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

5 years ago

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

Renazzo-type carbonaceous (CR) chondrites are distinct from most other chondrites in having younger chondrule ²⁶Al-²⁶Mg ages, but the significance of these ages and whether they reflect true formation times or spatial variations of the ²⁶Al/²⁷Al ratio within the solar protoplanetary disk are a matter of debate. To address these issues and to determine the timescales of metal-silicate fractionation and chondrule formation in CR chondrites, we applied the short-lived ¹⁸²Hf-¹⁸²W chronometer to metal, silicate, and chondrule separates 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 chondrule formation 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 ²⁶Al in the early solar system. As such, the young Al-Mg ages for CR chondrules do not reflect spatial ²⁶Al/²⁷Al heterogeneities but indicate that CR chondrules formed ∼1–2 Ma later than chondrules from 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-process carrier. 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 chondrule formation and chondrite accretion, indicating that the CR chondrite parent body is one of the youngest meteorite parent bodies. The relatively late accretion of the CR parent body is consistent with its isotopic composition (for instance the elevated ¹⁵N/¹⁴N) that suggests a formation at a larger heliocentric distance, probably beyond the orbit of Jupiter. As such, the accretion age of the CR chondrite parent body of ∼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 bodies from beyond its orbit into the inner solar system.

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DOI: S0016703717306774