BORON ISOTOPIC COMPOSITIONS OF SHIMOGAMO HOT-SPRINGS, IZN, JAPAN

Hot spring waters in Shimogamo, Izu Peninsula, Japan, were analyzed fo r the stable isotopes of boron as well as for chemical compositions. T he boron isotopic ratios, B-11/B-10, of the waters range from 4.190 to 4.216, corresponding to the variation in delta(11)B from +36.2 to +42 .6 permil. These values are very high compared to those of hot spring waters in other areas on the Japanese islands and close to that of sea water. The results of the isotopic and chemical analyses indicate tha t the Shimogamo hot spring waters are mixtures of surface water and a deep geothermal brine directly derived from sea water through the inte raction with heated silicate rocks.