A new geochronometer using lanthanum-138

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We have applied a new geochronometer, based on the decay of 138La to 138Ba, to a sample of Amitsoq gneiss from West Greenland. Using a value for the electron-capture decay constant, λEC, of 4.44 × 10−12 yr−1, the resulting age (2,408 ± 24 Myr) is in good agreement with that from Sm–Nd systematics (2,410 ± 54 Myr). Application of this geological clock is limited to REE (rare earth element)-rich minerals, but its merit is that, for Archaean rocks, the age can be evaluated from one specimen on the assumption that the initial Ba isotope ratios are effectively constant.