Continental Subduction and the Deep Carbon Cycle in Northern Tibet

Degassing of volatiles within convergent plate margins, investigated through systematic variations in gas geochemistry, provides crucial insights into the recycling process, the lithospheric structure, and the dynamics of plateau growth. To date, such processes in the India-Asia continental collision zone remains poorly constrained in northern Tibet due to a dearth of detailed geochemical data on volatiles. Here, we report new data on chemical compositions and He–C isotopic ratios of hydrothermal volatiles in the northern plateau. CO₂-rich samples exhibit elevated ³He/⁴He ratios (0.11 R_A–0.39 R_A) compared to crustal values, as well as displaying heavy δ¹³C (−4.66–0.02‰) and high CO₂/³He ratios ((36–9,400) × 10⁹), indicating in summary the occurrence of carbonate in the mantle-derived components. We have developed a coupled He–C isotope model incorporating depleted mantle (DM), recycled carbonate (RC), and crustal carbon endmember (CCE) reservoirs to explore quantitatively the nature of the hydrothermal volatiles emitted. The results of model calculations reveal an increasing proportion of RC together with a decreasing proportion of CCE from south to north that is accompanied by an increasing contribution from DM, all suggesting the presence of a carbonated mantle beneath northern Tibet. Degassing of helium from hydrothermal activities exhibits relatively high fluxes of total ³He (10⁴–10⁵ atoms/m²/s), indicating a tectonically active degassing of volatiles in the Tibetan Plateau under ongoing continental convergence between India and Asia. Systematic variations in He–C isotopes, mantle helium fluxes and RC/CEE proportions along hydrothermal activities spanning from the Himalayas to the Qaidam-Gonghe basin are consistent with a dual continental subduction model acting beneath the whole Tibetan Plateau.