Abstract
Strontium isotope stratigraphy (SIS) is widely used in geoscience as a chemo-stratigraphic tracer. In past SIS research, diverse sample dissolution methods have been developed and applied to bulk carbonate rocks to extract target Sr isotope signatures of primary carbonates, which are easily contaminated with Sr from extraneous phases. However, the accuracy of the various dissolution methods is debated because all carbonate materials in previous studies were collected directly from the field and did not undergo strict homogeneity examination before sample dissolution. To verify and evaluate the actual bias for carbonate in SIS research, in this study, we analysed a set of artificial carbonate mixed standard samples including carbonate certificate reference materials (CRMs) GBW03105a for limestone and JDo-1 for dolostone in addition to that for clay minerals, GBW03103. These samples were verified to be stable in their Sr concentrations and 87Sr/86Sr ratios and could be used as benchmarks to enable reasonable evaluation on the extraction method for carbonate materials. We conducted a subtle chemical leaching experiment with 12 steps on these samples after preleaching by ammonium acetate.Our results show that the acidic preleach is useful in diminishing the Sr contamination from the extraneous phases. Nevertheless, the Sr contamination from the clay mineral can not be completely removed by the chemical leaching method and it would be more substantial for carbonate samples with lower purity and relatively lower original 87Sr/86Sr. Our work suggests an optimised dissolution scheme for carbonate 87Sr/86Sr analysis: (1) an acidic preleach to dissolve about ~60% of the sample, (2) a further dissolution of ~20% of the sample to prepare for the 87Sr/86Sr analysis, (3) leaving at least 10% of the sample undissolved. A simplified dissolution protocol with advantage of rapid operational and relatively-high reliability was finally proposed for strontium isotope stratigraphy research, particularly for reconstruction of secular Sr isotope evolution curve.
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