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

[SGC37-02] Latent magmatism beneath non-volcanic area unveiled

*Heejun Kim¹, Hyunwoo Lee¹, Jungpyo Hong^1,2, Jung-Hun Song¹, Takanori Kagoshima³, Naoto Takahata⁴, Yuji Sano^4,5 (1.Seoul National University, 2.University of Michigan, 3.University of Toyama, 4.University of Tokyo, 5.Kochi University)

Keywords:Helium isotopes, Stable isotopes, Magmatism

The correlation between wave velocity anomaly and the geochemical property was often used to illustrate the behavior of mantle-derived material. The correlation was usually proved in a volcanic area such as the Rio Grande rift. On the other hand, since active volcanism had ceased for several Millenniums, the Korean peninsula has been regarded as a non-volcanic setting. However, severe low-velocity zones were reported beneath the Cenozoic volcanic area of the Korean peninsula, suggesting the presence of melt. To verify the correlation of volatile geochemistry at the surface with mantle tomography, 35 new water samples (e.g., natural spring, hot spring, and groundwater) were collected and analyzed in this study, from chemical composition to isotope compositions including noble gases. Stand on the correlation of ³He/⁴He emitting on the Korean peninsula and S-wave velocity tomography at depth, here we propose the possible occurrence of magmatism beneath the Korean peninsula.
N₂-Ar-He composition showed a two-component mixing of the atmospheric and MORB/crust components in all samples, without evidence of subducted slab signals. N₂ and CO₂ are the main components, where wide ranges of δ¹⁵N-N₂ (-8.0 to 3.3‰) and δ¹³C-CO₂ (-20.2 to -3.7‰) suggest that N₂ and CO₂ come from both organic matter in the sedimentary layer and mantle-derived sources. Similarly, ³He/⁴He ratios (0.12 Ra to 6.17 Ra) reflect varying degrees of mantle-derived helium contributed to this area. A negative correlation between S-wave velocity and ³He/⁴He ratios suggests that the presence of melt and volatiles has reduced seismic wave velocity, as indicated by increased helium isotope ratios.
In addition, the computed ³He fluxes are comparable to the global major fault-controlled hydrothermal systems. The Korean gas samples show that mantle and crustal ⁴He fluxes are similar to global volcanic regions rather than the tectonic areas. Therefore, combining geochemical results and geophysical information suggests that there could be latent magmatism beneath the non-volcanic Korean peninsula.