Precise determination of Sr isotope ratios in igneous rock samples and application to micro-analysis of plagioclase phenocrysts

概要

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A two-step chemical procedure to separate Sr from whole igneous rock sample has been developed for the analysis of Sr isotopes by thermal ionization mass spectrometry (TIMS). Firstly, a Sr-bearing fraction and a REE fraction are separated from the major elements of a decomposed sample solution using H+ form cation-exchange chromatography. Then the Sr fraction is purified in a second column using extraction chromatography and a Sr specific resin. For small amounts of sample (< 0.1 mg) obtained by micro-milling, only the second step of column separation is applied, directly separating the Sr fraction from the decomposed sample solution. The total procedural blank of these chemical separation procedures are very low, comparable to the lowest blank levels ever reported. The purified Sr is loaded on to a single Re-filament with a Ta ionization activator and measured using TIMS in static multi-collection mode. The mean value and reproducibility for standard 87Sr/86Sr in NIST SRM 987 with Sr loads of 100 ng and 5 ng are 0.710240 ± 0.000012(2σ, n = 12) and 0.710248 ± 0.000022 (2σ, n = 7), respectively. These values agree well with those from previous studies. Additionally, Sr isotope ratios measured in GSJ igneous rock reference materials JB-2, JA-1 and JR-1 are within error of values from previous studies, verifying the versatility of the technique reported in this study.