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Silicon isotope constraints on terrestrial planet accretion.

ABSTRACT: Understanding the nature and origin of the precursor material to terrestrial planets is key to deciphering the mechanisms and timescales of planet formation¹. Nucleosynthetic variability among rocky Solar System bodies can trace the composition of planetary building blocks^2-5. Here we report the nucleosynthetic composition of silicon (μ³⁰Si), the most abundant refractory planet-building element, in primitive and differentiated meteorites to identify terrestrial planet precursors. Inner Solar System differentiated bodies, including Mars, record μ³⁰Si deficits of -11.0 ± 3.2 parts per million to -5.8 ± 3.0 parts per million whereas non-carbonaceous and carbonaceous chondrites show μ³⁰Si excesses from 7.4 ± 4.3 parts per million to 32.8 ± 2.0 parts per million relative to Earth. This establishes that chondritic bodies are not planetary building blocks. Rather, material akin to early-formed differentiated asteroids must represent a major planetary constituent. The μ³⁰Si values of asteroidal bodies correlate with their accretion ages, reflecting progressive admixing of a μ³⁰Si-rich outer Solar System material to an initially μ³⁰Si-poor inner disk. Mars' formation before chondrite parent bodies is necessary to avoid incorporation of μ³⁰Si-rich material. In contrast, Earth's μ³⁰Si composition necessitates admixing of 26 ± 9 per cent of μ³⁰Si-rich outer Solar System material to its precursors. The μ³⁰Si compositions of Mars and proto-Earth are consistent with their rapid formation by collisional growth and pebble accretion less than three million years after Solar System formation. Finally, Earth's nucleosynthetic composition for s-process sensitive (molybdenum and zirconium) and siderophile (nickel) tracers are consistent with pebble accretion when volatility-driven processes during accretion and the Moon-forming impact are carefully evaluated.

SUBMITTER: Onyett IJ

PROVIDER: S-EPMC10356600 | biostudies-literature | 2023 Jul

REPOSITORIES: biostudies-literature

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Silicon isotope constraints on terrestrial planet accretion.

Onyett Isaac J IJ Schiller Martin M Makhatadze Georgy V GV Deng Zhengbin Z Johansen Anders A Bizzarro Martin M

Nature 20230614 7970

Understanding the nature and origin of the precursor material to terrestrial planets is key to deciphering the mechanisms and timescales of planet formation1. Nucleosynthetic variability among rocky Solar System bodies can trace the composition of planetary building blocks2-5. Here we report the nucleosynthetic composition of silicon (μ30Si), the most abundant refractory planet-building element, in primitive and differentiated meteorites to identify terrestrial p ...[more]

PMID: 37316662

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Project description:When corrected for the effects of cosmic ray exposure, Mo and Ru nucleosynthetic isotope anomalies in iron meteorites from at least nine different parent bodies are strongly correlated in a manner consistent with variable depletion in s-process nucleosynthetic components. In contrast to prior studies, the new results show no significant deviations from a single correlation trend. In the refined Mo-Ru cosmic correlation, a distinction between the non-carbonaceous (NC) group and carbonaceous chondrite (CC) group is evident. Members of the NC group are characterized by isotope compositions reflective of variable s-process depletion. Members of the CC group analyzed here plot in a tight cluster and have the most s-process depleted Mo and Ru isotopic compositions, with Mo isotopes also slightly enriched in r- and possibly p-process contributions. This indicates that the nebular feeding zone of the NC group parent bodies was characterized by Mo and Ru with variable s-process contributions, but with the two elements always mixed in the same proportions. The CC parent bodies sampled here, by contrast, were derived from a nebular feeding zone that had been mixed to a uniform s-process depleted Mo-Ru isotopic composition. Six molybdenite samples, four glacial diamictites, and two ocean island basalts were analyzed to provide a preliminary constraint on the average Mo isotope composition of the bulk silicate Earth (BSE). Combined results yield an average μ 97Mo value of +3 ± 6. This value, coupled with a previously reported μ 100Ru value of +1 ± 7 for the BSE, indicates that the isotopic composition of the BSE falls precisely on the refined Mo-Ru cosmic correlation. The overlap of the BSE with the correlation implies that there was homogeneous accretion of siderophile elements for the final accretion of 10 to 20 wt% of Earth's mass. The only known cosmochemical materials with an isotopic match to the BSE, with regard to Mo and Ru, are some members of the IAB iron meteorite complex and enstatite chondrites.

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Silicon isotope constraints on terrestrial planet accretion.

Publications

Silicon isotope constraints on terrestrial planet accretion.

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