Calibration of the Lutetium-Hafnium Clock

Abstract

Well-defined constants of radioactive decay are the cornerstone of geochronology and the use of radiogenic isotopes to constrain the time scales and mechanisms of planetary differentiation. Four new determinations of the lutetium-176 decay constant (λ 176 Lu) made by calibration against the uranium-lead decay schemes yield a mean value of 1.865 ± 0.015 × 10 −11 year −1 , in agreement with the two most recent decay-counting experiments. Lutetium-hafnium ages that are based on the previously used λ 176 Lu of 1.93 × 10 −11 to 1.94 × 10 −11 year −1 are thus ∼4% too young, and the initial hafnium isotope compositions of some of Earth's oldest minerals and rocks become less radiogenic relative to bulk undifferentiated Earth when calculated using the new decay constant. The existence of strongly unradiogenic hafnium in Early Archean and Hadean zircons implies that enriched crustal reservoirs existed on Earth by 4.3 billion years ago and persisted for 200 million years or more. Hence, current models of early terrestrial differentiation need revision.