13:45 〜 14:00
[SVC29-01] 阿蘇火山周辺における地下水の希土類元素組成が示す多様性
Rare earth elements in spring waters may provide information on the origin and/or processes during water circulation; e.g., material and generation process at the source, interaction during the fluid migration through a crack or a vein with host rocks, mixing with meteoric water in a shallow aquifer. A large-scale circulation processes, including the dehydration from the subducted Philippine Sea slab and the subsequent interactions with surrounding basement rocks and meteoric waters, were distinguished by the rare earth element compositions of the spring waters in the Arima and the Kii areas, southwest Japan (Nakamura et al., 2015; 2016), with the aid of statistical analyses (Iwamori et al., 2020). To understand the origin and the upwelling processes, we have analyzed rare earth element composition of groundwaters including hot springs, river waters around the Aso volcano in Kyushu, Japan.
The Aso volcano has a caldera structure formed with four gigantic eruptions and numerous small eruptions (e.g., Kaneko et al., 2007). After the caldera formation, the central cone named Naka-dake has been and is active. The activity of Naka-dake and the related magma supply system have been studied by geophysical and petrological methods (e.g., Hata et al., 2018; Abe et al., 2017; AIST, 2016)). The northward-dipping conductive anomaly toward Naka-dake cone and several anomalies interpreted as an aqueous fluid flow have been detected by electromagnetic study (Hata et al., 2018). Both basaltic and dacitic magmas are thought to be involved in the magma supply system by petrological study, and the magma seems to have evolved during four gigantic eruptions (AIST, 2016). Alternatively, geochemical studies of groundwaters were performed to identify a magma related fluid (e.g., Kazahaya et al., 2019, Morikawa et al., 2019), and of which major solute elements are comprehensively analyzed by using statistical method (Iwamori et al., 2021). Statistical study has been revealed a spatial distribution of major solute elements which clearly decreases toward outside of caldera as a concentric structure (Iwamori et al., 2021).
In this study, we have classified the rare earth element compositions of groundwaters based on the result of statistical analysis of major solute elements, and newly found that the behavior of rare earth element is partly decoupled from major solute elements. Therefore, the dataset of rare earth elements would be expected to include unique information during fluid generation and upwelling. Combining geophysical, petrological, and geochemical studies, we aim to reveal the fluid circulation and the spatial extent of magmatic component at the Aso volcano.
This study was supported by the Secretariat of the Nuclear Regulation Authority, Japan, “Research on Knowledge Development of Giant Eruption Processes, FY 2020”.
The Aso volcano has a caldera structure formed with four gigantic eruptions and numerous small eruptions (e.g., Kaneko et al., 2007). After the caldera formation, the central cone named Naka-dake has been and is active. The activity of Naka-dake and the related magma supply system have been studied by geophysical and petrological methods (e.g., Hata et al., 2018; Abe et al., 2017; AIST, 2016)). The northward-dipping conductive anomaly toward Naka-dake cone and several anomalies interpreted as an aqueous fluid flow have been detected by electromagnetic study (Hata et al., 2018). Both basaltic and dacitic magmas are thought to be involved in the magma supply system by petrological study, and the magma seems to have evolved during four gigantic eruptions (AIST, 2016). Alternatively, geochemical studies of groundwaters were performed to identify a magma related fluid (e.g., Kazahaya et al., 2019, Morikawa et al., 2019), and of which major solute elements are comprehensively analyzed by using statistical method (Iwamori et al., 2021). Statistical study has been revealed a spatial distribution of major solute elements which clearly decreases toward outside of caldera as a concentric structure (Iwamori et al., 2021).
In this study, we have classified the rare earth element compositions of groundwaters based on the result of statistical analysis of major solute elements, and newly found that the behavior of rare earth element is partly decoupled from major solute elements. Therefore, the dataset of rare earth elements would be expected to include unique information during fluid generation and upwelling. Combining geophysical, petrological, and geochemical studies, we aim to reveal the fluid circulation and the spatial extent of magmatic component at the Aso volcano.
This study was supported by the Secretariat of the Nuclear Regulation Authority, Japan, “Research on Knowledge Development of Giant Eruption Processes, FY 2020”.