Hf‐W chronology of a macrochondrule from the L5/6 chondrite Northwest Africa 8192

1Jan L. Hellmann,1,2Thomas S. Kruijer,1Knut Metzler,1Markus Patzek,3Andreas Pack,4Jasper Berndt,1Thorsten Kleine
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13571]
1Institut für Planetologie, University of Münster, Wilhelm‐Klemm‐Strasse 10, 48149 Münster, Germany
2Nuclear & Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue L‐231, Livermore, California, 94550 USA
3Geowissenschaftliches Zentrum, University of Göttingen, Goldschmidtstr. 1‐3, 37077 Göttingen, Germany
4Institut für Mineralogie, University of Münster, Corrensstrasse 24, 48149 Münster, Germany
Published by arrangement with John Wiley & Sons

A large, igneous‐textured, and 2 cm‐sized spherical object from the L5/6 chondrite NWA 8192 was investigated for its chemical composition, petrography, O isotopic composition, and Hf‐W chronology. The petrography and chemical data indicate that this object closely resembles commonly found chondrules in ordinary chondrites and is therefore classified as a “macrochondrule.* As a result of metal loss during its formation, the macrochondrule exhibits elevated Hf/W, which makes it possible to date this object using the short‐lived 182Hf‐182W system. The Hf‐W data provide a two‐stage model age for metal–silicate fractionation of 1.4 ± 0.6 Ma after Ca‐Al‐rich inclusion (CAI) formation, indicating that the macrochondrule formed coevally to normal‐sized chondrules from ordinary chondrites. By contrast, Hf‐W data for metal from the host chondrite yield a younger model age of ~11 Ma after CAIs. This younger age agrees with Hf‐W ages of other type 5–6 ordinary chondrites, and corresponds to the time of cooling below the Hf‐W closure temperature during thermal metamorphism on the parent body. The Hf‐W model age difference between the macrochondrule and the host metal demonstrates that the Hf‐W systematics of the bulk macrochondrule were not disturbed during thermal metamorphism, and therefore, that the formation age of such objects can still be determined even in strongly metamorphosed samples. Collectively, this study illustrates that chondrule formation was not limited to mm‐size objects, implying that the rarity of macrochondrules reflects either that this process was very inefficient, that subsequent nebular size‐sorting decimated large chondrules, or that large precursors were rare.

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