An upper limit on the spontaneous fission decay constant of 232Th derived from xenon in monazites with extremely high Th/U ratios

Abstract

We attempted to analyse xenon produced by spontaneous fission of 232Th in monazites with Th/U ratios on the order of several thousand and Th concentrations of ~4000-6000 ppm. The samples are from the Bayan Obo ore deposit in China and previously yielded precise Th-Pb internal isochron ages of ~400-500 Ma (Wang et al., 1994, Geochim. Cosmochim. Acta 58, 3155). The monazites retained between 20-50% of their radiogenic 4He, and we can assume that fission Xe has essentially been completely retained. The minute amounts of fission Xe detected are compatible with production entirely from the traces of 238U. We derive an upper limit of ~2(*)10-21 y-1 for the spontaneous fission decay constant of 232Th. This shows that the Th-derived contribution can be neglected when dating U-bearing minerals by fission Xe.