Natural analogue studies of long term alteration of volcanic glass and bentonite in alkaline conditions

The alteration characteristics of volcanic glass and bentonite in geological medium are investigated as natural analogues of geological disposal of high-level radioactive wastes. For the study of volcanic glass, volcanic ashes are collected from boreholes, which were drilled in the Kanto region by the National Research Institute for Earth Science and Disaster Prevention. The volcanic glass, under the temperature condition 70degC, was altered to montomorillonite and clinoptilolite, but another volcanic glass, under the temperature condition up to 50degC, was not altered. Both cases may be occurred in the connate marine water at last 0.5 Ma. For the study of bentonite, slight alkaline hot spring water (pH:up to 9) and rocks in Rausu area, Hokkaido Prefecture, have been studied to examine the zeolitization of montmorillonite. X-ray diffraction analysis show that montmorillonite, kaolinite, mordenite, and alunite were recognized as the alteration minerals. Alkaline (pH:up to 9) and high temperature (about 90degC) water (Na+-C- type) were found at several outcrops, chemical and isotopic analysis show that those alkali hot spring waters were meteoric origin, increasing of salinity were as a result of seepage through the marine sedimentary strata, and high temperature was derived from the heat source. Stability relationship for system anorthite-kaolinite-montmorillonite-silica minerals-aqueous solution shows that montmorillonite was stable in those spring water. (author)