The Chingshui geothermal field is located in the submetamorphic terrain of Taiwan. The area is mainly underlain by argillite and slate, with minor amounts of metasandstone of Miocene age. The oxygen isotope compositions of argillite and slate cuttings from the deep (3000 m) well No. 16 in this geothermal field were analyzed. Although the rock samples do not show significant alteration under the microscope, the δ18O-values show a systematic change from + 14.6 to + 11.7 ‰ with increasing depth. Combined with early published stable isotopic data for geothermal waters and carbonates from the same area, it is shown that the rocks in this field have undergone an increasing degree of O-isotope exchange with the geothermal waters at progressively higher temperatures (i.e. greater depth). The water/rock ratios in such a water-rock interaction system were estimated to be lower than 0.58. Such low values may be due to the very low permeability of the rocks. © 1993.
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