Discrimination of the Volcanic Ash Layers by Means of the Features of Volcanic Glass, with Special Reference to the Difference of the Titanium Contents in the Volcanic Glass of the Plio-Pleistocene Osaka Group, Japan

Abstract

The correctly identified pyroclastic layers are the best keys for the geologic chronology. The Plio-Pleistocene Osaka and Kobiwako groups are good sediments for the tephrochronology because of the high frequency of intercalations with many thin volcanic ash layers.The foundation of the tephrochronology is correct identification of tephra. Especially, the features of the volcanic glass are very important for identification, since main volcanic ash of the Osaka and Kobiwako groups consists chiefly of volcanic glass. The writer tried to determine the titanium content of volcanic glass by polarogram, in order to identify some tephra.In this paper, the method of polarographic determination of titanium used in cast iron with good accuracy, reported by Fukui & Arita (1962), was applied to the volcanic glass in a little modification. The sample was dissolved by heating in sulfuric acid-fluoric acid mixture. The fluoric acid was driven off by heating. Then, Fe3+ was reduced to Fe2+ by addition of metallic aluminum. The solution was treated with oxalic acid and the determination of titanium was carried out by recording a polarogram obtained in the range of 0--0.8V.The results are shown in Table 4. The individual volcanic ash layer has one's own titanium content. It may be sure that the determination of various metalic elements in the volcanic glass is a good method for identification of tephra.