ABSTRACT

Multivariate statistical analyses have been applied to a compositional data set for spring waters from the Arima and Kii areas in southwest Japan, for understanding sources and processes that produced the deep-seated Arima-type brine and the related spring waters in the areas. The data set consists of 14 rare earth elements (REEs), six major solute species (HCO₃, Cl, Na, Ca, K, Mg) and isotopic ratios of oxygen, hydrogen and helium (δD, δ¹⁸O, ³He/⁴He) from 19 sites. Independent Component Analysis (ICA) and k-means cluster analysis on the whitened data (KCA) are useful to extract independent features hidden in such a high-dimensional geochemical data set. As a result of ICA and KCA, four ICs and eight clusters were found to sufficiently and effectively describe the observed variability. Comparison of the results with δD, δ¹⁸O and ³He/⁴He provides geochemical interpretations on the extracted four ICs as follows: IC1 represents the overall REE concentration level; IC2 measures the amplitude of Eu anomaly inherited from local basement rocks/minerals; IC3 reflects the amplitude of (reversed) W-shaped pattern, which developed on mineral precipitation event(s) when the deep brine mixes with meteoric water in a relatively shallow aquifer under oxidative conditions, and IC4 measures the amplitude of Ce anomaly possibly by precipitation or mechanical filtering of CeO₂. Combination of the four ICs may create a complex and various REE pattern to explain the actual data. Although these geochemical features with Eu, Ce and W-shaped patterns have been broadly suggested to be important for understanding origin of the spring waters in the areas, this is the first time to demonstrate that the above features represent the independent processes and to compare them systematically with the regional compilation of the basement rock compositions. In addition, several clusters were found only in either the Arima area or the Kii area, which reflects broad geographical provenance of IC3 and IC4. These spatial variations can be attributed to compositional variations of the original deep brine, which possibly varies with slab depth and temperature to control the solute concentrations and subsequent geochemical evolution to produce different types of spring waters.