Ionium Dating for Volcanic Ejecta

Abstract

The date of volcanic ejection can be determined by the measurements on the state of radioactive disequilibrium between ionium and uranium in a mineral separated from the ejecta. The approach to the radioactive equilibrium state is shown in Fig. 1. From this figure we can conclude that the ionium dating requires uranium and thorium isotopic analysis on two different kinds of minerals. In order to make accurate dating, higher contents of uranium and thorium, and big difference of U/Th activity ratios in these minerals are desirable. In Table 2, contents of uranium and thorium, and U/Th activity ratios in minerals separated from pumice of Torihama pyroclastic flow in southern Kyushu are shown. In this example zircon and glass were most suitable for ionium dating. The results of ionium dating for volacnic ejecta, which are shown in Table 3, were in good agreement with the ¹⁴C ages and with the ages estimated from mass color of zircons except ¹⁴C age on pumice (PmI) of Ontake volcano. The zircons separated from pumice of Hijiori pyroclastic flow and PmI were not on the isochrones (Fig.3). This means that these zircons were not simultaneously crystallized with other minerals. Possible ways of introduction of these zircons in pumice are discussed in connection with the measured Io/Th and U/Th activity ratios.