ABSTRACT

Concentrations of uranium, and concentrations and isotopic ratios of thorium in zircon and glass were determined for pyroclastic dacite pumices in southern Kyushu, Japan, and lo-ages were calculated. Uranium and thorium distributions in other minerals were also determined. Uranium and thorium contents of minerals were highest for zircon and decreased in the order, zircon-apatite-glass-mafic minerals-plagioclase. Zircons showed the highest U/Th activity ratios compared with other minerals. Io-ages obtained by glass-zircon pair for pyroclastic flows in southern Kyushu, except for Ata pyroclastic flow were consistent with ¹⁴C and fission track ages and the stratigraphic sequence. Comparison of uranium and thorium concentrations in mineral constituents of acidic volcanic rocks showed that the glass-zircon pair is the most preferable for ionium dating.