High precision TIMS measurements of stable Sr isotopes (δ88/86Sr) of standards with different matrices and depth bound seawater samples from the Bay of Bengal using a new 87Sr–84Sr double-spike

Abstract

We report high precision stable Sr isotopic data (δ^88/86Sr) using an ⁸⁷Sr–⁸⁴Sr double spike TIMS technique for a variety of standards with different matrices (carbonate, silicate, seawater, and pure Sr). The composition of the double spike used in the present study, determined using a total evaporation technique using TIMS, is significantly different from other Sr double spike solutions reported in the literature; it is less expensive as the compositions of the ⁸⁴Sr (carbonate) and ⁸⁷Sr (metal) single spikes are slightly less pure compared to those reported in the literature. Despite the lesser purity of the single spikes, the long-term external reproducibility of δ^88/86Sr values, based on multiple analyses of a variety of standards with different matrices, is better than ±0.035‰ which is comparable to, if not better than, previous studies. We demonstrate that the δ^88/86Sr values are not affected by the nature of the loading filament (Ta versus zone refined Re) or the amount of Sr loaded on the filament (500 ng, 1 μg, 2 μg). Additionally, the δ^88/86Sr values do not vary over a wide range of spike/sample ratios, consistent with our modelling calculations. We report δ^88/86Sr values relative to the NIST SRM987 standard for an ICPMS Sr standard (0.101 ± 0.033‰, 2SD, n = 22), NASS-6 seawater (0.387 ± 0.034‰, 2SD, n = 4), JCp-1 coral carbonate (0.196 ± 0.006‰, 2SD, n = 2), JCt-1 clam shell (0.252 ± 0.004‰, 2SD, n = 2), BCR-2 basalt (0.264 ± 0.003‰, 2SD, n = 2), and SRM987 (0 ± 0.023‰, 2SD, n = 26) standards. Finally, we report δ^88/86Sr values of depth bound samples from the Bay of Bengal for the first time. The δ^88/86Sr values range from 0.373 to 0.411‰ with an average of 0.388 ± 0.025‰ (2SD, n = 10), which overlaps with δ^88/86Sr values of global oceans. Our results demonstrate that despite variations in ⁸⁷Sr/⁸⁶Sr in the Bay of Bengal, where radiogenic ⁸⁷Sr/⁸⁶Sr at 100–120 m depth is explained by submarine groundwater discharge, the δ^88/86Sr values are invariant and the average δ^88/86Sr of the Bay of Bengal overlaps with global seawater data.