Neutron capture on Pt isotopes in iron meteorites and the Hf-W chronology of core formation in planetesimals

The short-lived Hf-182-W-182 W isotope system can provide powerful constraints on the timescales of planetary core formation, but its application to iron meteorites is hampered by neutron capture reactions on W isotopes resulting from exposure to galactic cosmic rays. Here we show that Pt isotopes in magmatic iron meteorites are also affected by capture of (epi)thermal neutrons and that the Pt isotope variations are correlated with variations in W-182/W-184. This makes Pt isotopes a sensitive neutron dosimeter for correcting cosmic ray-induced W isotope shifts. The pre-exposure W-182/W-184 derived from the Pt-W isotope correlations of the IID, IVA and IVB iron meteorites are higher than most previous estimates and are more radiogenic than the initial W-182/W-184 of Ca-Al-rich inclusions (CAI). The Hf-W model ages for core formation range from +1.6 +/- 1.0 million years (Ma; for the IVA irons) to +2.7 +/- 1.3 Ma after CAI formation (for the IID irons), indicating that there was a time gap of at least similar to 1 Ma between CAI formation and metal segregation in the parent bodies of some iron meteorites. From the Hf-W ages a time limit of <1.5-2 Ma after CAI formation can be inferred for the accretion of the IID, IVA and IVB iron meteorite parent bodies, consistent with earlier conclusions that the accretion of differentiated planetesimals predated that of most chondrite parent bodies. (C) 2012 Elsevier B.V. All rights reserved.