A total of nine geothermal fields located along an 800-km long E-W transect of the North Anatolian Fault Zone (NAFZ), Turkey were monitored for three years (2002)(2003)(2004) inclusive; 3-sampling periods per year) to investigate any possible relationship between seismic activity and temporal variations in the chemistry and isotope characteristics of waters in the fields. The geothermal fields monitored in the study were, from west to east, Yalova, Efteni, Bolu, Mudurnu, Seben, Kurşunlu-Ç ankırı, Hamamözü, Gözlek and Reşadiye. The chemical (major anion-cation contents) and isotopic ( 18 O/ 16 O, D/H, 3 H) compositions of hot and cold waters of the geothermal sites were determined in order to both characterize the chemical nature of the individual fields and identify possible temporal variations associated with localized seismic activity. The geothermal waters associated with the NAFZ are dominantly Na-HCO 3 , whereas the cold waters are of the Ca-HCO 3 type. The oxygen-and hydrogen-isotope compositions reveal that the hot waters are meteoric in origin as are their cold water counterparts. However, the lower d 18 O, dD and 3 H contents of the hot waters point to the fact that they are older than the cold waters, and that their host aquifers are recharged from higher altitudes with virtually no input from recent (post-bomb) precipitation. Although no major earthquakes (e.g., with M C 5) were recorded along the NAFZ during the course of the monitoring period, variations in the chemical and isotopic compositions of some waters were observed. Indeed, the timing of the chemical/isotopic changes seems to correlate with the occurrence of seismic activity of moderate magnitude (3 < M < 5) close to the sampling sites. In this respect, Cl, 3 H and Ca seem to be the most sensitive tracers of seismically-induced crustal perturbations, and the Yalova and Efteni fields appear to be the key localities where the effects of seismic activity on the geothermal fluids are most pronounced over the monitoring period. The present study has produced a 'baseline' database for future studies directed at characterizing the effects of moderate-major earthquakes on the composition of geothermal waters along the NAFZ. Future work involving longer monitoring periods with more frequent sampling intervals should lead to a better understanding of the underlying mechanism(s) producing the observed chemical and isotopic variations.
This study presents the results of a real-time gas monitoring experiment conducted, via the use of a quadrupole mass spectrometer, in a mofette field within the Tekke Hamam geothermal site in western Anatolia (Turkey), a tectonically active region hosting several east-west trending grabens. The study is aimed to establish a baseline gas profile of the region. Within the framework of the experiment, gas compositions (CO 2 , N 2 , O 2 , H 2 , H 2 S, CH 4 , He, and Ar) and flow rate of a mofette were monitored during two observation periods: November 2007-January 2008 and April-July 2008. During the course of monitoring, the major gas component was CO 2 with concentration changing around 96 volume percent. Other gases, from the most abundant to the least, were N 2 , CH 4 , O 2 , H 2 S, Ar, H 2 , and He. The study produced a short-term, baseline gas profile of the region with daily/diurnal variations and temporal gas fluctuations appearing as instant signals. Although the temporal gas fluctuations did not reach the anomaly level (variations staying within the mean ± 2σ), some of the variations in more than one parameter in the gas compositions (exceeding the mean ± 1σ), accompanied by changes in the diurnal gas pulses lasting for long durations, were correlated with the seismic events selected according to the adopted seismic event elimination criteria. The variations were mainly attributed to changing gas mixing ratios in relation to porosity/permeability modifications possibly related to seismicity. Studies involving the continuous monitoring of meteorological parameters are necessary to assign these variations to geogenic events.
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 (δ13C), hydrogen (δ2H) and nitrogen (δ15N) isotopic data of CO2, CH4, and N2 from gas samples collected from the Kızıldere and Tekke Hamam geothermal fields, located along the eastern segment of the Büyük Menderes Graben, Turkey. The stable isotopic composition of carbon (δ13C) ranges from +0.30 to +0.99‰ (PDB) for CO2 from Kızıldere and is slightly more variable (−0.95 to +1.3‰) in samples from Tekke Hamam. Carbon isotope data in combination with CO2/3He data reveal that ~97% (Tekke Hamam) to ~99% (Kızıldere) 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 Kızıldere, compared to Tekke Hamam can be attributed to the higher temperatures of the Kızıldere reservoir and resulting amplified fluid–limestone interaction, as well as helium depletion during phase separation for Kızıldere samples. In contrast to the carbon isotopic composition of CO2, the δ13C values of methane from Kızıldere and Tekke Hamam are clearly distinct and vary between −23.6 and −20.8‰ for Kızıldere and −34.4 and −31.7‰ for Tekke Hamam, respectively. The δ2H-CH4 composition is also distinct, measured as −126.7‰ for Kızıldere and −143.3‰ for Tekke Hamam. CO2-CH4 carbon isotope geothermometry calculations based on the isotopic fractionation of δ13C between the dominant component CO2 and the minor component CH4 reveals temperatures 20–40 °C and 100–160 °C higher than the bottom–hole temperatures measured for Tekke Hamam and Kızıldere, respectively. Based on the CO2-CH4 carbon isotope disequilibrium, unusual high methane concentrations of ~0.3 to 0.4 vol.-% and CH4/3He-δ13C-CH4 relationships we suggest thermal decomposition of late (Tekke Hamam) to over-mature (Kızıldere) 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–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 δ15N isotopic ratios (varying between −4.44‰ and 4.54‰), 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 N2 gas content in the samples.
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