Hf–W chronometry of core formation in planetesimals inferred from weakly irradiated iron meteorites

Abstract

The application of Hf–W chronometry to determine the timescales of core formation in the parent bodies of magmatic iron meteorites is severely hampered by 182W burnout during cosmic ray exposure of the parent meteoroids. Currently, no direct method exists to correct for the effects of 182W burnout, making the Hf–W ages for iron meteorites uncertain. Here we present noble gas and Hf–W isotope systematics of iron meteorite samples whose W isotopic compositions remained essentially unaffected by cosmic ray interactions. Most selected samples have concentrations of cosmogenic noble gases at or near the lowermost level observed in iron meteorites and, for iron meteorite standards, have very low noble gas and radionuclide based cosmic ray exposure ages (<60Ma). In contrast to previous studies, no corrections of measured W isotope compositions are required for these iron meteorite samples. Their ε182W values (parts per 104 deviations from the terrestrial value) are higher than those measured for most other iron meteorites and range from −3.42 to −3.31, slightly elevated compared to the initial 182W/184W of Ca–Al-rich Inclusions (CAI; ε182W=−3.51±0.10). The new W isotopic data indicate that core formation in the parent bodies of the IIAB, IIIAB, and IVA iron meteorites occurred ∼1–1.5Myr after CAI formation (with an uncertainty of ∼1Myr), consistent with earlier conclusions that the accretion and differentiation of iron meteorite parent bodies predated the accretion of most chondrite parent bodies. One ungrouped iron meteorite (Chinga) exhibits small nucleosynthetic W isotope anomalies, but after correction for these anomalies its ε182W value agrees with those of the other samples. Another ungrouped iron (Mbosi), however, has elevated ε182W relative to the other investigated irons, indicating metal–silicate separation ∼2–3Myr later than in the parent bodies of the three major iron meteorite groups studied here.