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

Journal Article · Tue Oct 24 00:00:00 EDT 2017 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2017.10.014· OSTI ID:1430963

Budde, Gerrit ^[1]; Kruijer, Thomas S. ^[2]; Kleine, Thorsten ^[1]

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1. Univ. of Munster (Germany)
2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Univ. of Munster (Germany)

The 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 a heterogeneous distribution of ²⁶Al are not well understood. To better determine the timescales of CR chondrule formation and CR chondrite parent body accretion, we obtained Hf-W isotopic data for metal, silicate, and chondrule separates from four CR chondrites. We also obtained Mo isotopic data for the same samples, to assess potential genetic links among the components of CR chondrites, and between these components and bulk chondrites. The isotopic data demonstrate that metal and silicate in CR chondrites exhibit distinct nucleosynthetic W and Mo isotope anomalies, caused by the heterogeneous distribution of a single presolar s-process carrier. These isotope signatures are akin to the complementary anomalies found previously for chondrules and matrix in CV chondrites and indicate that the major components of CR chondrites are genetically linked and formed from a common reservoir of solar nebula dust. The obtained Hf-W age of 3.6±0.6 million years (Ma) after the formation of Ca-Al-rich inclusions (CAIs) most likely dates metal-silicate separation during chondrule formation and is consistent with Al-Mg and Pb-Pb ages for CR chondrules, indicating that CR chondrules formed ~1–2 Ma later than chondrules from most other chondrite groups. Moreover, chemical, isotopic, and chronological data imply close temporal link between chondrule formation and chondrite accretion, making the CR chondrite parent body one of the youngest meteorite parent bodies. Such a late accretion at ~3.6 Ma after CAIs is consistent with isotopic composition of CR chondrites (e.g., ¹⁵N/¹⁴N) that is indicative of a formation at a larger heliocentric distance, probably beyond the orbit of Jupiter. As such, the accretion age of the CR parent body provides the earliest possible time at which Jupiter could have migrated inwards, leading to scattering of carbonaceous meteorite parent bodies into the inner solar system. Finally, the concordant Hf-W and Al- Mg ages for CR chondrules, combined with Hf-W and Al-Mg data for bulk CAIs, angrites, and CV chondrules, provide strong evidence for a disk-wide, homogeneous distribution of ²⁶Al in the early solar system.

• Research Organization: Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC52-07NA27344
• OSTI ID: 1430963
• Report Number(s): LLNL-JRNL-734229
• Journal Information: Geochimica et Cosmochimica Acta, Vol. 222, Issue C; ISSN 0016-7037
• Publisher: The Geochemical Society; The Meteoritical Society
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
79 ASTRONOMY AND ASTROPHYSICS