Multi-stage core formation in planetesimals revealed by numerical modeling and Hf-W chronometry of iron meteorites

1,2W. Neumann,1,3T. S. Kruijer,2D. Breuer,1T. Kleine
Journal of Geophysical Research, Planets Link to Article [DOI: 10.1002/2017JE005411]
1Institut für Planetologie, Westfälische Wilhelms-Universität (WWU), Münster, Deutschland
2Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Berlin, Deutschland
3Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, California, USA
Published by arrangement with John Wiley & Sons

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 sulphur content and hence highest liquidus temperature. Here, 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 proto-core 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 a ≈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.


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