Li Zhang 1, Xiangxian Ma 1*, Zhongping Li 1, Chunhui Cao 1, Georgy Chelnokov 2, Vasilii Lavrushin 2,

 Guodong Zheng1,3*

(1. Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000,

China 2. Geological Institute, RussianAcademy of Sciences, Moscow 119017, Russia 3. School of

Environmental Studies, China University of Geosciences at Wuhan, Wuhan 430074, China)

Abstract: The Muji Basin, located at the tectonic zone between the northwestern Tibetan Plateau and the eastern Pamir Plateau, is characterized by abundant carbonic springs, gas emission sites, and tufa deposits, providing a natural window into deep gas release within a thick continental crust. This study investigates the associated gases of the Muji carbonic springs using gas compositions, δ13C_CO2, 3He/4He, and CO₂/3He ratios to constrain the origin of CO₂, spatial variations in gas compositions, and the role of fault systems in controlling gas release. 

The associated gases of the Muji carbonic springs are dominated by CO₂, followed by N₂, with minor He. Except for the N₂-rich sample MJXSP-03, most samples contain 60.9%–83.4% CO₂, and 15.9%–38.0% N₂. The CO₂/N₂ ratios vary markedly among spring sites, indicating spatial heterogeneity in deep CO₂ supply and conduit connectivity. Strong emission sites, such as HD-11 and HD-12, show high CO₂ contents and high  CO₂/N₂ ratios, whereas MJXSP-03 contains 69.7% N2 and only 29.2% CO₂, suggesting a transition from  strongly connected CO₂-dominated conduits to shallowly mixed or distal N₂-rich end-members. Carbon isotopic compositions further support this spatial differentiation. The CO2 volume fraction shows a positive correlation with δ13C_CO2 values, which range from –5.8‰ to –2.9‰ (Pearson r = 0.732, n = 12, p < 0.01),  indicating that high-CO₂ samples generally have heavier carbon isotopic compositions. Strong gas emission sites are spatially associated with fault intersections or secondary fault segments, where high CO₂ contents,  elevated CO₂/N₂ ratios, and relatively heavy δ13C_CO2 values indicate enhanced permeability and stronger  connectivity with deep gas reservoirs. The 3He/4He ratios are generally low, approximately 0.08–0.12 Ra,  indicating a dominant contribution from radiogenic crustal He. The He-CO₂ ternary mixing model further indicates that the mantle-derived CO₂ contribution is limited, whereas CO₂ is mainly derived from  metamorphic decarbonation or thermal decomposition of crustal carbonate rocks within the thickened crust. The presence of minor mantle-derived He and CO₂ signals suggests that deep-seated faults in the Muji area may act as restricted but discernible conduits for the upward migration of deep volatiles.

Overall, the Muji carbonic springs represent a distinct crustal CO₂ degassing system in a non-volcanic  continental collision zone. The study results provide new insights into deep gas migration in the tectonic transition zone of eastern Pamir–northwestern Tibetan Plateau and demonstrate the potential of CO₂-He gas geochemistry for identifying deep fault connectivity and regional tectonic fluid activity.

Keywords: Muji Basin; Carbonic springs; CO₂ degassing; He-C isotopes; Eastern Pamir This research was funded by the National Key Research and Development Program of China (No. 2019YFA0708501) and the RSF–NSFC Joint Research Project (No. 42261134534).