HAFNIUM-TUNGSTEN CHRONOMETRY AND THE TIMING OF TERRESTRIAL CORE FORMATION

THE accretion of the Earth and Moon within the solar nebula is thought(1-3) to have taken 50 to 100 million years. But the timing of formation of the Earth's core has been controversial, with some(4,5) proposing that it took place within the first 15 Myr of Earth's accretion history and others(6,7) proposing that it occurred after 50 Myr of accretion. Meteorite chronometry based on the Hf-182-W-182 system has the potential to resolve this debate, as segregation of a metal core from silicates should induce strong fractionation of hafnium from tungsten. Here we report tungsten isotope compositions for two iron meteorites, two carbonaceous chondrites, and a lunar mare basalt. We see clear W-182 deficits in both iron meteorites, in agreement with previous results(4,5). But the data for chondrites are inconsistent with the hypothesis of early core formation, suggesting that both this event and the formation of the Moon must have occurred at least 62+/-10 Myr after the iron meteorites formed.