Heterogeneous delivery of silicate and metal to the Earth by large planetesimals

Abstract

After the Moon’s formation, Earth experienced a protracted bombardment by leftover planetesimals. The mass delivered during this stage of late accretion has been estimated to be approximately 0.5% of Earth’s present mass, based on highly siderophile element concentrations in the Earth’s mantle and the assumption that all highly siderophile elements delivered by impacts were retained in the mantle. However, late accretion may have involved mostly large (≥ 1,500 km in diameter)—and therefore differentiated—projectiles in which highly siderophile elements were sequestered primarily in metallic cores. Here we present smoothed-particle hydrodynamics impact simulations that show that substantial portions of a large planetesimal’s core may descend to the Earth’s core or escape accretion entirely. Both outcomes reduce the delivery of highly siderophile elements to the Earth’s mantle and imply a late accretion mass that may be two to five times greater than previously thought. Further, we demonstrate that projectile material can be concentrated within localized domains of Earth’s mantle, producing both positive and negative 182W isotopic anomalies of the order of 10 to 100  ppm. In this scenario, some isotopic anomalies observed in terrestrial rocks can be explained as products of collisions after Moon formation.

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Fig. 1: Projectile’s core interaction during large terrestrial collisions.
Fig. 2: Post-impact distribution of projectile material.
Fig. 3: Collisionally driven compositional heterogeneities.
Fig. 4: Impact-driven W isotopic anomalies.

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Acknowledgements

We thank F. Nimmo, K. Pahlevan, D. Stevenson and A. Morbidelli for discussion and comments that greatly improved the paper. S.M. and R.M.C. thank NASA Exobiology grant NNX15AL26G and NASA SSERVI programmes for support. R.J.W. acknowledges NASA Emerging Worlds grant NNX16AN07G, and NASA SSERVI grant NNA14AB07A.

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S.M. conceived the work and analysed SPH results. R.M.C. performed SPH simulations and analysed the results. R.J.W. contributed HSE and W data and interpretation. All authors wrote the manuscript and discussed the results.

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Correspondence to S. Marchi.

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Marchi, S., Canup, R.M. & Walker, R.J. Heterogeneous delivery of silicate and metal to the Earth by large planetesimals. Nature Geosci 11, 77–81 (2018). https://doi.org/10.1038/s41561-017-0022-3

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