Oxygen isotopic composition of a compound Ca-Al-rich inclusion from Allende meteorite: implications for origin of palisade bodies and O-isotopic environment in the CAI forming region

Abstract

A calcium-aluminum-rich inclusion (CAI) containing big palisade bodies was found within the Allende meteorite. The bulk composition and mineral assemblages show that the CAI belongs to coarse-grained Type B. Oxygen isotopic distributions in palisade bodies in the CAI are similar to those observed in typical Type B CAIs, whereas all minerals including melilite in the host parts are enriched in ¹⁶O. The oxygen isotopic distribution in the CAI indicates that exchange of oxygen isotopes in the palisade bodies occurred before the trapping of palisades into the host. The palisade bodies were formed separately as small CAIs in the solar nebula and then accumulated together to form a large CAI. This suggests the possibility that during the CAI formation, O isotopic environment of CAI-forming region in the solar nebula repeatedly changed from ¹⁶O-rich to ¹⁶O-poor.