Tephrochronology and diagenesis of the manganese wad deposit at the Akan Yunotaki hotspring, Hokkaido, Japan

Abstract

An outcrop of manganese oxide about one meter thick, inter-bedded with several volcanic ash layers, is found at the Yunotaki hotspring, Hokkaido, Japan. The manganese wad is precipitated from hydrothermal water. On the basis of the previously known age of the inter-bedded volcanic ash layers, linked to well-known eruptions, it has been possible to determine the chronology of the manganese oxide precipitation. The ash layers were identified by the chemical composition of the separated volcanic glasses. Since the lowermost manganese oxide layer of this outcrop occurs immediately above the mud flow of Mt. Ponmachineshiri (one volcano of Mt. Meakan) without the presence of any intermediate soil layer between them, it's age of formation could be near the age of the mud flow, i.e. 4500 - 5000 years BP. The manganese oxide layers in the top 50 cm of the outcrop are comparatively softer than those occurring below 50 cm and commonly have the texture of the aggregate of a filament structure. Manganese oxide occurring below 50 cm is hard and has a diagenetically altered texture reflecting change in the filament structure to a grain aggregate structure. These oxides have lost their structural water and the mineralogy has begun to change from todorokite (10A phase) to birnessite (7A phase) during the diagenetic process.