title: Geochemistry: does U-Pb date Earth's core formation?
abstract: Constraining the timing of the formation of Earth's core, which defines the birth of our planet, is essential for understanding the early evolution of Earth-like planets. Wood and Halliday and Halliday discuss the apparent discrepancy between the U-Pb (60-80 Myr) and Hf-W clocks (30 Myr) in determining the timescale of Earth's accretion and core formation. We find that the information the authors present is at times contradictory (for example, compare Fig. 1 in ref. 1 with Fig. 1 in ref. 2) and confusing and could suggest that the U-Pb clock constrains core formation better than the Hf-W system. Here we point out the limitations of the U-Pb system and show that the U-Pb age cannot be used to argue for protracted accretion and/or core formation (>50 Myr) because this clock only records the processes that occurred during the last 1% of Earth's accretion and core formation in the Wood and Halliday mechanism.
authors.affiliation: Department of Geology, University of California, Davis, California 95616, USA. yin@geology.ucdavis.edu
authors.forename: Qing-zhu
authors.initials: QZ
authors.lastname: Yin
authors.forename: Stein B
authors.initials: SB
authors.lastname: Jacobsen