Volcanic Glass Inclusions in Rhyolite and Tuff from the Chitose Mine, Hokkaido

Abstract

Volcanic glass inclusions in the quartz phenocrysts of rhyolite and in the quartz grains of rhyolitic tuff from the Chitose gold-silver mine, Hokkaido, were studied by means of the microscopic observation, heating experiments, electron-probe microanalysis and laserprobe microanalysis. The glass inclusions in the rhyolite generally consist of transparent glass and a bubble, and some of them contain dendritic crystals. On the other hand, the glass inclusions in the tuff contain reddish brown hexagonal tabular crystals, short prismatic crystals, long prismatic or acicular crystals. The heating experiments revealed that the homogenization temperature of the glass inclusions in rhyolite was in the range between 1, 000°C and 1, 200°C, while that of tuff was in the range of 870°-1, 000°C.
The major compositions of glass inclusions in rhyolite and tuff were determined by the electron-probe microanalysis as follows; SiO₂:70-76wt.%, AL₂O23:±12wt.%, Na₂O:2-5wt.%, K₂O:2-4wt.%, CaO:±1wt.%, MgO:<0.2wt.%, FeO(total):±0.8wt.%. No distinct difference was recognized between the glass inclusion of rhyolite and that of tuff. The laser-probe microanalysis showed the presence of Cu, Ag, Ti and Zn in the glass inclusions.
Though the present data are not enough to extract the general conclusion on the formation of glass inclusions in volcanic rocks, it is presumed from the microscopic observation and heating experiments that glass inclusions would inform us of the approximate thermal history. This presumed history, however, would give us merely a general trend, because the phase changes such as the melting, nucleation and growth of crystals in glass inclusions occur metastably in various degrees. The devitrification of glass inclusions does not necessarily mean a slow cooling, because the hydrothermal actions would also cause the devitrification. Larger inclusions are more frequently devitrified than smaller inclusions.