Determining 88Sr/86Sr of barite using the Na2CO3 exchange method

Abstract

This study presents a method of high-precision stable Sr isotope measurement for barite using the Na₂CO₃ exchange reaction and MC-ICP-MS. The reliability of this method was strictly tested by measurement of the synthetic barite standard. The synthetic barites were made by mixing the natural barite standard GBW07814 with three strontium-rich matrix rocks (carbonate JLs-1, andesite AGV-2, and black shale SGR-1) in different proportions. The average ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr of synthetic barites are 0.713556 ± 0.000013 (2SD, n = 22) and −0.07 ± 0.02‰ (2SD, n = 22), respectively, which are consistent with the values of GBW07814 (⁸⁷Sr/⁸⁶Sr of 0.713555 ± 0.000023, 2SD, n = 46 and δ⁸⁸Sr of −0.06 ± 0.02‰, 2SD, n = 28). High recovery in the exchange reaction was achieved by using excessive Na₂CO₃ to avoid Sr isotope fractionation. Furthermore, the potential matrix effects in chemical purification were rigorously evaluated by analyzing the synthetic solution made by mixing SRM987 and matrix elements. The average δ⁸⁸Sr of the synthetic solution is −0.01 ± 0.04‰ (2SD, n = 4), consistent with the δ⁸⁸Sr of SRM987 (0.00‰). Based on long-term analyses of USTC-Sr (pure Sr solution), the external precision of δ⁸⁸Sr was found to be better than 0.03‰ (2SD, n = 92). We further measured the ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr of five natural barite standards (GBW07811, GBW07812, GBW07814, GBW07815 and GBW07816), revealing significant stable Sr isotope fractionation in natural barites by 0.37‰.