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The proto-Earth as a significant source of lunar material


A giant impact between the proto-Earth and a Mars-sized impactor named Theia is the favoured scenario for the formation of the Moon1,2,3. Oxygen isotopic compositions have been found to be identical between terrestrial and lunar samples4, which is inconsistent with numerical models estimating that more than 40% of the Moon-forming disk material was derived from Theia2,3. However, it remains uncertain whether more refractory elements, such as titanium, show the same degree of isotope homogeneity as oxygen in the Earth–Moon system. Here we present 50Ti/47Ti ratios in lunar samples measured by mass spectrometry. After correcting for secondary effects associated with cosmic-ray exposure at the lunar surface using samarium and gadolinium isotope systematics, we find that the 50Ti/47Ti ratio of the Moon is identical to that of the Earth within about four parts per million, which is only 1/150 of the isotopic range documented in meteorites. The isotopic homogeneity of this highly refractory element suggests that lunar material was derived from the proto-Earth mantle, an origin that could be explained by efficient impact ejection, by an exchange of material between the Earth’s magma ocean and the protolunar disk, or by fission from a rapidly rotating post-impact Earth.

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Figure 1: Titanium nucleosynthetic heterogeneity, ɛ50Ti=[(50Ti/47Ti)sample/(50Ti/47Ti)rutile−1]×104, for carbonaceous, enstatite, ordinary chondrites, and achondrites.
Figure 2: Correction of cosmogenic Ti isotope effects in lunar samples.

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We thank National Aeronautics and Space Administration (NASA), R. N. Clayton and G. E. Lofgren for supplying the Apollo lunar samples; the Field Museum, United States National Museum, Muséum National d’Histoire Naturelle and Chicago Center for Cosmochemistry for providing bulk meteorites; K Pahlevan for his comments on an earlier version of the text, and P. R. Craddock, T. J. Ireland, A. Pourmand, H. Tang and A. Liu for their help with the experiments. This work was supported by NASA, through grants NNX09AG59G (to N.D.), NNX09AG39G (to A.M.D.) and NNX08AE06G (to L. Grossman in support of A.F.); National Science Foundation, through grant EAR-0820807 (to N.D.); a Packard Fellowship (to N.D.); and the Swiss National Science Foundation (to I.L.).

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J.Z., N.D. and A.M.D. planned the project, J.Z. measured the Ti isotopic compositions of all samples, I.L. provided model predictions of cosmogenic effects. A.F., J.Z., N.D. and A.M.D. calculated the timescale for evaporative exchange. All authors contributed to discussion, interpretation of the results and writing of the manuscript.

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Correspondence to Junjun Zhang.

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Zhang, J., Dauphas, N., Davis, A. et al. The proto-Earth as a significant source of lunar material. Nature Geosci 5, 251–255 (2012).

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