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Effects of Planetesimal-Scale Evaporation on Pb Isotopic Evolution and Timing of the Last U/Pb Fractionation
  • Tong Fang,
  • Weidong Sun,
  • Robert E. Zartman
Tong Fang
Institute of Oceanology

Corresponding Author:fangtong2019@qdio.ac.cn

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Weidong Sun
Institute of Oceanology, Chinese Academy of Sciences
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Robert E. Zartman
Institute of Oceanology
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The excess of radiogenic lead (Pb) isotopes in the silicate Earth, which is referred to as “the first terrestrial Pb paradox” has remained a confusion for a long time. A large-scale U/Pb fractionation with an increase of μ value (238U/204Pb) compared with CI chondrite is proposed to be the main culprit. The volatile e.g., Pb diffuses into space from the planetesimal-scale collisional melting, which plays a critical role in Pb loss on the accreting proto-Earth. The N-body simulation describes the collisional history of terrestrial planets in the first 200 million years of the Solar System. The collisional information provides the degree of silicate melting and further obtains the volatile loss fraction. Within the early 20% accretion of proto-Earth, the cumulative fraction of Pb loss can reach 80%-90%. Meanwhile, the μ value could rise to 1.5-4 setting the initial value to be 0.2-0.6. Besides, the silicate melting with higher temperature and lower oxygen fugacity (relatively reduced condition) can bring about more Pb loss. Further increase of μ to 9.26 possibly caused by a late large-scale U/Pb fractionation can effectively explain the excess of radiogenic Pb isotopes in the bulk silicate Earth. The two-stage model with the planetesimal-scale evaporation predicts a young age of 240 million years of the last large-scale fractionation event. The last fractionation is more consistent with the “Hadean matte” event than a late Moon-forming giant impact.