Late accretion as a natural consequence of planetary growth

Abstract

Core formation should strip highly siderophile elements (HSEs) from planetary mantles according to the expected metal–silicate partition coefficients1,2. However, studies of Earth3, the Moon4 and Mars5 indicate mantles with HSE abundances in chondrite-relative proportions that exceed the values expected from metal–silicate partitioning. Competing hypotheses have been proposed to account for these observations, including metal–silicate partitioning at higher pressures and temperatures6 and late accretion7. Here we present petrological and geochemical analyses of diogenite meteorites that represent mantle and crustal materials from two or more differentiated asteroids. We find that diogenites show HSE abundances that are consistent with metal–silicate equilibration, followed by minor continued accretion. Isotope chronometry supports diogenite crystallization ages within 2–3 million years of Solar System formation, indicating that late accretion occurred earlier than postulated for Earth, the Moon and Mars. The early timing and occurrence on differentiated asteroids, as well as on the larger terrestrial planets, therefore ties late accretion to planetary growth. On asteroidal bodies, such as the diogenite parent bodies, variations in HSE compositions may reflect regional rather than global effects. In contrast, for Earth, the Moon and Mars, compositional variations in mantle materials seem to be consistent with more homogeneous distributions through prolonged melting and/or solid-state convection.

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Figure 1: O isotope systematics of diogenites, eucrites and howardites.
Figure 2: HSE patterns for diogenite meteorites versus estimates of terrestrial and lunar mantle compositions.
Figure 3: Late accretion to numerous diogenite parent bodies revealed from Os concentrations and O isotopic compositions.

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Acknowledgements

We are grateful to A. J. Irving, the Meteorite Working Group and the NASA Johnson Space Center curatorial staff for provision of samples, R. D. Ash and P. M. Piccoli for analytical assistance and R. W. Carlson for access to the DTM/CIW laboratory. T. Kleine and A. J. Irving are gratefully acknowledged for providing comments. This work was made possible by financial support from NASA (NNX11AG34G, NNX12AH75G to J.M.D.D.; NNX10AG94G to R.J.W. and NNX07A48G to D.R.).

Author information

J.M.D.D. and R.J.W. planned the project, J.M.D.D. and L.Q. carried out Mn–Cr isotope determinations, D.R. III carried out O isotope measurements and J.M.D.D. carried out all other measurements. All authors contributed to discussion and interpretation of results in the manuscript.

Correspondence to James M. D. Day.

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The authors declare no competing financial interests.

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Day, J., Walker, R., Qin, L. et al. Late accretion as a natural consequence of planetary growth. Nature Geosci 5, 614–617 (2012) doi:10.1038/ngeo1527

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