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Abstract

We report ¹⁸²W and ¹⁴²Nd isotopic compositions, ¹⁸⁷Re-¹⁸⁷Os systematics, and abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Rh, Pt, Pd, and Au) for a suite of komatiites and basalts from the ∼ 3.3Ga Ruth Well Formation and the ∼ 3.45Ga Warrawoona Group of the Pilbara Craton, Western Australia. The ¹⁸²W compositions from all samples are indistinguishable from each other, and more radiogenic than modern bulk silicate Earth, with a mean μ¹⁸²W value of + 9.1 ± 4.2 (2SD). By contrast, the ¹⁴²Nd values for all samples are indistinguishable from each other and terrestrial standards, with a mean μ¹⁴²Nd value of - 1.6 ± 3.2 (2SD). The ¹⁴⁶Sm-¹⁴²Nd and ¹⁸⁷Re-¹⁸⁷Os systematics are consistent with chondritic Sm/Nd and Re/Os ratios in the mantle source during the lifetime of ¹⁸²Hf, and the observed ¹⁸²W excesses therefore cannot be accounted for by early Hf-W fractionation by magma ocean processes, neither by silicate liquid-crystal fractionation nor by high P-T metal-silicate equilibration. The estimated abundances of HSE in the mantle source, however, are significantly lower than modern bulk silicate Earth, with only 51 ± 9% (1SD) of modern bulk silicate Earth abundances. These results are consistent with a partial lack of late-accreted material within the Pilbara source at ∼ 3.3Ga to account for the ¹⁸²W excesses. Further, widespread ¹⁸²W excesses of similar magnitude in other Archean mantle-derived rocks worldwide strongly suggests that a common process, most likely incomplete addition of late-accreted material, was responsible. The apparent mismatch between late-accreted ¹⁸²W-HSE systematics for some other localities likely reflects either the inherent difficulties associated with estimating source HSE abundances, and/or dissociation of W and HSE by mantle processes. Finally, the combined average ¹⁸²W-HSE systematics of Archean samples indicate that the pre-late accretion BSE likely had a μ¹⁸²W value similar to that of the lunar mantle, which strongly suggests post-giant impact Earth-Moon equilibration and indicates that the Moon formed after ¹⁸²Hf extinction.