K-Ar Ages of the Middle Pleistocene Takahara Volcano, Central Japan, and Argon Release Processes in Cooling Lava

Abstract

Samples of eleven andesites and one dacite were collected from six lava flows from Takahara Volcano, central Japan, and were dated by the K-Ar method. Some of these lavas are intercalated with intra-caldera Shiobara lake sediments, thus providing an estimation of the age of the sediments which contain a cool-temperate Pleistocene flora. The validity of the analytical technique for K-Ar age dating of young volcanic rocks is discussed, and some important problems concerning lava samples with anomalous argon contents and isotopic ratios are addressed. Processes of argon release during cooling of lava flows are also examined in relation to their petrographic charcteristics.