Stable Isotope Evaluation of Geothermal Gases from the Kızıldere and Tekke Hamam Geothermal Fields, Western Anatolia, Turkey

Volatiles transported from the Earth&rsquo:s interior to the surface through permeable faults provide insights on the gas composition of deep reservoirs, mixing and migration processes, and can also be applied as gas-geothermometer. Here, we present carbon (&delta:13C), hydrogen (&delta:2H) and nitrogen (&delta:15N) isotopic data of CO2, CH4, and N2 from gas samples collected from the K&#305:z&#305:ldere and Tekke Hamam geothermal fields, located along the eastern segment of the B&uuml:y&uuml:k Menderes Graben, Turkey. The stable isotopic composition of carbon (&delta:13C) ranges from +0.30 to +0.99&permil: (PDB) for CO2 from K&#305:z&#305:ldere and is slightly more variable (&minus:0.95 to +1.3&permil:) in samples from Tekke Hamam. Carbon isotope data in combination with CO2/3He data reveal that ~97% (Tekke Hamam) to ~99% (K&#305:z&#305:ldere) of CO2 derives from limestone sources, with the residual CO2 being magmatic in origin with no evidence for CO2 from organic sources. The slightly higher contribution of limestone-derived CO2 in K&#305:z&#305:ldere, compared to Tekke Hamam can be attributed to the higher temperatures of the K&#305:z&#305:ldere reservoir and resulting amplified fluid&ndash:limestone interaction, as well as helium depletion during phase separation for K&#305:z&#305:ldere samples. In contrast to the carbon isotopic composition of CO2, the &delta:13C values of methane from K&#305:z&#305:ldere and Tekke Hamam are clearly distinct and vary between &minus:23.6 and &minus:20.8&permil: for K&#305:z&#305:ldere and &minus:34.4 and &minus:31.7&permil: for Tekke Hamam, respectively. The &delta:2H-CH4 composition is also distinct, measured as &minus:126.7&permil: for K&#305:z&#305:ldere and &minus:143.3&permil: for Tekke Hamam. CO2-CH4 carbon isotope geothermometry calculations based on the isotopic fractionation of &delta:13C between the dominant component CO2 and the minor component CH4 reveals temperatures 20&ndash:40 &deg:C and 100&ndash:160 &deg:C higher than the bottom&ndash:hole temperatures measured for Tekke Hamam and K&#305:z&#305:ldere, respectively. Based on the CO2-CH4 carbon isotope disequilibrium, unusual high methane concentrations of ~0.3 to 0.4 vol.-% and CH4/3He-&delta:13C-CH4 relationships we suggest thermal decomposition of late (Tekke Hamam) to over-mature (K&#305:z&#305:ldere) organic matter and, to some extent, also abiogenic processes as principal source of methane. The N2/36Ar ratios of most samples reveal the existence of a non&ndash:atmospheric nitrogen component within the gas mixture issuing from both fields, in addition to a constant contribution of atmospheric derived nitrogen accompanied into the system via the meteoric recharge of the geothermal system. Based on the &delta:15N isotopic ratios (varying between &minus:4.44&permil: and 4.54&permil:), the non&ndash:atmospheric component seems to be a mixture of both sedimentary (crustal organic) and mantle nitrogen. The thick Pliocene sedimentary sequence covering the metamorphic basement is the likely major source for the thermogenic content of CH4 and crustal N2 gas content in the samples.