Ion Microprobe Analysis of Rb and Sr Isotopes in K-rich Minerals: An Attempt of Radiometric Dating

• INOUE Yumiko

  Center for Advanced Marine Research, Ocean Research Institute, University of Tokyo

• TSUTSUMI Yukiyasu

  Department of Geology and Paleontology, National Science Museum

• TERADA Kentaro

  Department of Earth and Planetary Sciences

• SANO Yuji

  Center for Advanced Marine Research, Ocean Research Institute, University of Tokyo

抄録

We report here in situ measurements of Rb and Sr isotopes in K-rich minerals (muscovite, phlogopite, and K-feldspar) using Sensitive High-Resolution Ion MicroProbe (SHRIMP). Several grains of the K-rich minerals were extracted from eight granitic rocks with various formation ages and mounted on epoxy-resin disks. Chemical compositions were measured by an electron probe microanalyzer (EPMA). Then they were set in the source chamber of SHRIMP. A 4-5 nA mass filtered O₂^- primary beam was used to sputter a 30-μm-diameter flat-bottomed crater and secondary positive ions were extracted by a 10 kV accelerating voltage for mass analysis. A mass resolution of 8900-9300 at 1% peak height was attained to reduce isobaric hydride interferences such as ⁸⁷RbH⁺ on ⁸⁸Sr⁺. The magnet was cyclically peak-stepped from mass 84 (⁸⁴Sr and ²⁸Si⁵⁶Fe) to mass 88 (⁸⁸Sr and ³⁰Si⁵⁸Fe), including the background, mass 85 for ⁸⁵Rb and ²⁹Si⁵⁶Fe, mass 86 for ⁸⁶Sr and ³⁰Si⁵⁶Fe, and mass 87 for ⁸⁷Rb, ⁸⁷Sr, and ²⁹Si⁵⁸Fe, respectively. No SiFe compounds of any mass number contributed significantly for the samples analyzed. Observed ⁸⁷A/⁸⁵A ratios (where A is an arbitrary element) were plotted against ⁸⁸A/⁸⁵A ratios for each sample. A least-square fitting was made in the diagram. The gradient and Y-intercept correspond to initial ⁸⁷Sr/⁸⁸Sr and initial ⁸⁷Rb/⁸⁵Rb ratios or present (⁸⁷Rb/⁸⁵Rb)+(⁸⁷Sr^*/⁸⁵Rb) ratios, respectively, where ^{“ * ”} denotes radiogenic. The initial ⁸⁷Sr/⁸⁸Sr ratios are mostly consistent with the upper mantle ⁸⁷Sr/⁸⁸Sr ratios of various ages within experimental error. There is a positive correlation between the initial ⁸⁷Rb/⁸⁵Rb ratios and formation ages of the granitic rock samples, as it is expected from ⁸⁷Rb decay. Further evaluation of the instrumental isotopic mass fractionation and relative sensitivities for Rb and Sr are required before the in situ ⁸⁷Rb-⁸⁷Sr measurement is routinely applied as a dating method.

収録刊行物

• 質量分析

  質量分析 52 (4), 182-188, 2004

  一般社団法人 日本質量分析学会

キーワード

• Ion microprobe
• <i>in-situ</i> Measurement
• Rb-Sr
• Dating
• Isotope

詳細情報

• CRID: 1390001206493923456
• NII論文ID: 10013522572
• NII書誌ID: AN0010555X
• DOI: 10.5702/massspec.52.182
• ISSN: 18804225
• ISSN: 18843271
• ISSN: 13408097
• NDL書誌ID: 7085773
• Web Site: https://ndlsearch.ndl.go.jp/books/R000000004-I7085773
• Web Site: http://www.jstage.jst.go.jp/article/massspec/52/4/52_4_182/_pdf
• 本文言語コード: en
• データソース種別:
  • JaLC
  • NDL
  • Crossref
  • CiNii Articles
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