Uranium determination by133Xe in mineral separates from igneous rocks comparative study with fission track and thermal release characters of133Xe

Samples of an andesite (Asama-yama, Japan) and a basalt (Kilauea, Hawaii) were finely fractioned by density and the U distributions among the separates were determined by homogenized fission track method. Groundmass was found to be enriched with U; one half of the andesite U and almost all of the basalt U are accounted for by the groundmass U. Cerium and other REE are correlated with U. In the andesite separates Na is accompanied by these elements, but Fe and Co are rather anticorrelated. Uranium concentrations in the bulk samples and some of the separates were then compared with those determined by¹³³Xe extracted by heating at 1600°C. Fissiogenic¹³³Xe seems to become labile in groundmass, especially of the andesite, during reactor irradiations for 5 h or more. Stepwise heating experiments suggested that¹³³Xe in U-rich groundmass tends to escape at low temperatures and the remaining¹³³Xe is mostly retained in highly refractory sites in phenocryst rather poor in U.