Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.484k&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 484
Other
17
Allende Meteorite, Metamorphism (Geology), Meteoritic Composition, Temperature Effects, Chemical Composition, Petrography, Scanning Electron Microscopy
Scientific paper
Dark lithic clasts or inclusions commonly occur in CV3 carbonaceous chondrites. They were interpreted to represent primary aggregates of dust in the solar nebula or products of thermal metamorphism and melting on the meteorite parent body. Kojima et al. recently studied two dark clasts in the Vigarano CV3 chondrite, and found evidence that they probably experienced thermal heating after intense aqueous alteration on the meteorite parent body. The results of our petrographic and scanning electron microscope (SEM) study of an unusual dark clast in Allende (termed All-AF) are presented. The present study reveals evidence suggesting that the clast probably experienced extensive aqueous alteration and subsequent thermal metamorphism on the meteorite parent body. Our observations reveal the following unusual characteristics: Some objects are composed of assemblages of oval-shaped sections or bars that are oriented parallel to each other, suggesting that they are pseudomorphs after porphyritic or barred olivine chondrules; veins of various thicknesses which consist of nepheline, Fe-rich olivine and Ca-pyroxene, occur all over the clast; and olivine in the clast commonly shows a swirly, fibrous texture similar to phyllosilicate. These characteristics are common with those in the Vigarano clasts, and suggest that All-AF is not a primary aggregate of dust in the solar nebula but experienced extensive aqueous alteration and subsequent thermal metamorphism on the meteorite parent body. The size distribution of rounded to oval-shaped objects and distribution density of Ca-Al rich inclusion CAI-like objects in All-AF suggest that the precursor of All-AF may be a CV3-type chondrite, possibly Allende itself. Oxygen isotopic data also support this view. If that is the case, All-AF provides evidence that water once existed in the CV chondrite parent body and aqueous alteration took place on a large scale somewhere on the CV parent body. Some dark inclusions previously interpreted to be primary aggregates may have common origins with All-AF.
Kojima Takeo
Tomeoka Kazushige
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