In situ analysis of oxygen isotopes and Fe/Mg ratios in olivine using SIMS: Preliminary results for an Allende chondrule

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A secondary ion mass spectrometer (SIMS; CAMECA ims-6f) was used for in situ spot analyses of chemical and oxygen isotopic compositions in olivine. San Carlos olivine and Burma spinel were used for oxygen isotope standards. Repeated analyses of oxygen isotopes were performed using these terrestrial standards by changing several factors which might control instrumental mass fractionation (such as entrance slit position, field aperture position, etc.) and conditions required for reproducible oxygen isotope analysis were investigated. For an optimum condition, the results of ∿40 min analyses were reproducible within ±3‰ (1σ) for ^<17>O/^<16>O and ±2‰ (1σ) for ^<18>O/^<16>O. For chemical composition (e.g., Fe/Mg ratio) analyses, calibration was made using various terrestrial olivine grains with EPMA data. The oxygen isotope and Fe/Mg ratio analyses were performed for olivine grains in an Allende chondrule, which seems to be a compound chondrule consisting of two parts, (I) and (II). The results show that most of the analyzed points (14 out of 17 points) have relatively ^<16>O-rich compositions (δ^<17>O=-14 to -4‰ and δ^<18>O=-10 to -5‰), while three points near the edge of the chondrule (two points belong to (I) and one belongs to (II)) have relatively ^<16>O-poor compositions (δ^<17>O=-6 to -1‰ and δ^<18>O=∿0‰). The latter three points are more FeO-rich (the Fe/(Fe+Mg) atomic ratios of 4.5-8.2%) than the others (1.6-4.4%; mostly <3.2%). The present results, though rather preliminary, suggest that there is heterogeneity in oxygen isotopic composition even within a single chondrule.
国立極地研究所の論文

In situ analysis of oxygen isotopes and Fe/Mg ratios in olivine using SIMS: Preliminary results for an Allende chondrule

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