Groundwaters and deep-seated fluid circulation around Aso Volcano, Southwest Japan, revealed by multivariate statistical analysis of the geochemical data

Abstract

To understand the fluid circulation under volcanoes and their surrounding area, geochemical compositions of 590 groundwater and spring/river water samples from the central Kyusyu in SW Japan, including hot and mineral spring waters around Aso and Kuju Volcanoes, have been analyzed for 12 major solutes as well as oxygen‑hydrogen and helium isotopic ratios. To extract independent processes and sources involved in formation of the groundwaters, whitened data-based k-means cluster analysis (KCA) has been applied to the data set of 12 major solutes. Eight clusters have been identified by KCA, which were mapped geographically and were compared with the isotopic compositions, temperature, and pH that were not included in KCA. By the geographical mapping, a concentric zonal structure of the clusters emerges in the Aso caldera, which resulted broadly from two fluid cycling systems from caldera rim and central cones. In the entire study area including Aso and Kuju Volcanoes, the mapping also shows geographical provenance, and the distribution of each cluster shows characteristics related to geological features, such as upwelling of deep-seated fluids along the prominent tectonic lines. On the other hand, the Piper diagram-based classification is significantly different from the KCA results and exhibits neither a concentric zonal structure within the Aso caldera nor a regional geographical provenance. These results suggest that multivariate statistical analysis such as KCA is potentially useful for investigating the origins of groundwaters and shows the advantages over classical Piper diagram-based evaluations.