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Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth

Abstract

Earth grew through collisions with Moon-sized to Mars-sized planetary embryos from the inner Solar System, but it also accreted material from greater heliocentric distances1,2, including carbonaceous chondrite-like bodies, the likely source of Earth’s water and highly volatile species3,4. Understanding when and how this material was added to Earth is critical for constraining the dynamics of terrestrial planet formation and the fundamental processes by which Earth became habitable. However, earlier studies inferred very different timescales for the delivery of carbonaceous chondrite-like bodies, depending on assumptions about the nature of Earth’s building materials5,6,7,8,9,10,11. Here we show that the Mo isotopic composition of Earth’s primitive mantle falls between those of the non-carbonaceous and carbonaceous reservoirs12,13,14,15, and that this observation allows us to quantify the accretion of carbonaceous chondrite-like material to Earth independently of assumptions about its building blocks. As most of the Mo in the primitive mantle was delivered by late-stage impactors10, our data demonstrate that Earth accreted carbonaceous bodies late in its growth history, probably through the Moon-forming impact. This late delivery of carbonaceous material probably resulted from an orbital instability of the gas giant planets, and it demonstrates that Earth’s habitability is strongly tied to the very late stages of its growth.

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Acknowledgements

We are grateful to NASA and the Institute of Meteoritics, University of New Mexico, for providing samples. We are also grateful to K. Metzler, G. Brennecka and A. Bischoff for discussions, C. Brennecka for comments on the paper, U. Heitmann for technical support, as well as R. Walker and K. Bermingham (University of Maryland) for providing their Mo solution standard. This study was supported by the European Research Council Consolidator Grant “ISOCORE” (contract 616564) and by the Deutsche Forschungsgemeinschaft (SFB/TRR 170 subproject B3-1). This is TRR publication no. 61.

Author information

G.B., C.B. and T.K. devised the study. G.B. prepared the samples for Mo isotope analyses and performed all measurements. All authors contributed to the interpretation of the data and preparation of the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to Gerrit Budde.

Supplementary information

Supplementary Information

Supplementary Text, Supplementary Figures 1–6, Supplementary Tables 1–4, Supplementary References and Supplementary Data 1 caption.

Supplementary Data 1

Data file for Supplementary Table 4. Summary of Mo isotope data and references for bulk meteorites as displayed in Fig. 1 and used for calculations of CC and NC lines.

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Fig. 1: Mo isotope dichotomy of meteorites in ε95Mo versus ε94Mo space.
Fig. 2: Two potential scenarios for reproducing the BSE’s Mo isotopic composition.
Fig. 3: Predicted Δ95Mo of the BSE versus the degree of impactor core re-equilibration during the Moon-forming impact.
Fig. 4: Probability of matching the BSE’s Δ95Mo for different compositions of proto-Earth’s mantle (pE), the Moon-forming impactor (GI) and the late veneer (LV) as a function of impactor core re-equilibration during formation of the Moon.