LAUSR

The data obtained during the fieldworks in July 2016 were used for the geochemical description of thermal manifestations of Ketoi and Pallas Peak volcanoes on Ketoi Island in the middle of the Kuril Island arc. It is shown that the older edifice of Ketoi Volcano hosts a high temperature hydrothermal system whose surface manifestations are concentrated in the eroded crater of the volcano. Two main types of thermal water are discharged there: the acid and low-acid waters of the Ca–SO4 composition in thermal pools that are diluted to varying degrees with meteoric water, and boron-rich neutral boiling water of the Na–Ca–SO4 composition. The isotopic composition of both water types is similar to that of the local meteoric waters. Calculation of mineral saturation indices for hydrothermal minerals, as well as the behavior of boron, suggest that the waters of the first type are formed nearer the surface, while the boiling sulfate waters are most likely derived from steam condensates formed at depth at a temperature of approximately 200°С. The thermal springs related to Pallas Peak Volcano discharge acid sulfate chloride water. The total measured hydrothermal output of Cl and SO4 from Ketoi and Pallas Peak volcanoes is 8.5 t/day and 30 t/day, respectively.