Precise and accurate Ga isotope analysis of solution standards and geological reference materials by MC-ICP-MS

Abstract

Gallium (Ga) isotopes are being increasingly applied as a new geochemical tracer, but the modest variations in Ga isotope ratios in most of the geological samples necessitate precise and accurate Ga isotope analysis. Additionally, a unified reference material for Ga isotope analysis has yet to be established. Herein, we report a novel procedure for complete separation of Ga from matrices in geological samples using AG1-X8 and AG50W-X8 exchange resins with yields of >99.5%. Isotope ratios were measured using a Neptune Plus multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) in low-resolution mode, employing internal standardization methods (C-SSBIN) for mass bias correction, and the results are reported as δ⁷¹Ga in units per mil against NIST SRM 3119a. Two pure Ga solutions (NIST SRM 994 and GBW(E) 080560) were used as secondary reference materials during Ga isotope analysis, with δ⁷¹Ga of −2.11 ± 0.05‰ (2SD, n = 106) and −1.00 ± 0.04‰ (2SD, n = 97) against NIST SRM 3119a, respectively. The Ga isotope compositions of a suite of geological reference materials (GSS-1, GSS-11, GSS-12, GXR-1, NIST SRM 2711a, GSD-12, NOD-P-1, BHVO-2, BCR-2, GSP-2, JG-1, JGb-2, GSR-1, GSR-6, AGV-2 and SGR-1b) were reported, with about half of them for the first time. The long-term external reproducibility is ±0.07‰ (2SD) based on replicated measurements of reference materials. Our results reveal that Ga isotopes are not significantly fractionated during igneous processes, although distinct variations are observed in gabbro and Mn nodules. Our analyses further confirm that NIST SRM 994 is isotopically heterogeneous with respect to Ga isotopes, and we recommend NIST SRM 3119a as the zero-point reference material for Ga isotope analysis.