<scp> ²⁶ Al </scp> ‐ <scp> ²⁶ Mg </scp> isotope systematics of Ca‐Al‐rich inclusions and Al‐rich chondrules in carbonaceous unequilibrated chondrite Yamato 81020

Abstract Ca‐Al‐rich inclusions (CAIs), amoeboid‐olivine aggregates (AOAs), and chondrules from the lowest petrographic type unequilibrated chondrites hold the potential to provide the best‐preserved records of the origin and cosmochemical evolution of the solar system. Six CAIs, and three chondrules from Yamato (Y) 81020 (CO3.05), and one AOA and one spinel‐bearing chondrule from Allan Hills (ALHA)77307 (CO3.03) were analyzed for 26 Al‐ 26 Mg (t 1/2 = 0.72 Ma) short‐lived now‐extinct radioisotope decay systematics. Five CAIs from Y‐81020 and an AOA from ALHA77307 show a small range of abundance of 26 Al/ 27 Al from ~4.5 × 10 −5 to 3.2 × 10 −5 . The inferred abundances in these relatively small‐sized CAIs and AOA suggest their formation and/or resetting during distinct episodes spanning a few million years. The inferred time of formation of these small CAIs and AOA from the lowest petrographic type in Y‐81020 and ALHA77307 is consistent with the previous results of high‐precision analyses of three CAIs from Y‐81020. The obtained results in CO chondrites are also in agreement with CR chondrites and with an order of magnitude larger‐sized CAIs in CV (Vigarano) chondrites. 26 Al/ 27 Al abundances in the three analyzed chondrules imply their formation within the typical range of ~1 to 2 million years after the formation of CAIs. The observed 26 Al/ 27 Al abundances and initial magnesium isotopic compositions of these small CAIs and AOA in the weakly metamorphosed CO chondrites are in consonance with the previous studies of CAIs and AOAs in CV chondrites that inferred the formation and evolution of these objects from a homogeneous reservoir that existed at the birth of the solar system.