Hydrogen and oxygen isotopic ratios and major element chemistry of Japanese thermal water systems

Abstract

More than 140 thermal waters of Japan were studied isotopically and chemically. Highly saline brines at Arima and Takarazuka, Hyogo-Ken, and Ishibotoke near Osaka indicate wide ranges of δ18O and δD values from meteoric values of δ18O = -8.2 and δD = -50.0 ‰ SMOW to highly shifted values of +6.5 and -27.8 ‰, respectively. The isotopic values of these brines vary proportionally with chloride concentration irrespective of temperature, carbonate concentration or locality. These saline waters are isotopically and chemically best explained as the mixtures of local meteoric waters and a saline brine of Cl-= 43, 700 ppm, δ18O = +8 ‰ and δD = -30 to -25 ‰. The latter is most likely the “residual magmatic, metamorphic or geothermal” fluid associated with upper Cretaceous rhyolitic and granitic rocks and Ryoke metamorphic rocks in which these brines are found. Thermal waters at Ikeda and in adjacent areas, Shimane-Ken and at Senami, Niigata-Ken, are similar to the Arima brines in the isotopic and major element chemistry, but are much more diluted by the respective local meteoric waters. Many of the thermal waters along the ocean coasts are isotopically intermediate between oceanic and local meteoric waters and are considered to be mixtures of the two types of water. As a result of hydrothermal mineral-sea water interaction, the coastal thermal waters differ considerably in the chemistry from fresh sea water and are typical of Na-Ca-Cl type. The coastal thermal waters isotopically and chemically may be similar, if not the same, to submarine hydrothermal ore fluids responsible for the Kuroko type mineralization. The isotopic values and their relationship to salinity, however, widely differ from one system to another depending on the hydrogeological conditions of each system. The coastal thermal waters at Ibusuki of Ata Caldera, Kagoshima-Ken, for instance, are significantly affected by the waters from three crater lakes, Lake Ikeda, Unagi-Ike and Kagami-Ike, in which the δ18O and δD values are meteorologically balanced at such high values as -2.6 and -19.4‰, respectively. Acid to neutral thermal waters of volcanic affiliation indicate varying degrees of isotopic shifts, but they are supposed from their δD values to be essentially derived from recycled meteoric water. Many thermal waters of neutral chloride type in the “green tuff” regions of the inner Honshu also are simple meteoric in origin without showing any significant isotopic shifts, although the waters are relatively high in salinity and SO4/Cl ratios.