138La β-decay constant estimated from geochronological studies

La-Ce ages are reported for two sets of Finnish pegmatites, Loevboele and Mustikkamaeki, and for an Amitsoq gneiss, Greenland. When λβ138La value (2.29 x 10-12 yr-1) obtained by radioactivity measurement is used for the chronological calculation, the La-Ce ages (2129, 2325, 3271 Myr) evaluated for these rocks are 18-35% older than the Sm-Nd ages for the same samples. To make the La-Ce age fit to the Sm-Nd age for the same sample, a new value of (2.77±0.21) x 10-12 yr-1 is evaluated for λβ138La. In this calculation, the La-Ce and Sm-Nd ages reported for a Bushveld gabbro have been also taken into account together with those for the Loevboele pegmatite and the Mustikkamaeki pegmatite, while the Amitsoq gneiss (GGU110999) has been comitted because of the complicated thermal history of this sample. (orig.)