A FUN-like hibonite inclusion with a large 26Mg-excess

Abstract

We found a unique hibonite (CaAl12O19) 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 26Mg and possibly in 41K most likely from the decays of now-extinct nuclides 26Al and 41Ca with the inferred initial ratios of (26Al/27Al)0∼5.3×10−5 and (41Ca/40Ca)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.