Online correction of matrix effects for boron isotope analysis in tourmaline using nano-secondary-ion mass spectrometry

Abstract

Boron isotopes serve as a effective tracers for fluid-related geological processes. Tourmaline is a boron-rich mineral, making it an ideal medium for B isotopic studies. However, matrix effects, particularly instrumental mass fractionation (IMF), can significantly affect the accuracy of B isotope analysis performed using secondary ion mass spectrometry (SIMS). Conventional correction methods typically rely on offline coupling of major element contents determined via electron probe microanalysis (EPMA) and B isotope ratios measured using SIMS; however, these methods are time-consuming and susceptible to spatial mismatch. This study introduces an online matrix effect correction method using NanoSIMS, eliminating the need for EPMA data. B isotope analysis revealed a strong linear correlation (R² > 0.93) between IMF and the FeO^T + MnO content of tourmaline, suggesting that Fe/Mn substitution is likely the primary factor governing IMF. Subsequently, an online matrix correction for the B isotope ratio was established by simultaneously measuring the ⁵⁸Fe⁺/¹⁰B⁺, ⁵⁵Mn⁺/¹⁰B⁺ ratios and the B isotope ratio (¹¹B⁺/¹⁰B⁺), utilizing a binary linear regression model. Nine tourmaline reference materials with diverse compositions were analyzed and corrected using this online correction method, yielding δ¹¹B values that are consistent with the recommended reference values within the uncertainty range. Overall, this approach enhances analytical efficiency and reliability, enabling high-precision B isotope tracing in complex geological processes.