Strontium isotopic identification of the widespread Aira-Tn ash-fall deposits (AT) in Japan

The widespread distribution of Aira volcanic ash (AT) from Aira caldera which is one of the largest scale in Japan, is well known, and the ash was found even on the sea floor surrounding Japan island arc. The age of eruption was estimated as 21,000 to 22,000 YBP, and the area of distribution covers more than 500,000 km2. Essentially, glassy materials are rich in AT, and those are the fragments of glass bubbles, colorless and transparent. Strontium isotopic identification is effectively applicable to the studies on the distribution of volcanic ash. 11 samples were collected for the strontium isotopic identification from land area as widely as possible, in order to exclude the effect of sea water on volcanic ash. A VG Isomass 54 spectrometer was used for the strontium isotopic measurement. The 87Sr/86Sr ratio of AT was measured as 0.7061, while the ash of Mt. Fuji and the ash of Shibanda which is located about 6.5 km NW of Gotemba, Shizuoka Prefecture, were 0.70352 and 0.70482, respectively. Comparing these values, the value of Shibanda was between two other values. It was estimated that about 20 % of AT was mixed with the ash of Fuji strontium isotopic identification is an effective method for the identification of AT and its distribution. Similarly, this method is applicable to Aso pyroclastic flow and ash, and to the volcanic ash of Ikeda caldera. (Ishimitsu, A.)