Study on the formation process of differentiated meteorites based on platinum group element abundances

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Platinum group elements (hereafter, PGE) are cosmochemically grouped in to refractory elements. They are geochemically grouped into siderophile elements, being concentrated into core in earth-type planets with relative abundances similar to solar system abundances. On the other hand, silicate layers represented by earth mantle and crust have extremely low concentrations of PGE with a large variation in their relative abundances. In this study, it was aimed that PGE were precisely determined for silicate fractions of various differentiated meteorites and that the formation processes of these meteorites were considered based on their relative and absolute abundances of PGE. For determining PGE, inductively-coupled plasma mass spectrometry was applied. In addition, activation analyses using neutrons and photons were also used for the determination of major and trace elements. As a representative group of differentiated meteorites, eucrites were targeted in 2005, following researches conducted in 2004. In addition to eucrites, silicate phases of mesosiderites also were studies in 2005. As a result, PGE in silicate portions of mesosiderites were observed to have genetic relation with iron meteorites. It is implied that metal grains present in silicate portions of mesosiderites were introduced as metal-sulfide melt, which was produced by partly melting of solid irons. This further suggests that mesosiderites formed by impact of iron meteorites onto the silicate target. Along these studies, PGE abundances were precisely and accurately determined for carbonaceous chondrites including CI chondrites so that relative abundances of PGE in various materials of meteorites can be figured reliably.