Improved alkaline fusion method for B isotope and concentration measurements of silicate materials

Abstract

Boron (B) is one of the most powerful tracers of fluid-mediated processes. However, challenges still exist for B analysis of silicate materials with the main difficulty in sample dissolution. Cai et al. (2021) reported a novel alkaline fusion method for boron isotope analysis using solid sodium peroxide and glassy carbon crucibles. Here we report a modified and improved version of this method using hydrated sodium peroxide and silver crucibles, which permits a lower flux-to-sample mass ratio (FSMR) (3 : 1), relatively low fusion temperature of 710 °C, and the use of cheaper silver crucibles. This modified method achieves complete sample dissolution while being safer and more efficient as it removes the need for grinding fresh sodium peroxide solid into powder. The B blank of the hydrated sodium peroxide solution can be determined more accurately and further reduced through chromatography. In addition to the improvements in the alkaline fusion procedure, we also propose a new procedure to dissolve the fusion cake, which greatly reduces reagent usage and minimizes the procedural blank as a result. Using these improved techniques, we analyzed B concentrations and B isotope compositions of eight silicate international standards. The results are consistent with published data within analytical uncertainties. Combined with the one-step chromatography using the Amberlite® IRA-743 B-specific resin to purify B and the Neptune plus® MC-ICP-MS for isotopic analysis, this method generates a batch of B concentration and isotope data within 48 hours with minimal need to handle hazardous chemicals. This safe and efficient method will greatly speed up research using B isotopes on silicate materials.