Multistage Core Formation in Planetesimals Revealed by Numerical Modeling and Hf‐W Chronometry of Iron Meteorites. Issue 2 (17th February 2018)

• Record Type: Journal Article
• Authors: Neumann, W.; Kruijer, T. S.; Breuer, D.; Kleine, T.
• Is Part Of: Journal of geophysical research. Volume 123:Issue 2(2018)
• Page Start: 421
• Page End: 444
• Publication Date: 2018-02-17
• Subjects: planetesimals -- iron meteorites -- core formation -- Hf‐W chronology -- melt percolation -- shallow magma ocean; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1002/2017JE005411
• Languages: English
• ISSNs: 2169-9097

Abstract

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 182 Hf‐ 182 W 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, 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 182 W compositions. Consequently, the final 182 W 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. Key Points: Differentiation of the IVB parent body by a complex metal‐silicate separation scenario is presented Differentiation models are in accordance with the Hf‐W model age Two‐stage core formation is necessary to fit the thermochronological data