Sources of CH4 with variable carbon isotopes from Changbaishan volcano in NE China: Implications for the feeding system

We report methane (CH4) with variable carbon isotopes (between −60.9‰ and − 26.8‰) from Changbaishan, the most dangerous volcano in NE China. Furthermore, recent chemical and isotopic data of He, CO2, and CH4 in hot spring gases at Changbaishan volcano are also presented. Bubbling gas samples were collected periodically from 2014 to 2019 in three hot spring areas in the Changbaishan volcanic field: one on the shoreline of the Tianchi caldera lake and two on the north and southwest flanks. The helium isotopes of the bubbling gases from the three hot spring areas suggested a major magmatic source, up to 5.8 Ra (where Ra represents the atmospheric 3He/4He ratios). However, the CH4 exhibited a biotic signature in carbon isotopes (δ13C) as low as −60.9‰, and the concentrations of CH4 among the three hot spring areas exhibited remarkable variations from 0.02% to 17.04%. After evaluating the influence of air contamination, crustal and organic contributions, and thermodynamic processes, we suggest that CH4 from the three hot spring areas primarily originated from two sources: (1) a previously formed gas reservoir in the crust with significant contributions from the ancient subductions and (2) the MORB-like materials from the present upwelling asthenosphere. Based on these observations, we propose a model to explain the processes that formed these two sources, which demonstrates the existence of ancient subduction fluids in the subcontinental lithospheric mantle. In addition, our study of volcanic gases does not support the conspicuous material connections between the subducting west Pacific slab and Changbaishan volcano. Finally, based on our results, suggestions for volcanic gas monitoring at the Changbaishan volcano are provided.