  The 182Hf-182W 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 expo-sure (CRE) induced
modifications of W isotope compositions, which can be achieved using in-situ
neutron 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 ~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 ~1-2 Ma after CAI formation,
whereas most carbonaceous (CC) iron meteorite parent bodies underwent core
formation ~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 26Al-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 ~1 Ma after CAI formation, but at different radial
locations in the disk.
