長野県の温泉についての同位体化学的調査報告

Water samples from 28 hotsprings and mineral springs in Nagano Prefecture, central Japan, were examined for their stable isotope ratios of hydrogen, oxygen, carbon, and sulfur. Spring waters of Kashio are highly saline and enriched in heavy isotopes of oxygen and hydrogen (δ(18)O=-2.5～-4.6‰, δD=-54～-57‰). Linear relationships among δD, δ(18)O, and Cl(-) suggest that spring waters are the mixtures of a deep brine and local surface water. Extrapolation of the linear relationships indicates that the deep brine is both isotopically and chemically very similar to the deep brine previously suggested for the springs of Arima, Takarazuka, and Ishibotoke of which δD, δ(18)O, and Cl(-) are estimated as -33‰, +8.0‰, and 44g/l, respectively. A common origin may be warranted among these postulated brines, while their provenance is yet to be worked out. The hot springs in Matsushiro are a Na-Ca-Cl type of high carbonate content. Their hydrogen and oxygen isotope ratios (δD=-71～-46‰, δ(18)O=-9.1～-2.0‰) are higher than the local surface water. On the basis of the relationships among δD, δ(18)O, and Cl(-), they are considered to be the mixtures of fossil sea water and certain water of meteoric origin of which Cl(-) is about 4g/l and δ(18)O is higher by about 3‰ than the local surface water. The latter may be meteoric water circulating in the marine sedimentary formations (Green Tuff formations) with soluble sea salts. Isotopic exchange with carbonate minerals in the formations explains its (18)O enrichment. Spring waters from Yashio and Isobe (Gunma Pref.) as well as Yunosawa and Yatate (Akita Pref.) were previously interpreted to be mixtures of fossil sea water and local surface water of low Cl(-) content. Re-examination of their data revealed that the meteoric waters responsible for these springs contain about 3g/l Cl(-), similar to the value obtained for Matsushiro. However, unlike Matsushiro, the meteoric waters in these areas are found to be isotopically similar to the local surface waters. Waters from other hot springs studied here are of simply meteoric origin, thus belonging to the GreenTuff type water previously defined.

原著論文 (Original Papers)