Indicators of aqueous alteration and thermal metamorphism on the CV parent body: Microtextures of a dark inclusion from Allende

Abstract

An unusual dark clast in the Allende CV3 chondrite (termed Allende-AF), which was previously interpreted as a primary aggregate formed in the solar nebula (Kurat et al., 1989; Palme et al., 1989), was re-examined. Our study reveals abundant evidence suggesting that it probably experienced extensive aqueous alteration and subsequent thermal metamorphism on the meteorite parent body. Allende-AF contains numerous rounded to oval-shaped inclusions embedded in a dark matrix. The inclusions, consisting predominantly of fine grains of Fe-rich olivine, have internal textures suggesting that they are pseudomorphs after chondrules. Several inclusions appear to be replaced CAIs. Veins filled with fibrous olivine grains occur abundantly in both inclusions and matrix; some veins (up to 4 mm in length) penetrate several inclusions, providing strong evidence that aqueous alteration occurred after accretion. The fibrous morphology of olivine in veins and inclusions suggests that the olivine was produced by dehydration and thermal transformation of phyllosilicate that had been formed by aqueous alteration. Olivine grains in the matrix contain numerous micro-inclusions of FeNi sulfide, which were probably incorporated during transformation from phyllosilicate. Allende-AF is probably related to the fine-grained variety of dark inclusions reported from CV3 chondrites that has been described as the type containing abundant porous aggregates of Fe-rich olivine by Johnson et al. (1990). Many dark inclusions previously described appear to be similar in texture and mineralogy to Allende-AF, and probably experienced similar secondary process on the meteorite parent body. The wide variation in texture of dark inclusions can be explained by different degrees of aqueous alteration that preceded thermal metamorphism. The size distribution of chondrule pseudomorphs and the abundance of CAI pseudomorphs suggest that the precursor of Allende-AF is a CV type chondrite, probably Allende itself. Oxygen isotopic and chemical compositions are consistent with this interpretation. The CV parent body has been commonly thought to have escaped major secondary processing. However, Allende-AF provides evidence that extensive aqueous alteration and thermal metamorphism have occurred locally on the CV parent body.