ABSTRACT

In the Hakone geothermal system a variety of waters including precipitation, surface water, ground water, thermal water and steam condensate were analyzed for δD and δ¹⁸O, dissolved sulfate and carbonate for δ³⁴S, δ¹⁸O and δ¹³C, and some sulfur-bearing gases for δ³⁴S. Fifteen samples were collected monthly to see if there is any monthly change in the isotopic composition of water. Except in precipitation and steam condensates, no significant monthly changes were observed. δ¹⁸O_SO4 and δ³⁴S_SO4 analyses indicate that surface oxidation of volcanic sulfur produces isotopically light sulfate in water occurring at relatively high elevations. Sulfate minerals in the basement rocks formed by the Miocene submarine volcanism are another source of dissolved sulfate in waters at lower elevations. CO₂ originally derived from decomposition of marine carbonate is suggested as a carbon source for dissolved bicarbonate at higher elevations, although contribution of organic carbon becomes significant in waters at lower elevations. In a δD versus δ¹⁸O plot, surface waters including precipitation and ground water lie on the line, δD = 8δ¹⁸O + 17. On the other hand, thermal waters lie on a regression line, δD = 2.1δ¹⁸O − 33.5. Around the intersect of the two lines, δD = -51 ‰ and δ¹⁸O = -8.5 ‰, there is a swarm of point for groundwaters. We call the ground water with δD = -51 ‰ and δ¹⁸O = -8.5 ‰ “representative” groundwater (RGW). From both chemical and isotopic view points, thermal waters are interpreted to be a mixture of RGW and high temperature dense steam (HTDS), the latter being ultimately evolved from hydrothermal interaction of RGW with rocks containing appreciable amounts of hydrous silicates. Values of δD and δ¹⁸O for HTDS have been interpreted to be a result of rock-RGW interaction with the rock/water weight ratio of about 10, on the basis of isotopic material balance in a closed system with the use of δD of hydrous silicates in rocks as well as δD and δ¹⁸O values of RGW. When the ratio of about 10 is compared with those of other geothermal areas, the Hakone geothermal system is rock-dominated. Interaction between meteoric water and rocks including hydrous silicates under a rock-dominated condition can account for both hydrogen and oxygen isotopic shifts found in thermal waters.