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

摘要

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 0, 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 fort-nation of this inclusion. Unlike FUN inclusions, however, Kz1-2 shows large excesses in Mg-26 and possibly in K-41 most likely from the decays of now-extinct nuclides Al-26 and 41 Ca with the inferred initial ratios of (Al-26/Al-27)(0)-5.3 x 10(-5) and (Ca-41/Ca-40)(0)similar to 1.1 x 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. (c) 2006 Elsevier B.V All rights reserved.