Evidence for solar nebula signatures in the matrix of the Allende meteorite

Carbonaceous chondrites of type 3 (e.g. Allende) are among the most primiti ve meteorites. They contain components that formed prior to the accretion o f a parent asteroid and thus record conditions of the ambient nebular gas, the source of the material from which the solid bodies of our solar system formed. Identification of nebular signatures is often difficult as thermal metamorphism and/or aqueous alteration on the meteorite parent body may hav e erased mineralogical evidence of nebular processes. The major fraction of Ca in the Allende matrix is contained in relatively large, 20-50 mu m, Ca, Fe-rich aggregates (CFA) commonly assumed to have formed by parent body pro cesses. To better constrain their origin a transmission electron microscopi c study of these CFA was performed. They consist mainly of hedenbergitic py roxenes with minor andradite and sulfide. We found that pyroxenes with low Mg content belong to the space group P2/n, whereas the expected C2/c struct ure is restricted to pyroxenes with higher Mg content. A hedenbergitic pyro xene with space group P2/n has never been reported in the literature and is considered metastable. The relationship between composition and space grou p can be explained best assuming ferrobustamite (with wollastonite structur e) as a precursor phase for the P2/n pyroxenes. Above 970 degrees C a two p hase field exists between ferrobustamite and augite. The miscibility gap wi dens towards higher temperatures. In one case intergrown P2/n with C2/c pyr oxenes were found. Their compositions fit well into the ferrobustamite-augi te two phase field above 1050 degrees C, Very fast cooling(>10 degrees C/h) controls the incomplete transformation from ferrobustamite to hedenbergite resulting in the observed P2/n space group of Mg-poor pyroxenes. Thus, the Ca,Fe-pyroxenes provide strong evidence for a high temperature origin (>10 50 degrees C) followed by rapid cooling(> 10 degrees/h), implying that the CFA in the Allende matrix formed in the solar nebula as the Allende parent asteroid has never seen such temperatures and possible cooling rates on a k ilometer sized body are orders of magnitude lower. The conditions required for the formation of the CFA suggest either transport from a high temperatu re to a low temperature environment or very localized heating events in the solar nebula. In addition strongly oxidized conditions (log fO(2) (bar) = -15 to -10) are required to stabilize andradite against hedenbergite. (C) 2 000 Elsevier Science B,V. All rights reserved.