Abstract

The β-decay constant of 138La has been geochronologically determined using Archean granites from the Yilgarn Block, Western Australia. RbSr and SmNd mineral isochron ages were obtained for the two granite samples, which are nearly identical with the ion microprobe UPb zircon ages. This agreement indicates that the closure date for these samples is the same, within analytical error, making them suitable for geochronological estimation of the β-decay constant for 138La. Applying a new method of Ce isotope analysis, with analytical reproducibility of ±0.008%, LaCe isotope data for the studied samples resulted in precise mineral isochrons. Based on these internal mineral isochrons and ages determined by other geological clocks, the β-decay constant for 138La was estimated to be (2.33 ± 0.24)·10 −12a −1 (2σ).

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