Analytical Sciences

Simultaneous Determination of Mass-dependent Isotopic Fractionation and Radiogenic Isotope Variation of Strontium in Geochemical Samples by Multiple Collector-ICP-Mass Spectrometry

Takeshi OHNO and Takafumi HIRATA

Department of Earth and Planetary Sciences, Graduate School of Science and Technology, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro, Tokyo 152-8551, Japan

We present a method to determine ⁸⁸Sr/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr simultaneously. The former variation reflects the mass-dependent isotopic fractionation through the physico-chemical processes, and the latter originates from β^--decay of the parent nuclide ⁸⁷Rb as well as the mass-dependent isotopic fractionation. In order to determine the mass-dependent isotopic fractionation, the mass-discrimination effect on ⁸⁸Sr/⁸⁶Sr was externally corrected by an exponential law using Zr. For the radiogenic growth of ⁸⁷Sr/⁸⁶Sr, the mass-dependent isotopic fractionation effect on ⁸⁷Sr/⁸⁶Sr was corrected by a conventional correction technique using the ⁸⁸Sr/⁸⁶Sr ratio. The reproducibility of the ⁸⁸Sr/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr measurements for a high-purity Sr chemical reagent was 0.06‰ (2SD, n = 20) and 0.07‰ (2SD, n = 20), respectively. Strontium isotopic ratios (⁸⁸Sr/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr) were measured on six geochemical reference materials (igneous rock: JB-1a and JA-2; carbonate mineral: JLs-1, JDo-1, JCp-1 and JCt-1) and one seawater sample. The resulting ⁸⁷Sr/⁸⁶Sr ratios obtained here were consistent with previously published data within the analytical uncertainties. The resulting ⁸⁸Sr/⁸⁶Sr ratios for igneous rock samples did not vary significantly within the samples, whereas the carbonate samples showed enrichments of the lighter Sr isotopes over the seawater sample. The ⁸⁸Sr/⁸⁶Sr ratio of geochemical samples could reflect the physico-chemical processes for the sample formation. Also, a combined discussion of ⁸⁸Sr/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr of samples will render multi-dimensional information on geochemical processes.

J-STAGE: View this article in J-STAGE