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

[SGC37-P08] Helium isotopic composition of basement rocks: Implication for temporal variations in groundwater chemistry before and after the 2024 Noto Peninsula earthquake

*Misaki Hasegawa¹, Tomoaki Morishita¹, Takanori Kagoshima², Naoto Takahata³, Toshiro Takahashi⁴, Yoshihiko Tamura⁵ (1.Kanazawa University, 2.University of Toyama, 3.Atmosphere and Ocean Research Institute, the University of Tokyo, 4.Niigata University, 5.Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology)

Our group has been continuously sampling groundwater before and after the 2024 Noto Peninsula earthquake, for investigating changes in groundwater geochemistry associated with earthquake activity. Umeda et al. (2024) reported the presence of gas with high helium isotope ratios in groundwater in the northern Noto Peninsula, indicating the possibility of a mantle-derived fluid flux in this region. We have observed decrease in helium isotope ratios in groundwater at several sites in the NE Noto Peninsula before the Noto Peninsula earthquake (M7.6) on 1 January 2024 (Kagoshima et al., 2024, JpGU Abstracts). When rocks are fractured by seismic activity, rock components can be released, which can significantly alter the composition of groundwater (Sano et al., 2016 Sci. Rep.). In order to understand the relationship between the groundwater geochemical variations and the Noto Peninsula earthquake swarm activity including the M7.6 earthquake, information of the helium isotope ratios of basement rocks that may affect the helium isotope ratios of groundwater is needed. In this study, helium isotope ratios were measured for rocks of the Hida Belt (metamorphic and granitic rocks) and Tertiary volcanic rocks that make up the Noto Peninsula crust, with the aim of providing constraints on the fluid origin and geochemical variation associated with the Noto Peninsula earthquake. The measurements of noble gases (³He/⁴He ratios, ⁴He/²⁰Ne ratios and helium concentrations) were conducted at the noble gas laboratory at the Atmosphere and Ocean Research Institute, the University of Tokyo.
The measurement results show that the helium isotope ratios of the basement rocks are much lower than those of the groundwater. Most of the sample results are comparable to the calculated helium isotope ratios of radiogenic component which was generated in the rock based on the major element composition of the rocks (Morikawa and Tosaki, 2013 GSJ Res. Mater.), suggesting that the helium in the rocks composing the crust is of radiogenic origin. Some samples show helium isotope ratios higher than calculated ratios of pure radiogenic origin, which may be the result of mixing of mantle-derived and radiogenic helium. The contribution of mantle-derived helium component is thought to be low in the basement rocks. Therefore, the high helium isotope ratio of groundwater cannot be explained by a supply from the basement rocks. From the above, it can be suggested that the groundwater with high helium isotope ratios in the northern Noto Peninsula is due to elevated mantle-derived fluids and decrease in helium isotope ratios in groundwater before the Noto Peninsula earthquake were mixed with helium of surrounding crustal rock origin. The causes of this decrease in the helium isotope ratio of groundwater are discussed using the results of helium isotope ratio measurements of basement rocks.