¹Steven J. Desch,¹Daniel R. Dunlap,³Emilie T. Dunham,⁴Curtis D. Williams,⁵Prajkta Mane
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2023.115607]
¹School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, 85287-1404, AZ, USA
²Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, 37830, TN, USA
³Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, PO Box 951567, Los Angeles, 90095-1567, CA, USA
⁴Earth and Planetary Sciences Department, University of California, Davis, One Shields Ave., Davis, 95616, CA, USA
⁵Lunar and Planetary Institute, USRA, 3600 Bay Area Blvd., Houston, 77058, TX, USA
Copyright Elsevier

We use rapidly cooled achondrites to test the assumption of 26Al homogeneity in the solar nebula, by checking if there is a single value of tSS, the absolute “Pb-Pb” age of the Solar System’s t=0, that makes concordant their ages from the Al-Mg and Pb-Pb systems. We find that values tSS=4568.42±0.24 Myr do make these ages concordant, and therefore the hypothesis of homogeneous 26Al is not falsified. This age, defined to be when the solar nebula had (26Al/27Al)=5.23×10−5, is significantly older than the ≈ 4567.3 Myr inferred from direct measurements of Pb-Pb ages in CAIs. Discrepancies between the Al-Mg and Pb-Pb chronometers in chondrules and CAIs have previously been interpreted as arising from heterogeneities in 26Al, under the presumption that the Al-Mg and Pb-Pb systems in CAIs closed simultaneously. We examine this assumption and show that resetting is to be expected in CAIs. In particular, we quantitatively demonstrate that it is plausible that Pb-Pb ages of CAIs were reset at late times, without resetting the earlier Al-Mg ages, if they were transiently heated in the same manner as chondrules. We critically examine Pb-Pb isochrons, refining data and suggesting best practices for their calculation and reporting. We advocate reporting chronometry as times of formation after t=0 rather than absolute ages, as only the former is useful for astrophysical models of the solar nebula. We advocate averaging of multiple samples, rather than anchoring to individual meteorites, to improve precision.