Formation processes of achondrite parent bodies as inferred from meteorites.

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Achondrites are differentiated meteorites developed on primitive protoplanets, which have been formed by crystallization of totally or partially molten primitive source materials in the early solar system. We have tried to reconstruct their protoplanets on the basis of mineralogy and chemistry of their component materials and deduced their formation conditions. A method of computer image processing techniques (CMA method) has been applied to utilize elemental distribution maps obtained by SEM in terms of RGB color images. The image are stored on MD discs to be used as image database of achondrites.A group of meteorites with achondritic textures, but with signatures of chondritic primitive materials have been related to S-type asteroids most abundant in the main belt asteroids. These meteorites can be formed by partial removal of Al-Ca-rich partial melts and relocation of Fe-Ni-S eutectic melts and crystal growth induced by planetesimal-scale collisions. Two new Antarctic meteorites we studied belong to this group and fill gaps between primitive achondrites and lodranites, stony iron meteorites of one of its kind.Brecciated basalt-like meteorites known as monomict eucrites show quickly grown textures but with homogeneous chemistry of their Mg-Fe distribution in pyroxenes. Application of the CMA method shows that they are products of thermal annealing on a protoplanet. All these eucrites have been classified into type 1 to 6 on the basis of degrees of homogenization of their pyroxenes by the thermal metamorphism related to the cratering events on the parent body.Two new lunar meteorites have been investigated by the CMA method and are turned out to be breccias of thick mare basalts. They include low Ti basalts not too well represented in the Apollo samples. Early crystallized Mg-rich mare components were also identified.