Lunar tungsten isotopic evidence for the late veneer

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Abstract

According to the most widely accepted theory of lunar origin, a giant impact on the Earth led to the formation of the Moon, and also initiated the final stage of the formation of the Earth’s core1. Core formation should have removed the highly siderophile elements (HSE) from Earth’s primitive mantle (that is, the bulk silicate Earth), yet HSE abundances are higher than expected2. One explanation for this overabundance is that a ‘late veneer’ of primitive material was added to the bulk silicate Earth after the core formed2. To test this hypothesis, tungsten isotopes are useful for two reasons: first, because the late veneer material had a different 182W/184W ratio to that of the bulk silicate Earth, and second, proportionally more material was added to the Earth than to the Moon3. Thus, if a late veneer did occur, the bulk silicate Earth and the Moon must have different 182W/184W ratios. Moreover, the Moon-forming impact would also have created 182W differences because the mantle and core material of the impactor with distinct 182W/184W would have mixed with the proto-Earth during the giant impact. However the 182W/184W of the Moon has not been determined precisely enough to identify signatures of a late veneer or the giant impact. Here, using more-precise measurement techniques, we show that the Moon exhibits a 182W excess of 27 ± 4 parts per million over the present-day bulk silicate Earth. This excess is consistent with the expected 182W difference resulting from a late veneer with a total mass and composition inferred from HSE systematics2. Thus, our data independently show that HSE abundances in the bulk silicate Earth were established after the giant impact and core formation, as predicted by the late veneer hypothesis. But, unexpectedly, we find that before the late veneer, no 182W anomaly existed between the bulk silicate Earth and the Moon, even though one should have arisen through the giant impact. The origin of the homogeneous 182W of the pre-late-veneer bulk silicate Earth and the Moon is enigmatic and constitutes a challenge to current models of lunar origin.

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Figure 1: Plot of ε182W versus ε180Hf determined for KREEP-rich samples.
Figure 2: ε182W data of KREEP-rich samples and terrestrial rock standards.
Figure 3: Plot of ε182W versus mass fraction of late-accreted material on Earth.

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Acknowledgements

We thank CAPTEM, NASA and R. Zeigler for providing the Apollo lunar samples for this study. We thank G. Brügmann for providing an HSE spike. C. Brennecka is acknowledged for comments on the paper, and we also thank A. Brandon for comments.

Author information

T.S.K. prepared the lunar samples for W isotope analyses and performed the measurements. All authors contributed to the interpretation of the data and preparation of the manuscript.

Correspondence to Thomas S. Kruijer.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Compilation of ε182W results obtained for three terrestrial rock standards, BCR-2, AGV-2 and BHVO-2.

Each of these standards was analysed together with the lunar samples. ε182W (6/4) indicates that the data have been normalized to 186W/184W = 0.92767 (denoted ‘6/4’); see Methods for details. Error bars indicate internal uncertainties (2 s.e.) for a single measurement of 200 cycles. The external uncertainty (2 s.d.), as inferred from replicate standard analyses, is shown as a grey-hatched area, and the corresponding 95% confidence interval of 2 p.p.m. as a solid grey area.

Extended Data Figure 2 CI-chondrite-normalized and Ir-normalized HSE concentrations of lunar samples 14321, 68115, and 68815.

HSE concentrations are first normalized to chondritic abundances, then to the chondrite normalized Ir concentration. Corresponding HSE concentrations are given in Extended Data Table 3.

Extended Data Figure 3 Change of ε182W of the proto-Earth’s mantle through addition of impactor material.

The varying amounts of impactor material are given as Mimp/M where Mimp and M are respectively the mass of impactor material and the Earth’s mass. Hatched area (red) shows the maximum possible difference between the (eventual) ε182W of the pre-late-veneer BSE and the Moon, as inferred from the difference between the lunar pre-exposure ε182W value (+0.27 ± 0.04) and that calculated for the BSE before addition of the late veneer (). Shown are the effects of different degrees of equilibration of the impactor core with the proto-Earth’s mantle, from full (k = 1) to no equilibration (k = 0) and for two different impactor compositions: a, volatile-element-enriched ‘Mars-like’ and b, volatile-element-depleted ‘Vesta-like’. In both cases the impactor was assumed to have core and mantle fractions of 32% and 68%, identical to the Earth. For the (proto-)Earth’s mantle we used a W concentration of 13 ± 5 p.p.b. (2σ) (see Methods); its ε182W is set to zero. For the Mars-like impactor, CI-chondritic Hf (107 p.p.b.) and W (93 p.p.b.) concentrations60 were assumed. As core formation in this impactor would have occurred under more oxidizing conditions, we chose a relatively low metal–silicate partition coefficient61 for W of DW = 6, which yields a relatively high W concentration of [W]IM = 41 p.p.b. in the impactor mantle (and a low Hf/W of 4.4), and a correspondingly low W concentration of [W]IC = 205 p.p.b. in the core. The metal–silicate differentiation age was set to 9 Myr after CAI formation, resulting in a ε182W of −2.7 in the metal core, and a ε182W of +0.32 in the silicate mantle (calculated using a Solar System initial 182Hf/180Hfi of (1.018 ± 0.043) × 10−4; ref. 33). For the Vesta-like impactor we assumed CV-chondritic Hf (200 p.p.b.) and W (175 p.p.b.) concentrations31. Core formation in this impactor would have occurred under more reduced conditions, so the metal–silicate partition coefficient for W was set to a relatively high value (DW = 42)61. This yields a low W concentration in the silicate mantle ([W]IM = 12 p.p.b.), a high Hf/W of 25 in the silicate mantle, and a relatively high W concentration in the metal core ([W]IC = 522 p.p.b.) The metal–silicate differentiation age was set to 2 Myr after CAI formation, resulting in a ε182WIC of −3.3 in the metal core, and a ε182WIM of +27 in the silicate mantle.

Extended Data Figure 4 Effect of mixing impactor core into the lunar accretion disk on the ε182W of the Moon.

Shown are the effects on ε182W for two different impactor compositions (green and blue lines). Hatched area (red) shows the maximum possible difference between the (eventual) ε182W of Earth and Moon as inferred from the difference between the lunar pre-exposure ε182W value (+0.27 ± 0.04) and that calculated for the BSE before addition of late veneer (). In the mass balance we considered the same two impactor compositions as used in the mass balance shown in Extended Data Fig. 3, including (1) an (oxidized) volatile-element-rich impactor (green line), and (2) a (reduced) volatile-element-poor impactor (blue line). We used the same Hf and W concentrations, partition coefficients, core and mantle fractions, and differentiation ages, so the resulting Hf/W and ε182W values of impactor mantle and core are identical to those above. The amount of impactor core material currently present in the Moon is assumed to be equivalent to the lunar core fraction, that is, 2.5% of its mass. For this reason the mixing lines intersect the ordinate (Δε182W = 0) at 2.5%. For simplicity we assume the proportion of impactor material present in the Moon to be 80%, that is, consistent with most ‘canonical’ giant-impact models1.

Extended Data Table 1 Tungsten isotope data for terrestrial rock standards
Extended Data Table 2 Tungsten isotope data for KREEP-rich samples
Extended Data Table 3 Hf, W, and HSE concentrations of KREEP-rich samples determined by isotope dilution
Extended Data Table 4 Late veneer (LV) compositions and their effect on the ε182W value of Earth’s mantle

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Kruijer, T., Kleine, T., Fischer-Gödde, M. et al. Lunar tungsten isotopic evidence for the late veneer. Nature 520, 534–537 (2015). https://doi.org/10.1038/nature14360

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