Petrology, trace element abundances and oxygen isotopic compositions of a compound CAI–chondrule object from Allende

Abstract

We report the petrology, trace element abundances and oxygen isotopic characteristics of a compound CAI–chondrule object, WI-025, found in the Allende CV3 chondrite. The WI-025 is an irregularly shaped inclusion consisting of three texturally and chemically distinct portions: the interior portion, the igneous rim and the intermediate zone located between these two portions. The interior portion consists of anorthite, spinel, olivine and Al-bearing low-Ca pyroxene. The major element chemistry of the interior portion corresponds to that of Al-rich chondrules and is of intermediate character between fine-grained spinel-rich CAIs and ferromagnesian chondrules. The interior portion has abundant 16O-rich spinel (Δ17O=−14.2 to −24.7) and displays a group II CAI-like REE composition. These observations indicate that the interior portion contains a CAI component formed by fractional condensation. The major and trace element chemistry of the interior portion indicate that the CAI had subsequently assimilated chondrule materials through partial melting. The maximum heating temperature of the partial melting is estimated at approximately 1400°C, similar to the maximum heating temperature of Type-B CAIs. The oxygen isotopic compositions of the olivine and low-Ca pyroxene (Δ17O=−6.3) in the interior portion indicate that the partial melting and chondrule assimilation took place under a moderately 16O-poor nebular gas. The igneous rim is texturally and chemically similar to ferromagnesian chondrules and entirely surrounds the interior portion. The oxygen isotopic compositions of the olivine and low-Ca pyroxene in the igneous rim are indistinguishable from those of the interior olivine and Al-bearing low-Ca pyroxenes. These observations indicate that a chondrule material, which was melted in the same nebular gas as the interior portion, was accreted to the interior portion. The intermediate zone represents a reaction zone accompanying the igneous rim formation. The formation history of WI-025 can be summarized by the following processes: (1) original CAI formation, (2) partial melting and chondrule assimilation, (3) igneous rim formation and (4) secondary alteration on the parent body.