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

Volatiles transported from the Earth'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 C-13), hydrogen (delta H-2) and nitrogen (delta N-15) isotopic data of CO2, CH4, and N-2 from gas samples collected from the Kizildere and Tekke Hamam geothermal fields, located along the eastern segment of the Buyuk Menderes Graben, Turkey. The stable isotopic composition of carbon (delta C-13) ranges from +0.30 to +0.99 parts per thousand (PDB) for CO2 from Kizildere and is slightly more variable (-0.95 to +1.3 parts per thousand) in samples from Tekke Hamam. Carbon isotope data in combination with CO2/He-3 data reveal that similar to 97% (Tekke Hamam) to similar to 99% (Kizildere) 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 Kizildere, compared to Tekke Hamam can be attributed to the higher temperatures of the Kizildere reservoir and resulting amplified fluid-limestone interaction, as well as helium depletion during phase separation for Kizildere samples. In contrast to the carbon isotopic composition of CO2, the delta C-13 values of methane from Kizildere and Tekke Hamam are clearly distinct and vary between -23.6 and -20.8 parts per thousand for Kizildere and -34.4 and -31.7 parts per thousand for Tekke Hamam, respectively. The delta H-2-CH4 composition is also distinct, measured as -126.7 parts per thousand for Kizildere and -143.3 parts per thousand for Tekke Hamam. CO2-CH4 carbon isotope geothermometry calculations based on the isotopic fractionation of delta C-13 between the dominant component CO2 and the minor component CH4 reveals temperatures 20-40 degrees C and 100-160 degrees C higher than the bottom-hole temperatures measured for Tekke Hamam and Kizildere, respectively. Based on the CO2-CH4 carbon isotope disequilibrium, unusual high methane concentrations of similar to 0.3 to 0.4 vol.-% and CH4/He-3-delta C-13-CH4 relationships we suggest thermal decomposition of late (Tekke Hamam) to over-mature (Kizildere) organic matter and, to some extent, also abiogenic processes as principal source of methane. The N-2/Ar-36 ratios of most samples reveal the existence of a non-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 N-15 isotopic ratios (varying between -4.44 parts per thousand and 4.54 parts per thousand), the non-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 N-2 gas content in the samples.