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 …