Tungsten isotopes and the origin of the Moon

Journal Article · Fri Aug 04 00:00:00 EDT 2017 · Earth and Planetary Science Letters

DOI: https://doi.org/10.1016/j.epsl.2017.07.021 · OSTI ID:1410033

Kruijer, Thomas S.  ^[1]; Kleine, Thorsten ^[2]

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1. Univ. of Munster, Munster (Germany); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
2. Univ. of Munster, Munster (Germany)

Here, the giant impact model of lunar origin predicts that the Moon mainly consists of impactor material. As a result, the Moon is expected to be isotopically distinct from the Earth, but it is not. To account for this unexpected isotopic similarity of the Earth and Moon, several solutions have been proposed, including (i) post-giant impact Earth–Moon equilibration, (ii) alternative models that make the Moon predominantly out of proto-Earth mantle, and (iii) formation of the Earth and Moon from an isotopically homogeneous disk reservoir. Here we use W isotope systematics of lunar samples to distinguish between these scenarios. We report high-precision ¹⁸²W data for several low-Ti and high-Ti mare basalts, as well as for Mg-suite sample 77215, and lunar meteorite Kalahari 009, which complement data previously obtained for KREEP-rich samples. In addition, we utilize high-precision Hf isotope and Ta/W ratio measurements to empirically quantify the superimposed effects of secondary neutron capture on measured ¹⁸²W compositions. Our results demonstrate that there are no resolvable radiogenic ¹⁸²W variations within the Moon, implying that the Moon differentiated later than 70 Ma after Solar System formation. In addition, we find that samples derived from different lunar sources have indistinguishable ¹⁸²W excesses, confirming that the Moon is characterized by a small, uniform ~+26 parts-per-million excess in ¹⁸²W over the present-day bulk silicate Earth. This 182W excess is most likely caused by disproportional late accretion to the Earth and Moon, and after considering this effect, the pre-late veneer bulk silicate Earth and the Moon have indistinguishable ¹⁸²W compositions. Mixing calculations demonstrate that this Earth–Moon ¹⁸²W similarity is an unlikely outcome of the giant impact, which regardless of the amount of impactor material incorporated into the Moon should have generated a significant ¹⁸²W excess in the Moon. Consequently, our results imply that post-giant impact processes might have modified ¹⁸²W, leading to the similar ¹⁸²W compositions of the pre-late veneer Earth's mantle and the Moon.

• Research Organization: Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC52-07NA27344
• OSTI ID: 1410033
• Report Number(s): LLNL-JRNL--731059
• Journal Information: Earth and Planetary Science Letters, Journal Name: Earth and Planetary Science Letters Journal Issue: C Vol. 475; ISSN 0012-821X
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English