Nucleosynthetic Pt isotope anomalies and the Hf-W chronology of core formation in inner and outer solar system planetesimals

The Hf-182-W-182 chronology of iron meteorites provides crucial information on the timescales of accretion and differentiation of some of the oldest planetesimals of the Solar System. Determining accurate Hf-W model ages of iron meteorites requires correction for cosmic ray exposure (CRE) induced modifications of W isotope compositions, which can be achieved using in-situneutron dosimeters such as Pt isotopes. Until now it has been assumed that all Pt isotope variations in meteorites reflect CRE, but here we show that some ungrouped iron meteorites display small nucleosynthetic Pt isotope anomalies. These provide the most appropriate starting composition for the correction of CRE-induced W isotope variations in iron meteorites from all major chemical groups, which leads to a similar to 1 Ma upward revision of previously reported Hf-W model ages. The revised ages indicate that core formation in non-carbonaceous (NC) iron meteorite parent bodies occurred at similar to 1-2 Ma after CAI formation, whereas most carbonaceous (CC) iron meteorite parent bodies underwent core formation similar to 2 Ma later. We show that the younger CC cores have lower Fe/Ni ratios than the earlier-formed NC cores, indicating that core formation under more oxidizing conditions occurred over a more protracted timescale. Thermal modeling of planetesimals heated by Al-26-decay reveals that this protracted core formation timescale is consistent with a higher fraction of water ice in CC compared to NC planetesimals, implying that in spite of distinct core formation timescales, NC and CC iron meteorite parent bodies accreted about contemporaneously within similar to 1 Ma after CAI formation, but at different radial locations in the disk. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The Hf-182-W-182 chronology of iron meteorites provides important insights into the accretion and differentiation timescales of some of the oldest planetesimals in the Solar System. Correcting for cosmic ray exposure-induced modifications of W isotopes using Pt isotopes reveals anomalies in some ungrouped iron meteorites that lead to revisions of previously reported Hf-W model ages. The revised ages suggest that core formation in non-carbonaceous and carbonaceous iron meteorite parent bodies occurred at different timescales, with CC cores forming later and under more oxidizing conditions.