IDENTIFICATION OF MINERALS WITH ISOTOPIC ANOMALIES OF LIGHT ELEMENTS

该项目的研究人员之一西村在20世纪80年代发现阿连德原始陨石的基体材料中同位素过量^<24>Mg。与其他没有Mg异常的样品相比，这些基质材料显示出更高的Mg/Si比率，并且这些样品的矿物估计是类似于森林石的。但是，当时没有进行矿物的详细鉴定。在本项目中，我们利用SIMS对Allende陨石中几个较大的包裹体进行了镁同位素比的研究。我们在两个样品中检测到^<24>Mg过量。对于这两个包裹体样品，我们在用SIMS进行同位素分析后，用EPMA分析元素组成。在本分析中，SIMS轰击点被打磨掉，在EPMA的微观视图中看不到。但EPMA分析表明，这两个包裹体分别为Fo_<95>和Fo_<85>的橄榄石。另一包裹体无Mg异常，EPMA分析表明为Fo_<98>橄榄石。结果表明，所有橄榄石矿物均不含同位素异常镁。此外，对从阿连德陨石中提取的少量（约1mg）基体材料进行了镁的同位素分析。采用直接载样离子源表面电离质谱仪（DLMS）进行同位素比的测定。在分析了几十个基体材料样品后，我们发现只有一个样品显示^<26>Mg过量。过量的^<26>Mg很可能对应于^<26>Al的衰变。对于该样品，我们仅通过质谱图比较方法估计矿物可能是辉石或角闪石。但这种同位素分析还不能直接鉴定矿物。

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.