A FUN-LIKE HIBONITE INCLUSION WITH A LARGE 26Mg-EXCESS

Abstract

We found a unique hibonite (CaAl 12 O 19) inclusion Kz1-2 from the Kainsaz (CO3) chondrite, which exhibits evidence for a strong link between "normal" CAIs and FUN inclusions. Kz1-2 shows strongly fractionated isotopic compositions of O, Ca, and Ti preferring heavy isotopes and a characteristic REE abundance pattern similar to those of FUN (or HAL-type hibonite) inclusions, suggesting an intensive distillation process in the formation of this inclusion. Unlike FUN inclusions, however, Kz1-2 shows large excesses in 26 Mg and possibly in 41 K most likely from the decays of now-extinct nuclides 26 Al and 41 Ca with the inferred initial ratios of (26 Al/ 27 Al) 0 ∼ 5.3 × 10 − 5 and (41 Ca/ 40 Ca) 0 ∼ 1.1 × 10 − 8 , respectively, and no detectable mass-independent isotopic anomalies in Ca and Ti, both of which are characteristic of "normal" CAIs. These observations suggest that the distillation process which produced isotopic fractionation in Kz1-2 occurred as early as the formation of "normal" CAIs. It is also inferred that the formation process of FUN inclusions is closely related to that of "normal" CAIs.