Oxygen, hydrogen and carbon isotopic study of the Amakusa pottery stone deposits in altered rhyolite dikes, Kyushu, southwest Japan

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¹⁸O/¹⁶O and D/H ratios, together with ¹³C/¹²C ratio for one carbonate sample, were measured for 9 whole-rock samples and 4 clay samples from the Amakusa pottery stone deposits distributed in Tertiary rhyolite dikes. The δ¹⁸O values of whole-rocks range from +13.1 to +15.8‰ relative to SMOW. Extrapolation of the δ¹⁸O values to fresh rhyolite suggests that an ¹⁸O-enrichment up to 8‰ was attained in the alteration processes. The δ¹⁸O values of altered rocks are higher than the value of sericite (+14.5‰), which could be caused by re-deposition of ¹⁸O-rich silica. The Amakusa clays are depleted in ¹⁸O by about 5‰, and enriched in D by about 10‰ relative to kaolinites of weathering origin. This may eliminate the possibility of weathering origin for the Amakusa clays. The hydrothermal water equilibrated with the Amakusa clays is estimated to have been enriched in both ¹⁸O and D as compared with present-day local meteoric waters. This hydrothermal water could be magmatic in origin, and have undergone mixing with meteoric waters and kinetic fractionation due to liquid-vapor separation.