IDENTIFICATION OF MINERALS WITH ISOTOPIC ANOMALIES OF LIGHT ELEMENTS

该项目的研究人员之一西村于1980年在S的研究中发现了阿连德原始陨石基质中的~(24)&Gt；Mg同位素过剩。这些基质物质的镁硅比值高于其他没有镁异常的样品，估计这些样品的矿物为镁橄榄岩。然而，当时还没有进行详细的矿物鉴定。在本项目中，我们利用SIMS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。我们在两个样品中检测到^&lt；24&gt；对于这两个包裹体样品，在用SIMS进行同位素分析后，用EPMA分析了它们的元素组成。在这次分析中，SIMS轰击的点被抛光了，在EPMA的显微镜下看不到。但这两个包裹体的EPMA分析表明，这两个包裹体分别是具有Fo_&lt；95&gt；和Fo&lt；85&gt；的橄榄石。对于另一种未发现镁异常的包裹体，EPMA分析表明该包裹体为橄榄石。因此，这一结果表明并不是所有的橄榄石矿物都含有同位素异常的镁。此外，还对从阿连德陨石中提取的少量(约1 mg)基质物质进行了镁的同位素分析。采用一种新型的表面电离质谱仪和直接进样离子源(DLMS)进行同位素比值的测量。在对几十个样品进行分析后，我们发现只有一个样品出现了镁超标。这种过剩的镁很可能与从铝中衰变的镁相对应。对于这个样品，我们只用质谱图比较法估计了矿物可能是辉石或角闪石。但在这种同位素分析中，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.