The study of genesis and formation condition of bentonite

Abstract

Origin and formation condition of bentonite were obtained by new methods at four mines: Tsukinuno, Kawasaki, Dobuyama and Kuroishi. The Tsukinuno bentonite was considered diagenetic and the others were hydrothermal origin. The formation age were obtained by fission-track and K-Ar methods. The eruption age of original rock of these four bentonite deposits were 12-17 Ma. The fission track length and the difference between fission-track and K-Ar age indicate the hydrthemal activity in the Kawasaki, the Dobuyama and the Kuroishi deposits.
The composition of plagioclase in bentonitized tuff is related to original volcanic rocks. Plagioclase were measured by refractive index. Plagioclase in the Tsukinuno bentonite had a little sodium rich composition than the other bentonites.
Formation temperature of bentonite were measured by oxygen and hydrogen isotopic study. Formation temperature of four bentonite were several dozen °C Formation temperature of Dobuyama bentonite was lowest, and Kuroishi was highest of four bentonites.
Thermal history of bentonite were measured by ESR and Thermoluminescence. Kawasaki, Dobuyama and Kuroishi bentonite were suffered some heat evidence until 1-2 million years ago. Tsukinuno bentonite was suffered heat until several hundred thousand years ago.