Precise and accurate boron and lithium isotopic determinations for small sample-size geological materials by MC-ICP-MS

Abstract

A low-blank, high-precision and highly reproducible technique for boron (B) and lithium (Li) isotope analyses by Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) was developed in this study. The new analytical protocol presented here involves effective sample dissolution and simultaneous purification of B and Li, together with high-precision B and Li isotopic determinations in silicate matrices. The sample solutions prepared from the low-temperature acid-dissolution have nearly 100% yields for B, Li, and Be. B and Li can be extracted from the same aliquot through ion chromatographic separations, followed by a modified micro-sublimation technique for further B purification. The low procedural blanks of Li and B using this new analytical protocol enable us to process silicate samples with trace amounts of B (<80 ng) and Li (<1 ng). The long-term external precisions of δ¹¹B (5 ng of B) and δ⁷Li (1 ng of Li) are better than ±0.35‰ (2SD) and ±0.27‰ (2SD), respectively, based on the measurements on a series of international reference materials over the last two years. The measured δ¹¹B and δ⁷Li values for rock standards with a variety of matrices, including JR-2, JA-1, AGV-2, JB-2, BCR-2, BHVO-2, and IAEA-B5, are in excellent agreement with the published values, demonstrating the robustness of our new analytical protocol. Applying this improved technique, we are able to generate accurate and high-precision δ¹¹B and δ⁷Li data for B- and Li-depleted natural samples, such as meteorite materials and ultramafic rocks, which can provide critical constraints on early solar system geochemistry and mantle compositions.