Kyushu Island in southern Japan is located within an area subjected to one of the highest rates of chemical weathering in the world. This area is purported to have the highest discharge rate of dissolved silica and other cations within the Japanese Island arc. To understand the timescale of chemical weathering in such a weathering ‘hot spot’ details of water–rock interaction are characterized using a conventional geochemical model for the Kirishima volcanic area. The model is focused on source water (spring water) with different residence times previously estimated by chlorofluorocarbons (CFCs) concentration analysis. The geochemical and geological features of the spring water samples suggest that the primary reactant minerals are pyroxenes and plagioclase. It was also found from activity-diagram analysis that kaolinite and Ca-smectite are the major precipitable secondary minerals from the system. The occurrence of these reactions was quantitatively confirmed by stoichiometric calculations of measured dissolved ions. In combination with previous estimates of spring water residence ages, it is concluded that saturation of dissolved silica and simultaneous transition of phase precipitation from kaolinite to Ca-smectite starts in ca. 20 years after aqueous recharge. Our findings imply the usefulness of geochemical data as a function of water-rock interaction timescale in certain field. In future work, these findings can be compared to data from other areas to improve understanding of global heterogeneity in chemical weathering rates.
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