Calibration of the lutetium-hafnium clock

Well-defined constants of radioactive decay are the cornerstone of geochron ology and the use of radiogenic isotopes to constrain the time scales and m echanisms of planetary differentiation. Four new determinations of the lute tium-176 decay constant (lambda Lu-176) made by calibration against the ura nium-lead decay schemes yield a mean value of 1.865 +/- 0.015 x 10(-11) yea r(-1), in agreement with the two most recent decay-counting experiments. Lu tetium-hafnium ages that are based on the previously used lambda Lu-176 of 1.93 x 10(-11) to 1.94 x 10(-11) year(-1) are thus similar to4% too young, and the initial hafnium isotope compositions of some of Earth's oldest mine rals and rocks become less radiogenic relative to bulk undifferentiated Ear th when calculated using the new decay constant. The existence of strongly unradiogenic hafnium in Early Archean and Hadean zircons implies that enric hed crustal reservoirs existed on Earth by 4.3 billion years ago and persis ted for 200 million years or more. Hence, current models of early terrestri al differentiation need revision.