Degassing of deep-sourced CO2 from Xianshuihe-Anninghe fault zones in the eastern Tibetan Plateau

A large number of gases are releasing from the medium-high temperature geothermal fields distributed along the large-scale strike-slip fault zones in the southeastern margin of the Tibetan Plateau. In this study, 11 hot spring water and the associated bubbling gas samples were collected along the Xianshuihe-Anninghe fault zones (XSH-ANHFZ) and analyzed for chemical and isotopic compositions. The $${\delta ^{18}}{{\rm{O}}_{{{\rm{H}}_2}{\rm{O}}}}$$ and $$\delta {{\rm{D}}_{{{\rm{H}}_2}{\rm{O}}}}$$ values indicate that hot spring waters are predominantly meteoric origin recharged from different altitudes. Most water samples are significantly enriched in Na+ and HCO3− due to the dissolution of regional evaporites, carbonates and Na-silicates. 3He/4He ratios of the gas samples are 0.025–2.73 times the atmospheric value. The 3He/4He ratios are high in the Kangding region where the dense faults are distributed, and gradually decrease with increasing distance from Kangding towards both sides along the Xianshuihe fault zones (XSHFZ). Hydrothermal fluids have dissolved inorganic carbon (DIC) concentrations from 2 to 42 mmol L−1, δ13CDIC from −6.9‰ to 1.3‰, $${\delta ^{13}}{{\rm{C}}_{{\rm{C}}{{\rm{O}}_2}}}$$ from −7.2‰ to −3.6‰ and Δ14C from −997‰ to −909‰. Combining regional geochemical and geological information, the CO2 sources can be attributed to deep-sourced CO2 from mantle and metamorphism of marine carbonate, and shallow-sourced CO2 from the dissolution of marine carbonate and biogenic CO2. The mass balance model shows that 11±6% of the DIC is sourced from the dissolution of shallow carbonate minerals, 9±8% formed by pyrolysis of sedimentary organic matter, 80±9% derived from deep metamorphic origin and mantle-derived CO2. Among them, the deep-sourced CO2 in Anninghe fault zones (ANHFZ) is merely metamorphic carbon, whereas ca. 12% and ca. 88% of the deep-sourced CO2 in the XSHFZ are derived from the mantle and metamorphic carbon, respectively. The average deep-sourced CO2 flux in the Kangding geothermal field is estimated to be 160 t a−1. If all the hot springs in various fault zones in the southeastern margin of the Tibetan Plateau are taken into account, the regional deep-sourced CO2 flux would reach ca. 105 t a−1. These results show that the deep-sourced CO2 released from non-volcanic areas might account for a considerable proportion of the total amount of global deep-sourced carbon degassing, which should be paid more attention to.