Water and gas geochemistry of springs in Ulleungdo volcano, South Korea: Implications for degassing of upper mantle-derived volatiles in Northeast Asia

Ulleungdo, a volcanic island located in the back-arc basin (East Sea/Sea of Japan) of the Japanese arc, is one of the Cenozoic intraplate volcanoes in Northeast (NE) Asia. Due to the unique tectonic characteristics of the transition from the past back-arc opening phase to the current intraplate setting, elucidating the magma origin of this volcano allows us to evaluate the possible influence of diverse upper mantle components on intraplate magmatism. Here we report the results of water and dissolved gas geochemistry sampled from 13 springs in Ulleungdo. The springs can be categorized into high CO₂ concentration (HC) and low CO₂ concentration (LC) types, each exhibiting different evolutionary pathways. The HC springs show high ³He/⁴He ratios (1.04 to 5.87 Ra) and δ¹³C-CO₂ values (–7.3 to –5.1‰), indicating the release of mantle-derived helium and CO₂. Although some of the LC springs also contain mantle-derived helium (0.98 to 4.51 Ra), their lower δ¹³C-CO₂ values (–22.3 to –17.3‰) and CO₂/³He ratios (3.79 x 10⁸ to 1.39 x 10¹⁰) reflect calcite precipitation during fluid ascent. The consistency of the ³He/⁴He ratios between the springs and the previously reported Ulleungdo volcanic rocks suggests the preservation of the helium isotope ratio in the Ulleungdo magma from at least 1.37 Ma to the present. Collectively, helium and carbon isotope compositions from Ulleungdo, Mt. Baekdu, and Wudalianchi in NE Asia could be determined by regional variations in continental lithospheric thickness. This may therefore be explained by a contribution from the subcontinental lithospheric mantle and/or interaction with the continental crust during fluid ascent, rather than an influx of subducted lithospheric material into magma sources. Our findings demonstrate that the interaction between magma derived from the upwelling asthenospheric mantle and the overlying continental lithosphere can be a key process in controlling the volatile geochemistry of the NE Asian Cenozoic intraplate magmatism.