Ionium dating of acidic volcanic rocks

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.