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

[SGC37-P03] Geochemistry of mantle-derived volatiles dissolved in springs and groundwater released through fluids in Jeju Island, South Korea

*Jeongyeon Yu¹、Heejun Kim¹、Jeonghoon Lee²、Hyejung Jung²、Naoto Takahata³、Yuji Sano^3,4、Hyunwoo Lee¹ (1.School of Earth and Environmental Sciences, Seoul National University, Republic of Korea、2.Department of Science Education, Ewha Womans University, Seoul, Republic of Korea、3.Atmosphere and Ocean Research Institute, University of Tokyo, Japan、4.Center for Advanced Marine Core Research, Kochi University, Japan)

キーワード：Jeju island, Helium isotope, Carbon isotope, Volatiles, Volcano

In Jeju Island, where no active volcanism has occurred for thousands of years, it was recently reported that a low-speed area (LVZ) exists in the subcontinental part, and groundwater in some areas of Jeju Island is known to contain mantle-derived carbon dioxide (CO₂). To identify potential volcanic activity in Jeju Island, we report the results of geochemical analysis from water collected from springs and groundwater wells and gases dissolved in water samples. In most springs and groundwater in Jeju Island, CO₂ and N₂ are the main dissolved gas components and contain more than 79.9 vol.%. In particular, among them, CO₂ of more than 99.5 vol.% is measured in carbonic springs. In addition, the relative abundances of N₂-Ar-He in the dissolved gases show that carbonic springs and some groundwater with high He concentrations are relatively well-preserved for and tend to be diluted by atmospheric components. The relatively high carbon isotope ratios of CO₂ in all carbonic springs (δ¹³C vs. V-PDB = -1.5 to -9.3‰) indicate the mantle contribution, but the other samples have lower δ¹³C values of -13.4 to -22.7‰, which implies biogenic CO₂. Similarly, the carbonic springs exhibit mantle-derived helium, with helium isotope ratios (³He/⁴He) ranging from 5.57 Ra to 6.25 Ra. Moreover, mantle-derived helium is also observed in some groundwater wells (5.59 to 6.79 Ra), which means that mantle-derived CO₂ is emitted only in certain areas, but helium is released throughout Jeju Island. Taken together, mantle-derived CO₂ and He are currently degassed in Jeju Island, which can be seen as indicators of potential magmatic activities considered together with the LVZ distribution, that continuous monitoring is necessary.