PETROLOGIC MODEL OF SHIRATAKI OBSIDIAN, NORTHERN HOKKAIDO, JAPAN; ITS STRUCTURE, COMPOSITION AND THE ORIGIN

Shirataki obsidian is the most important source of obsidian artifacts in Hokkaido, north of Japan. The Shirataki obsidian tools have spread throughout not only the Hokkaido island but also Sakhalin, southern Kurile and northern Honshu islands more than 500 km from the source in the Paleolithic era and the following Jomon era (e.g. Kimura, 2005; Kuzmin et al., 2002; Phillips and Speakman, 2009).

The Shirataki obsidian was formed by quenching of aphyric rhyolite magmas at least 10 lava units within Horoka-yubetsu caldera at about 2.2 Ma ago. The aphyric rhyolite magmas are divided into two magma series; one is Tokachi-ishizawa (TI) magmas and the other is Akaishiyama (AK) magmas; AK magmas are higher in FeO*, CaO, Ba, TiO₂/K₂O, and CaO/Al₂O₃ contents than TI magmas. We can see the internal-structure section of single rhyolite lava with the clastic surface zone, outer compact obsidian zone, vesiculated obsidian zone, and central thick rhyolite zone. The outer compact obsidian zone makes remarkably fresh obsidian.

The chemical compositions of major elements in obsidian glasses from the compact obsidian zone were quantitatively determined by electron probe micro-analyzer (EPMA) at Hokkaido University of Education. On the basis of FeO*-CaO diagram, AK magmas and TI magmas are sub-divided into two compositional groups respectively (AK-1, AK-2, TI-1, TI-2). These four compositional groups are corresponding to the geological lava units. We propose the Shirataki obsidian discrimination diagrams of FeO* versus CaO together with the TiO₂/K₂O versus CaO/Al₂O₃, which can identify unknown obsidian artifacts as which lava unit were they derived from.

No phenocryst in the Shirataki rhyolite-obsidian magmas shows a higher temperature of rhyolite magmas above the liquidus prior to the eruption, which means adiabatic uprize of the magmas without staying in the magma chamber with keeping above liquidus. The Shirataki obsidian in Monbetsu-Kamishihoro graben (Yahata, 1997) was probably formed in the tensional field of crust. These physical conditions would cause formation of fractures in the crust and then Shirataki rhyolite-obsidian magmas would have been uprized through the fractures easily to maintain high temperature.