Origin of fayalitic olivine rims and lath‐shaped matrix olivine in the CV3 chondrite Allende and its dark inclusions

Abstract

Abstract— We have studied an Allende dark inclusion by optical microscopy, scanning electron microscopy, electron microprobe analysis and transmission electron microscopy. The inclusion consists of chondrules, isolated olivines and matrix, which, as in the Allende host, is mainly composed of 5–20 μm long lath‐shaped fayalitic grains with a narrow compositional range (Fa42 ± 2) and nepheline. Olivine phenocrysts in chondrules and isolated olivine grains show various degrees of replacement by 5–10 μm wide fayalitic rims (Fa39 ± 2) and 100–1000 μm wide translucent zones, which consist of 5–20 μm long lath‐shaped fayalitic grains (Fa41 ± 1) intergrown with nepheline. These fayalitic olivines, like those in the matrix of the dark inclusion, contain 10–20 nm sized inclusions of chromite, hercynite, and Fe‐Ni sulfides. The fayalitic rims around remnant olivines are texturally and compositionally identical to those in Allende host, suggesting that they have similar origins. Chondrules are surrounded by opaque rims consisting of tiny lath‐shaped fayalitic olivines (<1–3 μm long) intergrown with nepheline. As in the Allende host, fayalitic olivine veins may crosscut altered chondrules, fine‐grained chondrule rims and extend into the matrix, indicating that alteration occurred after accretion.We infer that fayalitic olivine rims and lath‐shaped fayalites in Allende and its dark inclusions formed from phyllosilicate intermediate phases. This explanation accounts for (1) the similarity of the replacement textures observed in the dark inclusion and Allende host to aqueous alteration textures in CM chondrites; (2) the anomalously high abundances of Al and Cr and the presence of tiny inclusions of spinels and sulfides in fayalitic olivines in Allende and Allende dark inclusions; (3) abundant voids and defects in lath‐shaped fayalites in the Allende dark inclusion, which may be analogous to those in partly dehydrated phyllosilicates in metamorphosed CM/CI chondrites. We conclude that the matrix and chondrule rims in Allende were largely converted to phyllosilicates and then completely dehydrated. The Allende dark inclusions experienced diverse degrees of aqueous/hydrothermal alteration prior to complete dehydration. The absence of low‐Ca pyroxene in the dark inclusion and its significant replacement by fayalitic olivine in Allende is consistent with the lower resistance of low‐Ca pyroxene to aqueous alteration relative to forsteritic olivine. Hydro‐thermal processing of Allende probably also accounts for the low abundance of planetary noble gases and interstellar grains, and the formation of nepheline, sodalite, salite‐hedenbergite pyroxenes, wollastonite, kirschsteinite and andradite in chondrules and Ca,Al‐rich inclusions.