14:45 〜 15:00
[SVC33-05] Circulation of geofluids beneath the Aso caldera: constraints from multivariate statistical analysis of geochemical data of groundwaters
キーワード：地下水、地殻流体、統計解析
Resolving distribution and circulation of geofluids within and beneath volcanic calderas, including aqueous fluids and magmas, is important to understand magmatic processes that could be related to catastrophic eruption. The compositional variability of groundwater (including hot/mineral spring waters) in such volcanic calderas reflects the origin and circulation processes of geofluids, such as fluid sources, fluid-rock reactions, flow rates and residence times, which may provide useful information concerning the fluid processes within and beneath the calderas. In this study, the compositions of groundwaters collected over the area of about 120 km east-west and 80 km north-south surrounding the Aso caldera in central Kyushu (Takahashi et al., 2018, and unpublished data) were utilized to discuss the circulation of the geofluids within and beneath the Aso caldera.
We examined the high-dimensional composition data set (590 samples containing 12 major dissolved elements of Na, K, Mg, Ca, Li, Cl, SO4, HCO3, F, NO3, Br, and total C), based on multivariate statistical analyses. The statistical methods used were "whitened-data based k-means cluster analysis (KCA)" and "independent component analysis (ICA)" (Iwamori et al., 2017). Principal component analysis (PCA), a standard multivariate analysis, can extract information for independent sources and processes only if the data show a (joint) normal distribution. For data showing a non-normal distribution, the above KCA and ICA should be used. The data set used in this study shows clear non-normality, for which KCA and ICA are suitable. Based on the eigenvalue analysis of the data, we set the number of clusters and independent components to eight and attempted to detect statistically independent features in their compositional and geographical spaces.
As a result, a concentric spatial structure (distinguished with the 8 clusters) was found both in the Aso caldera and in the wide area surrounding the Aso volcano, with a broad decrease in the concentrations of Na, Li, Cl, HCO3, Br, total C toward the outside. When focusing on the inside of caldera, four clusters were mainly distributed concentrically. The hydrogen isotopic ratio, δD, can be used as an indicator of recharge altitude based on the calibration lines (Kagabu et al., 2011; Yamada et al., 2011). We newly found that the clusters exhibit a correlation with δD, in other words the recharge altitude, and that (1) one specific cluster with high Cl concentrations shows a higher recharge altitude and distribute themselves around the central volcanic edifice, whereas (2) the clusters along the caldera rim show systematic correlations with the sampling altitude, recharge altitude and the average solute concentrations including Na and Cl. These highly correlated variations are best interpreted that the groundwater of cluster (1) with high Cl concentrations and high recharge altitude was derived from the meteoric water from the central volcanic edifice, and the groundwaters of (2) were derived from the caldera rim flowing down through the caldera wall to the caldera bottom. The groundwater (1) was probably fluxed by the magmatic gas/fluids from magmas beneath the central volcanoes, including Naka-dake, resulting in a high Cl concentration. The groundwater (2) reacted with the caldera wall materials, mainly composed of pyroclastic deposits that provide cations readily to the groundwater flowing through, which created the systematic variation in solute concentration according to the flow path length, measured by the difference between recharge and sampling altitudes. The two types of groundwaters (1) and (2) meet in the caldera bottom, which produced a groundwater that exhibit the intermediate geochemical characteristics exactly between (1) and (2).
We also identified two more clusters that are not included any of the clusters mentioned above. These two clusters are found along the caldera rim and in a broad area outside the caldera. One of them shows a high F concentration. Another exhibits the highest Cl concentrations and a relatively high oxygen isotopic ratio, δ18O, relative to the meteoric line in the δ18O-δD diagram. These features suggest that the clusters represent deep-seated fluids, such as those reacted with the basement rocks and/or those derived originally from the underlying subducted slab.
This research was supported by the Nuclear Regulation Authority "Research on Knowledge Development of Giant Eruption Processes, FY 2021".
We examined the high-dimensional composition data set (590 samples containing 12 major dissolved elements of Na, K, Mg, Ca, Li, Cl, SO4, HCO3, F, NO3, Br, and total C), based on multivariate statistical analyses. The statistical methods used were "whitened-data based k-means cluster analysis (KCA)" and "independent component analysis (ICA)" (Iwamori et al., 2017). Principal component analysis (PCA), a standard multivariate analysis, can extract information for independent sources and processes only if the data show a (joint) normal distribution. For data showing a non-normal distribution, the above KCA and ICA should be used. The data set used in this study shows clear non-normality, for which KCA and ICA are suitable. Based on the eigenvalue analysis of the data, we set the number of clusters and independent components to eight and attempted to detect statistically independent features in their compositional and geographical spaces.
As a result, a concentric spatial structure (distinguished with the 8 clusters) was found both in the Aso caldera and in the wide area surrounding the Aso volcano, with a broad decrease in the concentrations of Na, Li, Cl, HCO3, Br, total C toward the outside. When focusing on the inside of caldera, four clusters were mainly distributed concentrically. The hydrogen isotopic ratio, δD, can be used as an indicator of recharge altitude based on the calibration lines (Kagabu et al., 2011; Yamada et al., 2011). We newly found that the clusters exhibit a correlation with δD, in other words the recharge altitude, and that (1) one specific cluster with high Cl concentrations shows a higher recharge altitude and distribute themselves around the central volcanic edifice, whereas (2) the clusters along the caldera rim show systematic correlations with the sampling altitude, recharge altitude and the average solute concentrations including Na and Cl. These highly correlated variations are best interpreted that the groundwater of cluster (1) with high Cl concentrations and high recharge altitude was derived from the meteoric water from the central volcanic edifice, and the groundwaters of (2) were derived from the caldera rim flowing down through the caldera wall to the caldera bottom. The groundwater (1) was probably fluxed by the magmatic gas/fluids from magmas beneath the central volcanoes, including Naka-dake, resulting in a high Cl concentration. The groundwater (2) reacted with the caldera wall materials, mainly composed of pyroclastic deposits that provide cations readily to the groundwater flowing through, which created the systematic variation in solute concentration according to the flow path length, measured by the difference between recharge and sampling altitudes. The two types of groundwaters (1) and (2) meet in the caldera bottom, which produced a groundwater that exhibit the intermediate geochemical characteristics exactly between (1) and (2).
We also identified two more clusters that are not included any of the clusters mentioned above. These two clusters are found along the caldera rim and in a broad area outside the caldera. One of them shows a high F concentration. Another exhibits the highest Cl concentrations and a relatively high oxygen isotopic ratio, δ18O, relative to the meteoric line in the δ18O-δD diagram. These features suggest that the clusters represent deep-seated fluids, such as those reacted with the basement rocks and/or those derived originally from the underlying subducted slab.
This research was supported by the Nuclear Regulation Authority "Research on Knowledge Development of Giant Eruption Processes, FY 2021".