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

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