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The lead isotopic age of the Earth can be explained by core formation alone

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Abstract

The meaning of the age of the Earth defined by lead isotopes has long been unclear. Recently it has been proposed1 that the age of the Earth deduced from lead isotopes reflects volatile loss to space at the time of the Moon-forming giant impact rather than partitioning into metallic liquids during protracted core formation. Here we show that lead partitioning into liquid iron depends strongly on carbon content and that, given a content of 0.2% carbon2,3, experimental and isotopic data both provide evidence of strong partitioning of lead into the core throughout the Earth’s accretion. Earlier conclusions that lead is weakly partitioned into iron arose from the use of carbon-saturated (about 5% C) iron alloys. The lead isotopic age of the Earth is therefore consistent with partitioning into the core and with no significant late losses of moderately volatile elements to space during the giant impact.

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Figure 1: Volatile element abundances in the BSE.
Figure 2: Experimental data for Pb partitioning into liquid Fe coexisting with liquid silicate.
Figure 3: Calculated and estimated lead isotopic composition of the BSE.
Figure 4: Rb–Sr model age of the Moon plotted versus the percentage loss of Rb from the Earth at the time of the giant impact.

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Acknowledgements

The assistance of N. Pearson (Macquarie), N. Charnley (Oxford) and J. Day (Cambridge) with microprobe and laser ICP-MS analysis is acknowledged with thanks. B.J.W. acknowledges the support of the Australian Research Council through Federation Fellowship FF 0456999 and the NERC (UK) through grant NE/F018266/1. A.N.H. acknowledges support from STFC. Experiments at the Bayerisches Geoinstitut were performed under the EU ‘Research Infrastructures: Transnational Access’ Programme (contract number 505320; RITA—High Pressure).

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Contributions

B.J.W. performed all the experiments and all the electron microprobe and laser ICP-MS analyses, and the Pb-isotopic modelling of Fig. 3. A.N.H. performed the Sr-isotope modelling depicted in Fig. 4. Both authors contributed to the writing of the manuscript.

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Correspondence to Bernard J. Wood.

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Wood, B., Halliday, A. The lead isotopic age of the Earth can be explained by core formation alone. Nature 465, 767–770 (2010). https://doi.org/10.1038/nature09072

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