Origins of volatiles and helium fluxes from hydrothermal systems in the Eastern Himalayan Syntaxis and constraints on regional heat and tectonic activities

•Hydrothermal helium is mainly from radiogenic production in the crust with 7.52 % is mantle derived.•CO2 is mainly constrained by both decarbonization of Tethyan marine carbonate rocks and mantle fluid.•The He-CO2 system is constrained by the regional magmatic intrusions and degassing process.•The contrasting slab subduction style and the south boundary of EHS are constrained by helium isotopes.•Location of mantle suture in EHS is extended and the south boundary is also constrained by helium isotopes. While understanding the origins of volatiles has strong implications on heat and tectonic processes, the relationship has not well understood, especially in the Eastern Himalayan Syntaxis (EHS) in the Tibetan Plateau, where tectonic framework, petrologic properties, stress field, and geomorphic water system changes greatly due to the strongest continental collision. Based on samples from warm/hot springs in the EHS, geothermal fluid is sourced from local precipitation and simultaneously recharged by deeply-sourced mantle volatiles. Volatiles from hydrothermal systems are mixture of crustal and mantle origin, in which helium is mainly from radiogenic production in the crust with only up to 7.52% is mantle derived whereas CO2 is mainly constrained by both decarbonization of Tethyan marine carbonate rocks and mantle volatiles. The helium fluxes are estimated to be almost 1–2 orders higher than some representative volcanically and tectonically active continental regions. The He-CO2 system is dominantly constrained by the regional magmatic intrusions and hydrothermal degassing process. The boundary of the Indian plate underplating beneath Tibet is extended eastward to across the EHS by evidences from He isotope and this boundary is probably the south boundary of the EHS. Differences of helium isotopes between the EHS and its southeastern region indicate the contrasting slab subduction style in the deep structure. These results present some quantitative constraints for the interpretations of regional tectonic processes on transportation of deeply-sourced volatiles and its indications on continental collision style and present essential basis for understanding volatiles degassing and cycling in the continental collision settings.