Influence of polyborate ions on the fractionation of B isotopes during calcite deposition

Abstract

Boron (B) isotopes are widely used in studies of water–rock interaction and earth surface processes. The δ11B is a potentially attractive proxy of seawater pH, as the relative abundance of the two primary aqueous B species in seawater (boric acid, H3BO3 and borate, B(OH4)−) is strongly pH-dependent. In the process of marine carbonate deposition, the fractionation of the B isotope is controlled by the proportion of H3BO3 and B(OH4)− in seawater. In addition to H3BO3 and B(OH4)−, there are polyborates (e.g., B3O3(OH)4−, B4O5(OH)42−, and B5O6(OH)4−) in salt-lake brine, and their impacts on B isotope fractionation are still unclear. Notable, significant differences are observed in the degrees of fractionation of B isotope during the deposition of carbonates in marine and salt-lake; nonetheless, studies on the internal reasons for this significant difference between the marine and salt-lake environments are scarce, limiting the application of B isotope in salt lakes. In this study, a series of calcite sedimentary experiments in mother liquor with different BO compounds was conducted at a constant pH of 8.20 (±0.05). The results indicate the incorporation of H3BO3 through polyborates into calcite during the precipitation of calcite, thus reducing the B isotope fractionation degree between the mother liquor and calcite. This study provides an experimental basis for explaining the differences in B isotope fractionation between marine and terrestrial environments and the inversion of paleo-salinity using the B isotope of sedimentary carbonate.