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

铂族元素（以下简称 PGE）在宇宙化学上被归为难熔元素。它们在地球化学上被分为亲铁元素，集中在地型行星的核心中，其相对丰度与太阳系的丰度相似。另一方面，以地幔和地壳为代表的硅酸盐层的PGE浓度极低，且相对丰度变化较大。本研究的目的是精确测定各种不同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。为了测定 PGE，应用了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元素和微量元素。作为差异化陨石的代表组，继2004年进行研究之后，2005年又将目标瞄准了锂辉石。除了辉辉石之外，2005年还对中菱铁矿的硅酸盐相进行了研究。结果发现，中辉石硅酸盐部分中的PGE与铁陨石有亲缘关系。这意味着中菱铁矿的硅酸盐部分中存在的金属颗粒是以金属硫化物熔体的形式引入的，该金属硫化物熔体是通过部分熔化固态铁而产生的。这进一步表明中菱铁矿是由铁陨石撞击硅酸盐目标而形成的。通过这些研究，精确测定了包括 CI 球粒陨石在内的碳质球粒陨石的 PGE 丰度，从而可以可靠地计算出各种陨石材料中 PGE 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.