## 5 Discussion
### Isotopic Characteristics and Genesis of Spring Gas  
To constrain the origin and migration pathways of gases within the Maidan Fault system, we integrated noble gas and carbon isotope data with geophysical evidence, particularly Poisson's ratio anomalies. This multi-proxy approach enables distinction between mantle- and crustal-derived volatiles and reveals structural controls on fluid ascent. In natural environment, tritium (\({}^{3}\)He) is a primordial isotope, while deuterium (\({}^{4}\)He) predominantly originates from the radioactive decay of uranium (U) and thorium (Th). The \({}^{3}\)He/\({}^{4}\)He ratio serves as a tracer for helium sources: mantle-derived helium (1.1 \(\times\) 10\({}^{-5}\) to 1.4 \(\times\) 10\({}^{-5}\)), crustal helium (2 \(\times\) 10\({}^{-8}\)), and atmospheric helium (1.39 \(\times\) 10\({}^{-6}\)) [22; 64]. The mantle helium percentage is estimated using the atmospheric correction (Rc/Ra) derived from the observed \({}^{3}\)He/\({}^{4}\)He values. The combined analysis of \({}^{3}\)He/\({}^{4}\)He and \({}^{4}\)He/\({}^{20}\)Ne ratios provides insights into helium origin and atmospheric mixing (Figure 6a). Sample S1 exhibits 46.48% mantle-derived helium, indicating significant interaction with deep-sourced gases during ascent. This is likely driven by active tectonism or geothermal flux that facilitates mantle gas migration along fault conduits. Conversely, S13 contains only 5.01% mantle helium, indicating limited deep gas interaction. Instead, S13 helium primarily derives from U-Th decay, indicating stronger crustal influence. This implies a relatively quiescent tectonic environment, or alternatively, enhanced trapping of radiogenic helium with shallow fracture networks. The \(\delta^{13}\)C CO\({}_{2}\) values in thermal springs provide further constraints on carbon sources. Different reservoirs have distinct \(\delta^{13}\)C CO\({}_{2}\) values: Mid-Ocean Ridge Basalt (MORB) has a \(\delta^{13}\)C CO\({}_{2}\) value of \(-\)6.5%, carbonate has a \(\delta^{13}\)C CO\({}_{2}\) value of \(-\)30%, and organic sources have a \(\delta^{13}\)C CO\({}_{2}\) value of \(-\)30% [30; 64]. However, isotopic fractionation during CO\({}_{2}\) migration complicates unambiguous source attribution [11; 64]. Thus, dual-parameter plots of \(\delta^{13}\)C CO\({}_{2}\) versus CO\({}_{2}\)/\({}^{3}\)He ratios are employed to apportion carbon sources (Figure 6b). Quantitative deconvolution of CO\({}_{2}\) origins in S1 (Table 1) reveals (1) Carbonate dissolution (82.7%): the moderately enriched \(\delta^{13}\)C CO\({}_{2}\) value (\(-\)4.2%) and high CO\({}_{2}\)/\({}^{3}\)He ratio (3.49 \(\times\) 10\({}^{10}\)  
Figure 6: (a) Plot of \({}^{4}\)He/\({}^{20}\)Ne versus Rc/Ra ratio. Mixing lines between the atmosphere and upper mantle, as well as between the atmosphere and crust, were calculated using the following end-member compositions. Air: \({}^{3}\)He/\({}^{4}\)He = 1.4 \(\times\) 10\({}^{-6}\); \({}^{4}\)He/\({}^{20}\)Ne = 0.318, upper mantle: \({}^{3}\)He/\({}^{4}\)He = 12 \(\times\) 10\({}^{-6}\); \({}^{4}\)He/\({}^{20}\)Ne = 100,000, old continental crust: \({}^{3}\)He/\({}^{4}\)He = 0.02 \(\times\) 10\({}^{-6}\); \({}^{4}\)He/\({}^{20}\)Ne = 100,000 [64]. (b) Plot of \(\delta^{13}\)C CO\({}_{2}\) values versus CO\({}_{2}\)/\({}^{3}\)He ratio. The end-member compositions for Organic (O), MORB (M), and Carbonate (C) are as follows: O: \(\delta^{13}\)C CO\({}_{2}\) = \(-\)30%; CO\({}_{2}\)/\({}^{3}\)He = 1 \(\times\) 10\({}^{13}\), M: \(\delta^{13}\)C CO\({}_{2}\) = \(-\)6.5%, CO\({}_{2}\)/\({}^{3}\)He = 2 \(\times\) 10\({}^{9}\); C: \(\delta^{13}\)C CO\({}_{2}\)= 0%; CO\({}_{2}\)/\({}^{3}\)He= 1 \(\times\) 10\({}^{13}\). Binary mixing trajectories between M and C, M and O, and C and O are illustrated in the diagram [63]. are indicative of extensive fluid-rock interaction with carbonate-rich basement lithologies. (2) MORB input (4.3%): slight \(\xi^{13}\)C depletion and Rc/Ra = 3.71 (46.48% mantle He) suggest active mantle degassing through reactivated faults at the TFF-MDF intersection.