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

铂族元素（以下简称PGE）在宇宙化学上属于难熔元素。它们在地球化学上被归类为亲铁元素，在地球型行星中集中在核心，相对丰度与太阳系丰度相似。另一方面，以地幔和地壳为代表的硅酸盐层具有极低的PGE浓度，其相对丰度变化很大。本研究的目的是精确测定各种差异化陨石的硅酸盐组分中的PGE，并根据PGE的相对和绝对丰度来考虑这些陨石的形成过程。对于测定PGE，应用电感耦合等离子体质谱法。此外，使用中子和光子的活化分析也用于测定主要和微量元素。作为一组有代表性的分化陨石，eucrites在2004年进行的研究之后，于2005年成为目标。除了钙铝榴石，2005年还研究了中菱铁矿的硅酸盐相。结果表明，中菱铁矿硅酸盐部分中的PGE与铁陨石有成因联系。这意味着，金属颗粒中的硅酸盐部分介菱铁矿引入的金属硫化物熔体，这是由固体铁的部分熔融。这进一步表明，中菱铁矿是由铁陨石撞击到硅酸盐靶上形成的。沿着这些研究，精确和准确地测定了碳质陨石（包括Cl陨石）的PGE丰度，从而可以可靠地计算出PGE在各种陨石材料中的相对丰度。

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.