09:25 〜 09:50
[AHW30-02] Geochemical characteristics of groundwater related with deep-seated fluid in Hokkaido
★Invited Papers
キーワード：熱水、地下水、リチウム、水素酸素安定同位体比
In Japan, which is located in a subduction zone, fluids dehydrated from slab ascend to near the surface and are referred to as deep-seated fluids. The deep-seated fluids mix with regional groundwater systems and are utilized as geothermal resources. However, they may also contribute to natural disasters, such as the Noto Peninsula earthquake in January 2024, thereby exerting various impacts on human life. Therefore, estimating the origin and extent of deep-seated fluids is essential for assessing both the sustainable use of geothermal resources and disaster risks.
To estimate the extent of deep-seated fluid influence, it is crucial to understand the ascent processes from the slab. Three primary types of deep-seated fluids have been identified: diagenetic dehydration fluids, mineral dehydration fluids, and magmatic water (AIST, 2017; Tomioka et al., 2020). The ascent processes of these fluids differ between southwestern Japan (e.g., the Kinki and Shikoku regions) and northeastern Japan (AIST, 2017). In the NE Japan Arc, where the Pacific Plate subducts, mineral dehydration fluids are transported deeper by mantle convection and contribute to magma formation. Consequently, the primary sources of deep-seated fluids reaching the surface are diagenetic dehydration fluids and magmatic water. Furthermore, their spatial distribution has been clarified: diagenetic dehydration fluids are predominantly found on the forearc side, while magmatic water is mainly distributed along the volcanic front and the back-arc side (AIST, 2017).
Hokkaido, located at the northern extension of the NE Japan Arc, has a complex tectonic setting because it is situated at the arc-arc junction of the NE Japan and Kuril arcs. As a result, the applicability of these dehydration mechanisms has not been fully verified in this region, and knowledge regarding the origin and extent of deep-seated fluids remains limited.
This study aims to estimate the origin and extent of deep-seated fluids in Hokkaido by characterizing the geochemical signatures of potential deep-seated fluid components in groundwater. Geochemical methods are widely employed for evaluating deep-seated fluid origins and distributions, and the effectiveness of various chemical tracers—such as stable water isotopes, noble gas isotopes, and dissolved element compositions—has been demonstrated in previous studies (e.g., Matsubaya et al., 1973; Amita et al., 2014; Kazahaya et al., 2014; Morikawa et al., 2016). In particular, lithium (Li) and boron (B) in aqueous phases exhibit strong temperature dependence in water-rock interactions (e.g., You et al., 1996; James et al., 2003) and have been utilized to infer the origin and extent of deep-seated fluids that have experienced high-temperature and high-pressure conditions (Amita et al., 2014; Kazahaya et al., 2014). In this study, groundwater samples were classified based on chloride ion concentration, helium isotope ratios, and hydrogen-oxygen stable isotope ratios, which are widely used for identifying deep-seated fluid origins (Tomioka et al., 2020). Subsequently, the Li and B compositions of potential deep-seated fluids in groundwater were discussed.
To estimate the extent of deep-seated fluid influence, it is crucial to understand the ascent processes from the slab. Three primary types of deep-seated fluids have been identified: diagenetic dehydration fluids, mineral dehydration fluids, and magmatic water (AIST, 2017; Tomioka et al., 2020). The ascent processes of these fluids differ between southwestern Japan (e.g., the Kinki and Shikoku regions) and northeastern Japan (AIST, 2017). In the NE Japan Arc, where the Pacific Plate subducts, mineral dehydration fluids are transported deeper by mantle convection and contribute to magma formation. Consequently, the primary sources of deep-seated fluids reaching the surface are diagenetic dehydration fluids and magmatic water. Furthermore, their spatial distribution has been clarified: diagenetic dehydration fluids are predominantly found on the forearc side, while magmatic water is mainly distributed along the volcanic front and the back-arc side (AIST, 2017).
Hokkaido, located at the northern extension of the NE Japan Arc, has a complex tectonic setting because it is situated at the arc-arc junction of the NE Japan and Kuril arcs. As a result, the applicability of these dehydration mechanisms has not been fully verified in this region, and knowledge regarding the origin and extent of deep-seated fluids remains limited.
This study aims to estimate the origin and extent of deep-seated fluids in Hokkaido by characterizing the geochemical signatures of potential deep-seated fluid components in groundwater. Geochemical methods are widely employed for evaluating deep-seated fluid origins and distributions, and the effectiveness of various chemical tracers—such as stable water isotopes, noble gas isotopes, and dissolved element compositions—has been demonstrated in previous studies (e.g., Matsubaya et al., 1973; Amita et al., 2014; Kazahaya et al., 2014; Morikawa et al., 2016). In particular, lithium (Li) and boron (B) in aqueous phases exhibit strong temperature dependence in water-rock interactions (e.g., You et al., 1996; James et al., 2003) and have been utilized to infer the origin and extent of deep-seated fluids that have experienced high-temperature and high-pressure conditions (Amita et al., 2014; Kazahaya et al., 2014). In this study, groundwater samples were classified based on chloride ion concentration, helium isotope ratios, and hydrogen-oxygen stable isotope ratios, which are widely used for identifying deep-seated fluid origins (Tomioka et al., 2020). Subsequently, the Li and B compositions of potential deep-seated fluids in groundwater were discussed.