Hydrogen emission characteristics of Changbaishan Volcano: Spatial and temporal distribution and genesis

Active volcanoes provide conduits for deep gas migration and are potential targets for hydrogen (H₂) resources. Changbaishan Volcano is one of the largest and most potentially hazardous active volcanoes in Asia, with strong volcanic gas emissions, but systematic studies on the characteristics of H₂ emissions are lacking. The source and controlling factors of H₂ in Changbaishan Volcano, as well as the relationship between H₂ and magmatism, are still unclear. In this work, we analysed the characteristics of H₂ emissions from hot springs, geothermal wells, and the soils of Changbaishan Volcano. In addition, we examined the genesis of H₂ by combining He isotopes and water–rock interaction experiments. The results show that the average concentration of H₂ in the hot springs is 9.5 ppm. Isotopic characterization revealed that the Changbaishan volcanic gases were emitted from the mantle and mixed with small amounts of crustal material. However, during the 2002–2005 volcanic unrest, the H₂ concentration increased abruptly to 321.5 ppm, and the corresponding isotopic characterization was converted to a single magmatic genesis. These findings indicate that the hot spring H₂ originated mainly from the magma chamber and was controlled by deep magmatism. The measurement results of H₂ concentrations reveal that >24 % of the surveyed area has values exceed the background threshold. The average H₂ concentration in the anomaly area reached 45.51 ppm, much higher than that in the hot springs and geothermal well during the volcanic quiescent period, suggesting that the soil gas was unlikely to be originated from magmatic degassing alone. Considering the widely-distributed basalts in Changbaishan Volcano, combined with the results of water–rock interaction experiments, we believe that alteration of basaltic rocks probably provides the main source for the H₂ in the soil. Global comparative analyses indicate that lithology is an important factor for soil H₂ emissions in volcanic areas. Volcanoes with higher soil H₂ emissions are predominantly composed of mafic volcanic rocks.