ABSTRACT

Comparison of oxygen isotopic ratios in whole rocks and some constituent minerals were made between a small (300m × 100m) hydrothermal alteration area and the surrounding diagenetically altered area in the Tsugaru-Yunosawa geothermal field. The δ¹⁸O values of whole rocks are distinctly higher in the area subjected to hydrothermal alteration than those in the only diagenetically altered area. The oxygen isotope fractionations among the alteration minerals are nearly identical in the two areas. This suggests an isotopic equilibrium between the minerals and waters responsible for alteration. Therefore, the difference of 7‰ in δ¹⁸O values of minerals between hydrothermally and diagenetically altered areas may correspond to the difference in isotopic composition between waters responsible for these two types of alteration. The origin of the water responsible for the hydrothermal alteration seems to be related to the fossil seawater postulated as the source of the hot springs in and around this geothermal field. The δ¹⁸O value of the hydrothermal water responsible for the alteration was estimated to be in a range from about 0 to +7‰, based on the δ¹⁸O values of cristobalites in the hydrothermally altered area, the range of the estimated alteration temperature and the δ¹⁸O values of the present-day hot spring waters. The δ¹⁸O value of the water during the diagenetic alteration is lower by 7‰ than that of the hydrothermal water, but is higher than that of meteoric water.