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Title: Multistage Core Formation in Planetesimals Revealed by Numerical Modeling and Hf-W Chronometry of Iron Meteorites

Abstract

Iron meteorites provide some of the most direct insights into the processes and timescales of core formation in planetesimals. Of these, group IVB irons stand out by having one of the youngest 182Hf-182W model ages for metal segregation (2.9 ± 0.6 Ma after solar system formation), as well as the lowest bulk sulfur content and hence highest liquidus temperature. Here in this paper, using a new model for the internal evolution of the IVB parent body, we show that a single stage of metal-silicate separation cannot account for the complete melting of pure Fe metal at the relatively late time given by the Hf-W model age. Instead, a complex metal-silicate separation scenario is required that includes migration of partial silicate melts, formation of a shallow magma ocean, and core formation in two distinct stages of metal segregation. In the first stage, a protocore formed at ≈1.5 Ma via settling of metal particles in a mantle magma ocean, followed by metal segregation from a shallow magma ocean at ≈5.4 Ma. As these stages of metal segregation occurred at different times, the two metal fractions had different 182W compositions. Consequently, the final 182W composition of the IVB core does not correspond tomore » a single differentiation event, but represents the average composition of early- and late-segregated core fractions. Our best fit model indicates an ≈100 km radius for the IVB parent body and provides an accretion age of ≈0.1–0.5 Ma after solar system formation. The computed solidification time is, furthermore, consistent with the Re-Os age for crystallization of the IVB core.« less

Authors:
ORCiD logo [1];  [2]; ORCiD logo [3];  [4]
  1. Univ. of Munster, Munster (Germany). Inst. fur Planetologie; Deutsches Zentrum fur Luft-und Raumfahrt (DLR), Berlin (Germany)
  2. Univ. of Munster, Munster (Germany). Inst. fur Planetologie; Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States). Nuclear and Chemical Sciences Division
  3. Deutsches Zentrum fur Luft-und Raumfahrt (DLR), Berlin (Germany)
  4. Univ. of Munster, Munster (Germany). Inst. fur Planetologie
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE; German Research Foundation (DFG)
OSTI Identifier:
1430986
Report Number(s):
LLNL-JRNL-739210
Journal ID: ISSN 2169-9097
Grant/Contract Number:  
AC52-07NA27344
Resource Type:
Accepted Manuscript
Journal Name:
Journal of Geophysical Research. Planets
Additional Journal Information:
Journal Volume: 123; Journal Issue: 2; Journal ID: ISSN 2169-9097
Publisher:
American Geophysical Union
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; planetesimals; iron meteorites; core formation; Hf‐W chronology; melt percolation; shallow magma ocean

Citation Formats

Neumann, W., Kruijer, T. S., Breuer, D., and Kleine, T. Multistage Core Formation in Planetesimals Revealed by Numerical Modeling and Hf-W Chronometry of Iron Meteorites. United States: N. p., 2018. Web. doi:10.1002/2017JE005411.
Neumann, W., Kruijer, T. S., Breuer, D., & Kleine, T. Multistage Core Formation in Planetesimals Revealed by Numerical Modeling and Hf-W Chronometry of Iron Meteorites. United States. doi:10.1002/2017JE005411.
Neumann, W., Kruijer, T. S., Breuer, D., and Kleine, T. Thu . "Multistage Core Formation in Planetesimals Revealed by Numerical Modeling and Hf-W Chronometry of Iron Meteorites". United States. doi:10.1002/2017JE005411. https://www.osti.gov/servlets/purl/1430986.
@article{osti_1430986,
title = {Multistage Core Formation in Planetesimals Revealed by Numerical Modeling and Hf-W Chronometry of Iron Meteorites},
author = {Neumann, W. and Kruijer, T. S. and Breuer, D. and Kleine, T.},
abstractNote = {Iron meteorites provide some of the most direct insights into the processes and timescales of core formation in planetesimals. Of these, group IVB irons stand out by having one of the youngest 182Hf-182W model ages for metal segregation (2.9 ± 0.6 Ma after solar system formation), as well as the lowest bulk sulfur content and hence highest liquidus temperature. Here in this paper, using a new model for the internal evolution of the IVB parent body, we show that a single stage of metal-silicate separation cannot account for the complete melting of pure Fe metal at the relatively late time given by the Hf-W model age. Instead, a complex metal-silicate separation scenario is required that includes migration of partial silicate melts, formation of a shallow magma ocean, and core formation in two distinct stages of metal segregation. In the first stage, a protocore formed at ≈1.5 Ma via settling of metal particles in a mantle magma ocean, followed by metal segregation from a shallow magma ocean at ≈5.4 Ma. As these stages of metal segregation occurred at different times, the two metal fractions had different 182W compositions. Consequently, the final 182W composition of the IVB core does not correspond to a single differentiation event, but represents the average composition of early- and late-segregated core fractions. Our best fit model indicates an ≈100 km radius for the IVB parent body and provides an accretion age of ≈0.1–0.5 Ma after solar system formation. The computed solidification time is, furthermore, consistent with the Re-Os age for crystallization of the IVB core.},
doi = {10.1002/2017JE005411},
journal = {Journal of Geophysical Research. Planets},
number = 2,
volume = 123,
place = {United States},
year = {2018},
month = {2}
}

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