Al‐Mg isotopic study of spinel‐rich fine‐grained CAIs

Glenn J. MacPherson, Alexander N. Krot, Kazuhide Nagashima

Meteoritics & Planetary Science
First published: 23 December 2020


“High‐precision SIMS analyses of seven spinel‐rich fine‐grained CAIs from the CV3 chondrites Efremovka, Kaba, and Vigarano and the LL3.0 chondrite Semarkona, two known to have volatility‐fractionated Group II REE patterns (likely condensates) and three more probably Group II, all give initial 26Al/27Al (26Al/27Al0) close to the solar system initial value of 5.2 × 10–5. Our data thus provide no evidence for multiple condensation events in the early solar system. Other work by Ushikubo et al. (2017) and Kawasaki et al. (2019a, 2019b) did find some similar inclusions whose values are significantly lower than the solar system initial value. Because the latter data are too few to indicate whether or not the lower ratios cluster or rather spread out over a range, more work will be required to determine if nebular condensation occurred following only a few major, punctuated thermal events or rather happened more frequently. Regardless, it is now clear that CAIs having the Group II REE pattern formed very early in solar system history, contemporaneous with the formation of CAIs having unfractionated REE patterns. Whatever the mechanism that formed Group II, it occurred under conditions that caused spinel to condense prior to (at a higher temperature than) melilite. This is contrary to the predictions of equilibrium nebular condensation.”