Determination of δ88/86Sr Using Matrix Correction by MC-ICP-MS

Thursday, 18 December 2014
Bi Zhu1, Tao Yang2, Xiao-peng Bian2 and Zhi-yong Zhu2, (1)Hohai University, School of Earth Sciences and Engineering, Nanjing, China, (2)Nanjing University, State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing, China
Stable Sr isotopic compositions (δ88/86Sr) in marine carbonates potentially provide key information on paleoseawater temperature (Rüggeberg et al. 2008). Traditional methods for δ88/86Sr determination by 87Sr-84Sr double-spike TIMS or MC-ICP-MS require chemical purification of Sr before spectrometric measurements because of matrix effects. Recent studies suggested that the matrix-matching method, in which matrix-matched standard solutions were used to bracket untreated water samples, gave precise and accurate results for sulfur isotopic ratios by MC-ICP-MS (Lin et al., 2014). The obvious advantage of this method is that there is no need for chemical purification, thus eliminating the possibility of isotope fractionation during the ion chromatography and expediting sample throughput. In this study, we applied the matrix-matching method to δ88/86Sr determination by MC-ICP-MS. NIST 987 Sr solution and a purified seawater sample (collected from the South China Sea) were selected for this study. Given that major matrices in carbonate come form Ca2+, NIST 987 and SW solutions containing 40 ppm Ca2+ were prepared by adding high-purity Ca solution. All solutions used contained 200 ppb Sr and the 88Sr/86Sr ratios were measured using a Neptune MC-ICP-MS. The purified SW was first determined using SSB method, in which pure NIST 987 was used as standard to bracket SW and yielded δ88/86Sr value of 0.366 ± 0.008‰ (2SE, n = 10). The δ88/86Sr values of Ca-bearing SW were then measured by using pure NIST 987 solution as the working standard to investigate matrix effects. The determined δ88/86Sr value (0.039 ± 0.021‰; 2SE, n = 10) deviated obviously from the reference value. Finally, the matrix-matched NIST 987 was applied as the working standard to bracket the Ca-bearing SW, and the measured δ88/86Sr value is 0.351 ± 0.009‰ (2SE, n = 10), consistent with the reference value within uncertainties. The consistent δ88/86Sr values and comparable external precision suggested that the matrix-matching method is applicable to δ88/86Sr determination when Ca2+ is the major matrix inference. However, its application to natural carbonate samples with more complicated matrices needs further exploration. [1] Lin et al. 2014, Rapid Commun. Mass. Sp. [2] Rüggeberg et al. 2008. Earth. Planet. Sc. Lett.