open access

https://hdl.handle.net/2115/51784

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

英語

2013-02-01

Elsevier

Geochimica et Cosmochimica Acta

102

261-279

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. 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 between these two portions. The interior portion consists of anorthite, spinel, olivine and Al-bearing low-Ca pyroxene. Major element chemistry of the interior portion corresponds to that of Al-rich chondrules and is intermediate between fine-grained spinel-rich CAIs and ferromagnesian chondrules. The interior portion has abundant 16O-rich spinel (Δ17O = -14.2 to -24.7) and shows a group II CAI-like REE composition. These observations indicate that the interior portion contains a CAI component formed by fractional condensation. 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 to be about 1400℃, 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 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 is a reaction zone accompanying the igneous rim formation. The formation history of WI-025 can be summarized as 1) original CAI formation, 2) partial melting and chondrule assimilation, 3) igneous rim formation and 4) secondary alteration on the parent body.

学術雑誌論文

Accepted Manuscript

0016-7037