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

Multistage Core Formation in Planetesimals Revealed by Numerical Modeling and Hf‐W Chronometry... 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, 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. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Geophysical Research: Planets Wiley

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

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Publisher
Wiley Subscription Services, Inc., A Wiley Company
Copyright
©2018. American Geophysical Union. All Rights Reserved.
ISSN
2169-9097
eISSN
2169-9100
D.O.I.
10.1002/2017JE005411
Publisher site
See Article on Publisher Site

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, 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.

Journal

Journal of Geophysical Research: PlanetsWiley

Published: Jan 1, 2018

Keywords: ; ; ; ; ;

References

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