IDENTIFICATION OF MINERALS WITH ISOTOPIC ANOMALIES OF LIGHT ELEMENTS

1998 Fiscal Year Final Research Report Summary

• Project/Area Number: 07404030
• Research Category: Grant-in-Aid for Scientific Research (A)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Petrology/Mineralogy/Science of ore deposit
• Research Institution: NARUTO UNIVERSITY OF EDUCATION
• Principal Investigator: NISHIMURA Hiroshi Naruto University of Education, Faculty of Natural Science Education, Professor, 学校教育学部, 教授 (30029722)
• Co-Investigator(Kenkyū-buntansha): OZAWA Hiroaki Naruto University of Education, Faculty of Natural Science Education, Research A, 学校教育学部, 助手 (60253241) ; MURATA Mamoru Naruto University of Education, Faculty of Natural Science Education, Associate, 学校教育学部, 助教授 (80239532)
• Project Period (FY): 1995 – 1998
• Keywords: mass spectrometry / isotopic ratio of light element / isotopic anomaly / meteorite / secondary ion mass spectrometry / direct sample loading ion source

Research Abstract

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.