First indications on CH4 origin in the CO2-dominated gas discharges of the western Eger Rift (Czech Republic)

Tectonic structures such as faults and fractures act as preferential pathways for gas ascent and their consequent release into the atmosphere. Magma-derived gases are widespread throughout the western Eger Rift (Czech Republic), an intraplate region without active volcanism but with the occurrence of mid-crustal earthquake swarms. Geogenic CO₂ discharges from the Počatky–Plesná fault zone (PPZ), Mariánské Lázně Fault (MLF), Bad Brambach (BB), and a deep local fault (DLF) have been sampled since 2021. Gases were analysed for their chemical (CO₂, N₂, O₂, Ar, He, CH₄, and H₂) and isotopic contents (noble gases, CO₂, and CH₄). Results showed that CO₂ is the dominant gas species (concentrations > 99.4%), with the remaining gases being present in minor amounts. The He isotopic composition for gas samples from the PPZ and MLF is typical for the subcontinental lithospheric mantle (SCLM - with ³He/⁴He ratios between 5 and 6 R_A), while gases from BB and the DLF show a lower mantle input (³He/⁴He is 3.2 and 2.4 R_A, respectively). δ¹³C_CO2 data reflect a SCLM CO₂ signature (-4 to -1 ‰ vs. V-PDB). First CH₄ isotopic data present values between -52.0 and -47.1 ‰ vs. V-PDB for δ¹³C_CH4 and between -307 and -284 ‰ vs. V-SMOW for δ²H_CH4. With the exception of samples collected from the MLF that show a clear thermogenic CH₄ origin, all the other samples present isotopic values and CH₄/(C₂H₆+C₃H₈) ratios that suggest a likely biogenic origin, with secondary processes playing a crucial role on the gases’ isotopic signature. It should be noted that low CH₄concentrations (<100 μmol/mol) correspond to low CH₄/³He ratios (around 10⁴) in CO₂-rich magmatic gases, suggesting a possible common origin for these gases. Therefore, multiple origins of CH₄ (biogenic and volcanic-geothermal) cannot be excluded.

This research is a part of the MoCa - “Monitoring Carbon” project and this work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 461419881.