Reassessing the origin and chronology of the unique achondrite Asuka 881394: Implications for distribution of 26Al in the early Solar System

1Josh Wimpenny, 1Matthew E.Sanborn, 2Piers Koefoed, 3Ilsa R.Cooke, 3Claudine Stirling, 2Yuri Amelin, 1Qing-ZhuYin
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.10.006]
1Department of Earth and Planetary Sciences, University of California at Davis, One Shields Avenue, Davis, CA 95616, USA
2Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia
3Department of Chemistry, University of Otago, Dunedin 9016, New Zealand
Copyright Elsevier

The achondrite Asuka 881394 has mineralogy broadly similar to that of eucrites but is isotopically, chemically and texturally distinct from them. Previous U-Pb chronology shows that it is very old; forming within the first 0.8 Ma of the formation of calcium-aluminum rich inclusions (CAIs). However, the age difference between Asuka 881394 and other very old Solar System materials (CAIs, quenched angrites) measured with the 26Al-26Mg and 53Mn-53Cr extinct radionuclide chronometers, and 207Pb/206Pb chronometer, is not the same. This could be interpreted to reflect heterogeneity in the distribution of 26Al and 53Mn in the early Solar System. Given the significant implications for the early Solar System chronology if 26Al and 53Mn are indeed heterogeneously distributed, in this study we further investigate the origin of Asuka 881394 and the apparent age discordance between short-lived and absolute chronometers, by focusing on measurement of its ε54Cr composition, renewed measurements of the absolute Pb-Pb age and new, high precision measurements of its 26Al-26Mg systematics.
New Cr isotope data places additional constraints on the origin of Asuka 881394; its ε 54Cr value of -0.37 ± 0.10ε is resolvable outside of uncertainty from HED meteorites (-0.72 ± 0.10ε), reinforcing evidence from oxygen isotopic analyses that suggest it originated from a distinct parent body, unlikely to be 4 Vesta. New Pb-Pb analyses, combined with using a directly measured 238U/235U ratio for age calculation, result in a recalculated Pb-Pb age of 4564.95 ± 0.53Ma, ∼1.5Ma younger than previously reported age. With this age Asuka 881394 remains one of the oldest known achondrites in our Solar System. New high precision 26Al-26Mg data produce an initial 26Al/27Al ratio of 1.48 ± 0.12 × 10-6, within error of previous data. This ratio corresponds to an Al-Mg age of 4563.69 ± 0.36 Ma or 4564.83 ± 0.21 Ma relative to CAIs and the D’Orbigny angrite, respectively. Thus, depending on which age anchor is used, the 26Al-26Mg age is either 1.26 Ma or 0.12 Ma younger than the new Pb-Pb age, the latter being unresolved within analytical uncertainty.
Though a potential age discrepancy between 26Al-26Mg and Pb-Pb could be a result of heterogeneous distribution of 26Al, we demonstrate with our new high precision Mg isotope data, in conjunction with petrographic evidence, that the Mg isotope system has been disturbed in Asuka 881394. We suggest that the 26Al-26Mg system closed to diffusion after the U-Pb system, either due to slow cooling on the parent body or low-grade metamorphic re-equilibration of Mg. Thus, we can satisfactorily explain the observed age discrepancy between 26Al-26Mg and U-Pb systems in Asuka 881394 without invoking heterogeneous distribution of 26Al/27Al ratio in the early Solar System.
Comparison of the Asuka 881394 data with that of other anomalous achondrites from distinct parent bodies suggests that these could also have evolved from a source region with a canonical 26Al/27Al ratio. Because these achondrites have significant differences in their ε54Cr-Δ17O systematics, which could be indicative of location within the early protoplanetary disk, it is consistent with homogeneous distribution of 26Al in the early Solar System. Angrites remain an outlier; either because they evolved from a source with a lower 26Al/27Al ratio or because their 26Al-26Mg or Pb-Pb data are problematic. In either case, this suggests that basaltic angrites are questionable as the age anchor for the entire Solar System as a whole, and other very old, well preserved achondrites should be considered for that role.

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