La–Ce dating of Lewisian granulites to constrain the 138La β-decay half-life

The isotope 138La makes up 0.089% of natural lanthanum and exhibits a branched decay, by β− to 138Ce and electron capture to 138Ba. The La–Ce isotope scheme is a potentially powerful adjunct to the Sm–Nd method for petrogenetic studies, but the best determination by counting of the β-decay half-life of La (3.02 x 1011 yr)1 is in conflict with geological half-life determinations based on La–Ce and Sm–Nd mineral isochrons2–4. To test these alternatives, I determined a six-point La–Ce isochron on whole-rock samples of Lewisian granulite-facies Archaean gneiss from NW Scotland. Here I report, based on a half-life of 3.02 × 1011yr, a La–Ce age of 2,990 ±400 Myr (2σ), which falls very well within the error of a whole-rock Sm–Nd date on Lewisian granulites of 2,895±100 Myr (2σ). This half-life (equivalent to a La β-decay constant of 2.30 × 10−12 yr-1) is therefore recommended for geological application of the Ce-isotope method.