Geothermal He and CO₂ variations at Changbaishan intra‐plate volcano (NE China) and the nature of the sub‐continental lithospheric mantle

We report new He and CO 2 data for geothermal fluids from Changbaishan Volcano located on the border of China and North Korea. Helium isotope ratios reach a maximum of 5.4 R A (where R A = 3 He/ 4 He in air) whereas carbon isotope values ( δ 13 C) fall between −2 and −7 ‰ (vs. PDB). The ratio of CO 2 / 3 He varies between 0.5–7.1 (×10 9 ) for gas samples but is considerably higher (∼10 12 ) in waters. The combined He‐CO 2 systematics reveal the clear imprint of a dominant subducted slab contribution to the total CO 2 inventory. We conclude that these geothermal volatiles originate from the sub‐continental lithospheric mantle (SCLM) in NE China and represent ancient fluids captured by prior metasomatic events.