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 W-182 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 ICREEP-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 W-182 compositions. Our results demonstrate that there are no resolvable radiogenic W-182 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 W-182 excesses, confirming that the Moon is characterized by a small, uniform similar to + 26 parts-per-million excess in W-182 over the present-day bulk silicate Earth. This W-182 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 W-182 compositions. Mixing calculations demonstrate that this Earth-Moon W-182 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 W-182 excess in the Moon. Consequently, our results imply that post-giant impact processes might have modified W-182, leading to the similar W-182 compositions of the pre-late veneer Earth's mantle and the Moon. (C) 2017 Elsevier B.V. All rights reserved.
