High-precision SIMS analyses of initial 26Al/27Al in un-melted refractory inclusions: The search for multiple condensation episodes

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.