Natural Analogue Study of Volcanic Glass; A Case Study of Basaltic Glasses in Pyroclastic Fall Deposits of Fuji Volcano, Japan

Arai, Takashi; Yusa, Yasuhisa; Sasaki, Noriaki; Tsunoda, Naomi; Takano, Hitoshi*

A considerable range in alteration rates of basaltic glasses in various environments has been reported in previous studies. However, these studies paid only cursory attention to the environmental conditions under which the glass had been altered. In this study, the alteration of basaltic glasses was investigated and the environmental conditions and the alteration rate were discussed. Two sample ages were represented: 280 years and 2800 years. Basaltic glasses and their alteration layers were analyzed by electron probe microanalyzer (EMPA) and the thickness of the alteration layers were measured by scanning electron microscope (SEM). The ground water collected near the sampling point of Zunazawa Scoria (2800 years) and the pore water of both samples were analyzed. The alteration temperature and flow rate of water are estimated to be about 13C and 0.2 l/m/y respectively on the basis of meteorological data. The alteration layers of young aged basaltic glasses in freshwater conditions are similar to those of leached borosilicate glasses. The alteration rates of these basaltic glasses are estimated to be several m/1000y. The elemental concentrations in the ground water can be roughly explained as the result of leaching of the glasses.