Abstract
Oxygen and hydrogen stable isotopic ratios are measured about swelling montmorillonite occurring in Su-mikawa and Toroko landslide areas in Hatimantai geothermal area, Akita Prefecture and also Jizukiyama land-slide area, Nagano City. Formation temperatures of montmorillonite are estimated to be 50-100°, 27t and 50° in Sumikawa, Toroko and Jizukiyama areas, respectively. In Sumikawa area, isotopic ratios of water that yields the montmorillonite have feature of mixture of the present local meteoric water and fumarolic steam of Yake volcano where the Sumikawa landslide area is located on the hillside. On the other hand, in Toroko and Jizuki-yama area, the isotopic characters are far different from the expectation for the present local meteoric water, and shows intermediate one between expectation for meteoric water and sea water.
From these results, formation environment of montmorillonite in Sumikawa landslide area is geothermal one, which may continue for recent a few thousand years. Toroko and Jizukiyama landslide areas might be in diage-netic condition, and the higher isotopic ratios may be attributed to admixing with sea water, meteoric water at warmer climate or evaporation effect from water pool.
