Publication Date:
2023-02-01
Description:
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.
Description:
Published
Description:
452
Description:
1TR. Georisorse
Description:
JCR Journal
Keywords:
geothermal field
;
source of gas
;
stable isotopes
;
thermogenic methane
;
05.09. Miscellaneous
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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