Hf-W chronology of CR chondrites: Implications for the timescales of chondrule formation and the distribution of 26Al in the solar nebula

Abstract Renazzo-type carbonaceous (CR) chondrites are distinct from most other chondrites in having younger chondrule26 Al- 26 Mg ages, but the significance of these ages and whether they reflect true formation times or spatial variations of the 26 Al/ 27 Al ratio within the solar protoplanetary diskare a matter of debate. To address these issues and to determine the timescales of metal-silicate fractionation and chondruleformation in CR chondrites, we applied the short-lived 182 Hf- 182 W chronometer to metal, silicate, and chondruleseparates from four CR chondrites. We also obtained Mo isotope data for the same samples to assess potential genetic links among the components of CR chondrites, and between these components and bulk chondrites. All investigated samples plot on a single Hf-W isochron and constrain the time of metal-silicate fractionation in CR chondrites to 3.6 ± 0.6 million years (Ma) after the formation of Ca-Al-rich inclusions (CAIs). This age is indistinguishable from a ∼3.7 Ma Al-Mg age for CR chondrules, suggesting not only that metal-silicate fractionation and chondruleformation were coeval, but also that these two processes were linked to each other. The good agreement of the Hf-W and Al-Mg ages, combined with concordant Hf-W and Al-Mg ages for angrites and CV chondrules, provides strong evidence for a disk-wide, homogeneous distribution of 26 Al in the early solar system. As such, the young Al-Mg ages for CR chondrulesdo not reflect spatial 26 Al/ 27 Al heterogeneities but indicate that CR chondrulesformed ∼1–2 Ma later than chondrulesfrom most other chondrite groups. Metal and silicate in CR chondrites exhibit distinct nucleosynthetic Mo and W isotope anomalies, which are caused by the heterogeneous distribution of the same presolar s- processcarrier. These data suggest that the major components of CR chondrites are genetically linked and therefore formed from a single reservoir of nebular dust, most likely by localized melting events within the solar protoplanetary disk. Taken together, the chemical, isotopic, and chronological data for components of CR chondrites imply a close temporal link between chondruleformation and chondrite accretion, indicating that the CR chondrite parent bodyis one of the youngest meteorite parent bodies. The relatively late accretion of the CR parent bodyis consistent with its isotopic composition (for instance the elevated 15 N/ 14 N) that suggests a formation at a larger heliocentricdistance, probably beyond the orbit of Jupiter. As such, the accretion age of the CR chondrite parent bodyof ∼3.6 Ma after CAI formation provides the earliest possible time at which Jupiter's growth could have led to scattering of carbonaceous meteorite parent bodiesfrom beyond its orbit into the inner solar system.