Formation and evolution processes of S-asteroids as inferred from primitive achondrites

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In order to find relationship between the most common S-asteroids in the main asteroid beet and primitive chondritic source materials in the Solar System, mineralogical distributions of several new primitive achondrites have been investigated by electron and X-ray microanalysis techniques. The samples studied include five lodranites-acapulcoites with four coarse-grained recrystallized textures (Lodran, MAC88177, Y74357, Y791491, EET84302) and two with fine-grained ones (Acapulco, ALH81187) and two silicate inclusions in iron meteorites (Caddo County and Landes) . Electron probe microanalyzer and microfocus X-ray fluorescent analysis techniques have been applied to obtain two-dimensional elemental distribution maps of the above samples. The maps were then converted into mineral distribution maps. These meteorites consist of common component minerals in ordinary chondrites such as oliveine, orthopyroxene, augite, Plagioclase, FeNi metal, troilite etc. Their chemical compositions are nearly uniform within a grain and distribute within a small range between H and E chondrites. However, their modal abundances obtained from the mineral distribution maps differ geatly between the samples. Lodranites consist mainly of olivine and orthopyroxene and are depleted in plagioclase and some acaplucoites such as Acapulco and EET84302 and the Caddo County iron meteorite are enriched in plagioclase and FeNi metal and troilites. The processes of their formation and evolution can be explained by partial melting of low temperature melts and depletion of the melt from a high temperature region, and by migration of the melts and recrystallization around the depleted area. The heat source may be decay of ^<26>Al and collisions of planetesimals. Such process is what is expected during the growth of planetesimals by collisions and accreation to from fairly large S-asteroids.