Separate Determination of Strontium and Barium Mass Fractions in Calcite and Dolomite in Carbonate Rocks by a Multi-step Sequential Leaching Procedure

Abstract

Trace element partitioning in carbonate minerals has shown potential in palaeoenvironment reconstruction, while one of its preconditions of application – the respective extraction of trace element mass fractions in diverse mineral phases in carbonate rocks – leaves room for methodological improvement. We tested a ten-step sequential leaching method to selectively dissolve and extract calcite and dolomite based on their different reactivities to dilute acetic acid, with a focus on strontium and barium release. A group of mineral samples and a group of rock samples were selected for the experiment. The constant release of Sr from both calcite and dolomite mineral samples indicates its uniform distribution in the two carbonate minerals, which is consistent with prior conclusions that Sr is incorporated in the carbonate lattice, substituting for Ca. For carbonate rocks, the Sr and Ba contents present linear correlations with the ratio of calcite and dolomite, as well as Ca/(Mg+Ca) ratio dissolved in each leaching step, suggesting that the determination of their mass fractions in the two minerals can be achieved by regression. Although the Ba abundance in mineral samples is similar to that of Sr, its pattern of release from rock sample powder shows significant non-carbonate contributions. Based on the results of the sequential leaching, we design a multi-step procedure for the separate extraction of Sr and Ba in calcite and dolomite, and a case study is conducted to demonstrate its applicability, the results of which show a significantly lower Ba content in the carbonate minerals compared with bulk Ba concentrations, emphasising the importance of being cautious when using element abundances in carbonate rocks to reconstruct geochemical properties of the palaeoenvironment.