Alpha-decay of 184Os revealed by radiogenic 180W in meteorites: Half life determination and viability as geochronometer

Abstract

The decay of the rare nuclide 184Os by alpha emission to 180W has been theoretically predicted, but was previously never observed in experiments. Variable excesses of 180W were recently observed for iron meteorites, but the contribution to these excesses by 184Os-decay was regarded as insignificant. Here, we present combined 180W and Os–W concentration data for meteorites and terrestrial rocks, now indicating that the 180W heterogeneities can be explained by α-decay of 184Os. A combined 184Os–180W isochron for iron meteorites and chondrites yields a decay constant value of λ184Os(α) of 6.49±1.34×10−14 a−1 (half life 1.12±0.23×1013 yr), in good agreement with theoretical estimates. The 184Os–180W decay system may constitute a viable tracer and chronometer for important geological processes like core formation, silicate differentiation or late accretion processes. This is illustrated by a measured 180W-deficit in terrestrial basalts relative to chondrites by 1.16±0.69 parts in 10 000, consistent with core formation ∼4.5 Ga ago.