SESSION 13 - Obsidian - methodological issues of obsidian provenance studies and a new perspective of archaeological obsidian
Characteristics of rock micro-texture of obsidian for some obsidian sources of the Islands of Japan

To understand the obsidian formation process it is important to describe the micro-texture of various types of obsidian. We examined characteristics of petrological description of obsidian from Shirataki (northern Hokkaido), Rishiri (eastern Hokkaido), Kozushima (one of the Izu islands south of Tokyo), and Shinshu (Nagano prefecture) obsidian sources in Japan. The difference in phenocryst content, microlite texture and number density, groundmass texture, and glass composition are recognized among these obsidians, showing the diversity of rock texture; Shirataki obsidian has no phenocrysts and abundant magnetite microlites showing a clear jet-black luster, Rishiri obsidian has many phenocrysts and microlites, Kozushima obsidian has biotite phenocrysts, and Shinshu obsidian has no phenocrysts and rare microlites showing a high degree of transparency.

Heat experiments were performed on obsidian in an electric furnace to transform the perlite into a form like pumice in vitreosity, because the remaining water in obsidian becomes gas and makes air bubbles by heating. We examined the foaming temperature of the obsidians from different sources, and the relations between its foaming temperature and the water content of the obsidians. In addition, the heating temperature and the heating time were varied and the resulting differences in foaming form of the perlite were examined. Finally, relations between foaming process of the obsidians and the internal structure of the perlite were considered through microscopicanalysis of the perlite.