Abstract:
Carbonaceous chondrites of type 3 (e.g. Allende) are among the most primitive meteorites. They contain components that formed prior to the accretion of 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 have erased mineralogical evidence of nebular processes. The major fraction of Ca in the Allende matrix is contained in relatively large, 20-50 μm, Ca,Fe-rich aggregates (CFA) commonly assumed to have formed by parent body processes. To better constrain their origin a transmission electron microscopic study of these CFA was performed. They consist mainly of hedenbergitic pyroxenes 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 structure is restricted to pyroxenes with higher Mg content. A hedenbergitic pyroxene with space group P2/n has never been reported in the literature and is considered metastable. The relationship between composition and space group can be explained best assuming ferrobustamite (with wollastonite structure) as a precursor phase for the P2/n pyroxenes. Above 970°C a two phase field exists between ferrobustamite and augite. The miscibility gap widens towards higher temperatures. In one case intergrown P2/n with C2/c pyroxenes were found. Their compositions fit well into the ferrobustamite-augite two phase field above 1050°C. Very fast cooling (>10°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 (>1050°C) followed by rapid cooling (>10°/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 kilometer sized body are orders of magnitude lower. The conditions required for the formation of the CFA suggest either transport from a high temperature to a low temperature environment or very localized heating events in the solar nebula. In addition strongly oxidized conditions (log#fO2 (bar)=-15 to -10) are required to stabilize andradite against hedenbergite.