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Late formation and prolonged differentiation of the Moon inferred from W isotopes in lunar metals

Abstract

The Moon is thought to have formed from debris ejected by a giant impact with the early ‘proto’-Earth1 and, as a result of the high energies involved, the Moon would have melted to form a magma ocean. The timescales for formation and solidification of the Moon can be quantified by using 182Hf–182W and 146Sm–142Nd chronometry2,3,4, but these methods have yielded contradicting results. In earlier studies3,5,6,7, 182W anomalies in lunar rocks were attributed to decay of 182Hf within the lunar mantle and were used to infer that the Moon solidified within the first 60 million years of the Solar System. However, the dominant 182W component in most lunar rocks reflects cosmogenic production mainly by neutron capture of 181Ta during cosmic-ray exposure of the lunar surface3,7, compromising a reliable interpretation in terms of 182Hf–182W chronometry. Here we present tungsten isotope data for lunar metals that do not contain any measurable Ta-derived 182W. All metals have identical 182W/184W ratios, indicating that the lunar magma ocean did not crystallize within the first 60 Myr of the Solar System, which is no longer inconsistent with Sm–Nd chronometry8,9,10,11. Our new data reveal that the lunar and terrestrial mantles have identical 182W/184W. This, in conjunction with 147Sm–143Nd ages for the oldest lunar rocks8,9,10,11, constrains the age of the Moon and Earth to  Myr after formation of the Solar System. The identical 182W/184W ratios of the lunar and terrestrial mantles require either that the Moon is derived mainly from terrestrial material or that tungsten isotopes in the Moon and Earth’s mantle equilibrated in the aftermath of the giant impact, as has been proposed to account for identical oxygen isotope compositions of the Earth and Moon12.

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Figure 1: ε182W of lunar metals analysed in this study compared with data from refs 3 and 5.
Figure 2: Plot of ε 182 W against source 180 Hf/ 184 W.
Figure 3: ε 182 W difference between bulk silicate Moon and Earth as a function of time of core formation.

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Acknowledgements

We thank the Curation and Analysis Planning Team for Extraterrestrial Materials (CAPTEM), NASA curatorial staff; G. Lofgren for supplying the Apollo lunar samples; L. Borg and A. Brandon for reviews; and F. Nimmo and J. Van Orman for discussions. This research was supported by a EU Marie Curie postdoctoral fellowship to T. Kleine.

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Correspondence to M. Touboul.

Supplementary information

Supplementary Information

The file contains Supplementary Notes including descriptions of: the lunar metals, the analytical methods, the cosmogenic effects on W isotope ratios in lunar samples and the Hf/W fractionation in the crystallizing lunar magma ocean. It also contains Supplementary Table 1 summarizing the calculated cosmogenic corrections for the data presented in the main text, as well as Supplemental Figure 1 illustrating the two stage model age of the lunar mantle. (PDF 250 kb)

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Touboul, M., Kleine, T., Bourdon, B. et al. Late formation and prolonged differentiation of the Moon inferred from W isotopes in lunar metals. Nature 450, 1206–1209 (2007). https://doi.org/10.1038/nature06428

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