IDENTIFICATION OF MINERALS WITH ISOTOPIC ANOMALIES OF LIGHT ELEMENTS

20世纪80年代，西村在阿连德原始陨石的基质材料中发现了过量的Mg同位素。<24>这些基质材料表现出较高的Mg/Si比相比，在其他样品中没有Mg异常，和矿物估计这些样品的镁橄榄石。本项目利用二次离子质谱（SIMS）技术对阿连德陨石中几个较大的包裹体进行了镁同位素比值研究，在两个样品中发现了过量的Mg。<24>对这两个包裹体样品进行了SIMS同位素分析后，用电子探针分析了元素组成，在电子探针分析中，SIMS轰击点被磨掉了，在电子探针显微镜下看不见，但电子探针分析表明这两个包裹体分别为Fo_和Fo_的橄榄石。<95><85>对另一个不含Mg异常的包裹体进行了电子探针分析，结果表明该包裹体为Fo橄榄石。<98>因此，这些结果表明，所有的橄榄石矿物都不含同位素异常镁。从阿连德陨石中提取的基质物质。采用一种新型的直接进样离子源表面电离质谱仪（DLMS）进行同位素比测量。在分析了数十个基体材料样品后，我们发现只有一个样品显示出过量的镁。<26>这种过量的^Mg可能对应于^Al的衰变。对于该样品，我们仅用质谱模式比较法估计矿物可能是辉石或角闪石。<26><26>但同位素分析尚不能直接鉴定矿物。

One of the investigator of this project, Nishimura, found isotopic excesses of ^<24>Mg in matrix materials of the Allende primitive meteorite, in 1980's. These matrix materials showed higher Mg/Si ratios compared to that in the other samples without Mg anomalies, and minerals estimated for those samples were forsterite-like. However, detail identification of minerals were not be performed at that time.In this project, we carried out magnesium isotopic ratio investigation for a few comparatively large inclusions in the Allende meteorite with SIMS.We detected ^<24>Mg excesses in two samples. For these two inclusion samples, we analyzed elemental composition by the use of EPMA, after the isotopic analysis with SIMS.In this analysis, bombarded points by SIMS were polished out and could not be seen in the microscopic view of EPMA.But this EPMA analysis showed that those two inclusions are olivine with Fo_<95> and Fo_<85>, respectively. For another inclusion without Mg anomaly, EPMA analysis indicated that this is Fo_<98> olivine. So, this results show that all of olivine minerals does not contain isotopically anomalous magnesium.Moreover, isotopic analysis of magnesium were performed for the small amount (ca. 1mg) of matrix material picked out of the Allende meteorite. A new type of surface ionization mass spectrometer with direct sample loading ion source (DLMS) were used for this isotpoic ratio measurement. After several tens samples of matrix materials were analyzed, we found only one sample showing ^<26>Mg excess. This excess ^<26>Mg is likely to correspond to that was decayed from ^<26>Al. For this sample, we have only estimated that mineral might be pyroxyne or hornblende by the mass spectrum pattern comparison method. But direct identification of mineral could not yet be done in this isotpic analysis.