Abstract
Aira-Tn tephra is one of the most widespread tephra deposits of late Pleistocene age in Japan. It was erupted from Aira Caldera, about 25, 000 years ago. The Osumi Pumice fall, Tsumaya pyroclastic flow, Kamewarizaka breccia, Ito pyroclastic flow and its co-ignimbrite ash, Aira-Tn ash (AT) were formed successively. AT is distributed in a wide area as far as 1, 400 km from the vent. The proximal deposits are subdivided into 4 units ; AT 1-4, in ascending order, among which AT 3 and AT 4 are also distributed in the distal area. Glass shards are the main constituent with less voluminous free crystals of plagioclase, quartz, orthopyroxene, opaque minerals, clinopyroxene and amphibole, and a minor amount of lithic fragments.This study shows the lateral variations of size distribution and heavy mineral abundance of the AT in 24 localities.Thickness and median grain-size of glass shards and crystals of both AT 3 and AT 4 decrease almost systematically with distance from the vent. The abundance of each heavy mineral varies as a function of the distance from the vent, especially in the coaser grain size. Gravitational separation is considered to be the main cause of these variations. The variation profiles of AT 3 and AT 4 are quite similar to each other, suggesting that both layers were formed nearly concurrently before the climatic conditions changed. Some contamination of heavy minerals from other volcanic sources is also discussed.
