Study on Measurements of Lithium Isotopic Compositions for Common Standard Reference Materials Using Multi-Collector Inductively Coupled Plasma-Mass Spectrometry

Non-traditional lithium isotope is a frontier in isotope geochemistry and has been applied to the studies of Solid Earth Sciences, such as the lithosphere and fluids from the Earth's surface to the mantle. Lithium isotopic compositions of 7 standard reference materials BHVO-2, JB-2, BCR-2, AGV-2, NKT-1, L-SVEC and IRMM-016 are reported in this paper. The rock standards were digested with HNO₃-HF mixed acids. Cation exchange resin AG50W-X8 was used for the separation of Li from other elements. Li isotope ratios were determined by Multi-Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS), and isotopic mass fractionation was calibrated by standard-sample bracketing. The Li isotopic compositions and the analytical uncertainties (reported in 2SD) of analyzed standard reference materials were: ⁷Li_{BHVO-2-L-SVEC}=4.7‰±1.0‰ (n=53), ⁷Li_JB-2-L-SVEC=4.9‰±1.0‰ (n=20), ⁷Li_BCR-2-L-SVEC=4.4‰±0.8‰ (n=8), ⁷Li_AGV-2-L-SVEC=6.1‰±0.4‰ (n=14), ⁷Li_NKT-1-L-SVEC=9.8‰±0.2‰ (n=3), ⁷Li_{L-SVEC-L-SVEC}=-0.3‰±0.3‰ (n=10), ⁷Li_{IRMM-016-L-SVEC}=0.0‰±0.5‰ (n=10). The results were in good agreement with published data within analytical uncertainties. The accuracy of this method is about ±0.5‰ and the long-term external precision (2SD) of replicate analyses including the entire pretreatment procedure is less than ±1.0‰, and is similar to the state of the art method. The reported data of 7 standard reference materials provided a new standard framework for Li isotope research, and enabled the evaluation of the quality of data retrieved from geological samples. The influence of sample size was also assessed by loading different amounts of separated IRMM-016 solutions. The results show that when the lithium concentration has reached 100 μg/L, the measured lithium isotopic compositions are in coincidence with the accepted value within uncertainty, and the sample size does not affect the accuracy of measurements.