydrocarbon reservoirs being very rich in CH4 (> 78 %) and N2 (> 13%). The water composition of these samples is also distinctive of saline connate waters (Cl- and B-rich, SO4-poor). Isotopic composition of methane (δ13C ~ -65‰) indicates a biogenic origin while He-isotopic composition points to a prevailing crustal signature for one (R/Ra 0.16) of the sites and a small mantle contribution for the other (R/Ra 0.98) (Fig. 1). The three free gas samples (Group III), taken at two sites within the ophiolitic basement west of the basin, have the typical composition of gas generated by low temperature serpentinisation processes with high hydrogen (37–50 %) and methane (10–61 %) concentrations. While all three gases show an almost identical δD-H2 of ~ -750‰, two of them display an isotopic composition of methane (δ13C ~ -5‰; δD ~ -105‰) and a C1/[C2+C3] ratio (~100) typical of abiogenic hydrocarbons and a significant contribution of mantle-type helium (R/Ra: 1.33). The composition of these two gasses is comparable to that of the gasses issuing in similar geologic conditions (Chimera-Turkey, Zambales-Philippine and Oman ophiolites). The gas composition of the other site evidences a contribution of a crustal (thermogenic) component (δ13C-CH4 ~ -30‰; δD-CH4 ~ -325‰; C1/[C2+C3] ~ 3000). Such crustal contribution is also supported by higher N2 contents (40% instead of 2%) and lower He-isotopic composition (R/Ra 0.07) (Fig. 1). These first results highlight contributions of mantle-derived volatiles possibly drained towards shallow levels by the DSF and other parallel structures crossing the basin showing a tectonic control of the fluids circulating within the Basin .

Twenty-two gas samples were collected in August 2012 in the area of Amik basin (Turkey). Two samples were collected from gas seeps, one was a bubbling gas in a thermal spring, while the remaining were dissolved gases from cold and thermal groundwaters (T 16-43 °C). All gases were analysed for their chemical composition (He, H2, O2, N2, CH4 and CO2) and for their He isotopic composition. Dissolved gases were also analysed for the carbon isotopic composition of the total dissolved carbon (TDC), while free gases also for their higher hydrocarbon (C1 – C5) content and for D of H2 and CH4, 13C of CH4 Basing on their chemical composition, the gases can be roughly subdivided in three groups. Most of the dissolved gases (16) belonging to the first group were collected from springs or shallow wells (< 150 m depth). All these samples contain mainly atmospheric gasses with very limited H2 (< 80 ppm) and CH4 (1 – 2700 ppm) contents and minor concentrations of CO2 (0.5 – 11.2 %). The isotopic composition of TDC evidences an almost organic contribution. The only exception is represented by the CO2-richest sample where a small but significant mantle contribution is found. Such contribution can also be evidenced in its 3He rich isotopic composition. Further three samples of this group evidence a small mantle contribution. These samples were collected in the northern part of the basin along the main tectonic structures delimiting the basin and close to areas with quaternary volcanic activity. A second group is composed by two dissolved gases collected from deep boreholes (> 1200 m depth). Their composition is typical of hydrocarbon reservoirs being very rich in CH4 (> 78 %) and N2 (> 13%). Also the water composition is typical of saline connate waters (Cl- and B-rich, SO4-poor). C-isotopic composition of methane ( 13C -65% ) points to a biogenic origin while He-isotopic composition indicates a prevailing crustal signature for one (R/Ra 0.16) of the sites and small mantle contribution for the other (R/Ra 0.98). To the last group belong four gas samples taken at two sites within the ophiolitic basement that crops out west of the basin. These gases have the characteristic composition of gas generated by low temperature serpentinisation processes with high hydrogen (37 – 50 %) and methane (10 – 61 %) concentrations. While all gases show an almost identical D-H2 of -750h those of one of the two sites display an isotopic composition of methane ( 13C -5h D -105% ) and a C1/[C2+C3] ( 100) ratio typical of abiogenic hydrocarbons and mantle-type helium (R/Ra: 1.33), while those of the other site evidence a contribution of a crustal (thermogenic) component ( 13C-CH4 -30h D -325h C1/[C2+C3] 3000). Such crustal contribution is also supported by higher N2 contents (40% instead of 2%) and lower He-isotopic composition (R/Ra 0.07). The preliminary results highlight contributions of mantle-derived volatiles to the fluids vented along the Amik Basin. The main tectonic structure of the area, the Death Sea Fault, and other parallel structures crossing the basin seem to be the responsible for deep-originated volatiles drainage towards shallow levels.