Cooling of the Earth and core formation after the giant impact

Beat the clock One aspect of early Earth evolution that remains puzzling is the discrepancy between the more rapid formation time of the core given by the decay of hafnium to tungsten than by the clock based on the decay of uranium to lead. Wood and Halliday suggest that the explanation may be that the Hf–W clock represents the principal phase of core formation before the giant impact that formed the Moon. The upheaval introduced oxidation and the formation of a sulphur-rich metal in to which lead would have dissolved preferentially, in effect resetting the U–Pb clock to a younger date. Kelvin calculated the age of the Earth to be about 24 million years by assuming conductive cooling from being fully molten to its current state 1 . Although simplistic 2 , his result is interesting in the context of the dramatic cooling that took place after the putative Moon-forming giant impact, which contributed the final ∼10 per cent of the Earth's mass 3 , 4 . The rate of accretion and core segregation on Earth as deduced from the U–Pb system 5 is much slower than that obtained from Hf–W systematics 6 , 7 , 8 , and implies substantial accretion after the Moon-forming impact, which occurred 45 ± 5 Myr after the beginning of the Solar System. Here we propose an explanation for the two timescales 5 , 9 . We suggest that the Hf–W timescale reflects the principal phase of core-formation before the giant impact. Crystallization of silicate perovskite in the lower mantle during this phase produced Fe 3+ , which was released during the giant impact 10 , and this oxidation resulted in late segregation of sulphur-rich metal into which Pb dissolved readily, setting the younger U–Pb age of the Earth. Separation of the latter metal then occurred 30 ± 10 Myr after the Moon-forming impact. Over this time span, in surprising agreement with Kelvin's result, the Earth cooled by about 4,000 K in returning from a fully molten to a partially crystalline state.