[S-GC37] Volatiles in the Earth - from Surface to Deep Mantle

[SGC37-P02] Water and dissolved gas geochemistry of cold CO₂-rich springs in Ulleungdo volcano, South Korea

*Wonhee Lee¹、Hyunwoo Lee¹、Heejun Kim¹、Jungpyo Hong^1,2、Jeonghoon Lee³、Hyejung Jung³、Naoto Takahata⁴、Yuji Sano^4,5 (1.School of Earth and Environmental Sciences, Seoul National University, Republic of Korea、2.Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan, USA、3.Department of Science Education, Ewha Womans University, Republic of Korea、4.Atmosphere and Ocean Research Institute, University of Tokyo, Japan、5.Center for Advanced Marine Core Research, Kochi University, Japan)

キーワード：Ulleungdo, Cenozoic intraplate volcanism, Volatiles, Stable isotopes

Volatiles are released from deep into the atmosphere through fluids from springs of volcanoes and hydrothermal systems. Thus, chemical and isotope compositions of water and dissolved gases of springs may provide information on the source and evolution of volcanic gases. Ulleungdo, located in South Korea, is a volcanic island formed by Cenozoic intraplate volcanism and is an important site for understanding geological processes of East Asia after the formation of the back-arc basin in the rear of the Japanese arc. Here, we report the first results of water and dissolved gas geochemistry in nine cold springs (T = 9.5 to 22.7 ℃) in Ulleungdo to account for the origin of volatiles and constrain the conditions of the current hydrothermal system for the volcano. In particular, this presentation focuses on six springs with high CO₂ content (89.3 to 99.9 vol.%). The collected water and gas samples were analyzed for chemical and stable isotope compositions (e.g., d¹⁸O-H₂O, dD-H₂O, d¹³C-CO₂, and ³He/⁴He). The dD and d¹⁸O values of the spring water samples (-9.0 to -7.6‰ and -51.5 to -41.9‰ vs. V-SMOW, respectively) are plotted parallel to the global and local meteoric water lines, indicating the meteoric origin. The CO₂-rich springs are characterized by the Ca-(Na)-HCO₃ water type and travertine depositions. Their high total dissolved inorganic carbon (TDIC) contents (28.4 to 579.2 mg/L) and positively correlated cation concentrations imply silicate weathering due to acidic pH possibly caused by the addition of deep-derived CO₂-rich fluids. The d¹³C-CO₂ values (-7.3 to -5.1‰ vs. V-PDB) and the ³He/⁴He ratios (1.40 to 5.87 Ra) of CO₂-rich springs indicate the CO₂ and He input from deep sources (e.g., mantle) rather than the shallow crust. Interestingly, two N₂-rich springs exhibit atmospheric ³He/⁴He ratios (0.98 to 0.99 Ra), while one spring shows ³He/⁴He ratios (3.45 to 4.51 Ra) as high as the CO₂-rich springs. These high ³He/⁴He ratios of both spring types fall within the range reported in olivine and clinopyroxene phenocrysts in Ulleungdo basalts (4.5 to 6.0 Ra). In conclusion, this suggests that the spring gases may have originated from the same magma source that formed the volcanic rocks, and magma releasing volatiles still exists beneath Ulleungdo volcano.