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Tungsten isotopic evidence for disproportional late accretion to the Earth and Moon

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Abstract

Characterization of the hafnium–tungsten systematics (182Hf decaying to 182W and emitting two electrons with a half-life of 8.9 million years) of the lunar mantle will enable better constraints on the timescale and processes involved in the currently accepted giant-impact theory for the formation and evolution of the Moon, and for testing the late-accretion hypothesis. Uniform, terrestrial-mantle-like W isotopic compositions have been reported1,2 among crystallization products of the lunar magma ocean. These observations were interpreted to reflect formation of the Moon and crystallization of the lunar magma ocean after 182Hf was no longer extant—that is, more than about 60 million years after the Solar System formed. Here we present W isotope data for three lunar samples that are more precise by a factor of ≥4 than those previously reported1,2. The new data reveal that the lunar mantle has a well-resolved 182W excess of 20.6 ± 5.1 parts per million (±2 standard deviations), relative to the modern terrestrial mantle. The offset between the mantles of the Moon and the modern Earth is best explained by assuming that the W isotopic compositions of the two bodies were identical immediately following formation of the Moon, and that they then diverged as a result of disproportional late accretion to the Earth and Moon3,4. One implication of this model is that metal from the core of the Moon-forming impactor must have efficiently stripped the Earth’s mantle of highly siderophile elements on its way to merge with the terrestrial core, requiring a substantial, but still poorly defined, level of metal–silicate equilibration.

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Figure 1: Values of μ182W of lunar metals separated from KREEP-rich impact melts analysed by negative thermal ionization mass spectrometry in this study.
Figure 2: HSE abundances (normalized to Ir and CI chondrite abundances33) of metal separates from sample 68815,394 and 68815,396.

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Acknowledgements

This work was supported by NASA Cosmochemistry grant NNX13AF83G. We thank the Lunar Sample Laboratory Facility at Johnson Space Center for the provision of appropriate samples for this study.

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Authors

Contributions

M.T. conducted the W isotopic measurements and was involved in both the interpretations and the writing of the manuscript. I.S.P. conducted the measurements of the highly siderophile elements and Os isotopes, and was involved in both the interpretations and the writing of the manuscript. R.J.W. was involved in both the interpretations and the writing of the manuscript.

Corresponding authors

Correspondence to Mathieu Touboul, Igor S. Puchtel or Richard J. Walker.

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Competing interests

The authors declare no competing financial interests.

Additional information

The data presented here can be found in the EarthChem library (http://www.earthchem.org/library/browse/view?id=849).

Extended data figures and tables

Extended Data Figure 1 Plot of μ182W versus source 180Hf/184W ratio.

182W values shown as open symbols are weighted averages of the data obtained in ref. 1 for low-Ti (triangle down) and high-Ti (triangle up) mare basalts; error bars, 2 s.e. of the samples of each group. The red symbol corresponds to the average of our new high-precision data for metals separated from 68115 and 68815 impact melts; error bars, 2 s.d. of the data. Based on mineral–melt partition coefficients for minerals in a crystallizing magma ocean, significant Hf–W fractionations are expected among the products of the LMO6,20, resulting in high Hf/W in the source of high-Ti mare basalts (>40), low Hf/W in KREEP (10 ± 10) and intermediate Hf/W (26.5 ± 1.1) in the source of low-Ti mare basalts. Reference isochrons (blue dashed lines) corresponding to different times after the start of the Solar System are shown.

Extended Data Figure 2 Plot of μ182W versus total HSE content relative to the present-day mantle, PM.

This is based on the assumption that before late accretion, the mantle was HSE-free and had a μ182W of +10 to +30 p.p.m., assuming total contributions of late accretion to be between 0.3% and 0.8% of the mass of the mantle (see labels), as determined from HSE abundances in the Earth’s mantle4 and using W contents of 200 p.p.b. for chondrites14 and 13 p.p.b. for the current mantle38. With the addition of chondritic materials, the total HSE abundances present in the mantle increase and the W isotopic composition decreases to present-day values. Evolution of the mantle composition by late accretion, or mixing between pre-late accretionary mantle and current accessible mantle, are represented by the grey field. Estimate for the HSE content of the lunar mantle (red circle) is taken from ref. 3; error bars, 2 s.d. of the data from the three rocks examined. Diamond symbol indicates the composition of the present-day mantle.

Extended Data Table 1 Tungsten contents and isotopic compositions of mixing components

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Touboul, M., Puchtel, I. & Walker, R. Tungsten isotopic evidence for disproportional late accretion to the Earth and Moon. Nature 520, 530–533 (2015). https://doi.org/10.1038/nature14355

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