¹Glenn J. MacPherson, ²Alexander N. Krot, ²Kazuhide Nagashima, ³Marina Ivanova
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.01.001]
¹US National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, USA
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
³Vernadsky Institute, Kosygin St. 19, Moscow 119991, Russia
Copyright Elsevier

Refractory inclusions formed via high temperature events during the earliest stages of the solar system evolution. Studies of short-lived radionuclide systems in the inclusions provide constraints on the timing and nature of these thermal events. High-precision SIMS data for initial 26Al/27Al ratio [(26Al/27Al)0] in a suite of seven un-melted refractory inclusions (fine-grained spinel-rich and Fluffy Type A CAIs) from CV (Vigarano type) carbonaceous chondrites – which we interpret as primary nebular condensates or their very close derivatives – yield six values close to the canonical ratio of 5.2 × 10−5 and one marginally lower but still almost within error of 5.0 × 10–5. We specifically looked for but did not find much lower values like those reported recently by Kawasaki et al. (2020), as low as 3.4 × 10–5. Interpreted in terms of chronology, the accumulated high precision data acquired by us and others within the past 15 years for normal, 26Al-rich CAIs show no evidence for a significant condensation event that would correspond to (26Al/27Al)0 of (3–4) × 10–5. Rather, there appears to have been one major thermal event resulting in extensive evaporation and condensation in the CAI-forming region corresponding to (26Al/27Al)0 of 5.2 × 10–5 resulting in formation of most normal refractory inclusion precursors. Subsequent smaller events over the succeeding ∼200,000 years caused thermal modification and melting of many of them. Inclusions such as that studied by Kawasaki et al. (2020) could have formed either in an early event prior to significant isotopic mixing in the CAI-forming region, or later than most refractory inclusions during a thermal event that is not well represented in the meteorite record. Refractory inclusions characterized by low (26Al/27Al)0, < 1 × 10–5, such as FUN (Fractionation and Unidentified Nuclear effects) inclusions, PLACs (Platy Hibonite Crystals), and some corundum-, hibonite-, and grossite-rich CAIs formed during a much earlier heating event, likely prior to homogenization of 26Al in the early solar system. The initial 26Al/27Al values of such objects provide no quantitative chronological constraints.

¹M. Mijjum,²B. J. Andrews,²T. J. McCoy,²C. M. Corrigan,^1,3M. W. Caffee,¹M. M. Tremblay
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14309]
¹Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, Indiana, USA
²Department of Mineral Sciences, Smithsonian National Museum of Natural History, Washington, DC, USA
³Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana, USA
Published by arrangement with John Wiley & Sons

Cosmic ray exposure (CRE) ages provide information about the parent bodies and source regions of meteorite classes. Cosmogenic noble gases are often used to quantify exposure time scales ranging from tens of ka to hundreds of Ma. The production rate of cosmogenic noble gases is primarily controlled by a meteorite’s chemical composition. Historically, an average chemical composition for an entire meteorite class or subgroup was used to calculate production rates. At the scale needed for noble gas measurements, however, some meteorites exhibit mineral abundance variabilities that translate into chemical heterogeneities, necessitating subsample-specific production rates. We find that the metal and sulfide content can vary significantly between ~100 and 300 mg subsamples of the same enstatite (E) chondrite, leading to >10% differences in cosmogenic 21Ne production rates between subsamples. We demonstrate an approach to determining subsample-specific production rates using E chondrites. We use electron microprobe analysis and X-ray computed microtomography to quantify the chemical composition and abundances, respectively, of metal, sulfide, and silicate minerals in six E chondrites and calculate subsample-specific production rates of 3He and 21Ne. By applying this method to more E chondrite subsamples alongside noble gas measurements, we may begin to address broader questions, such as whether peaks in the E chondrite CRE age distribution can be attributed to distinct impact events.