Accurate lithium isotopic analysis of twenty geological reference materials by multi-collector inductively coupled plasma mass spectrometry

The lithium (Li) isotopes can be used to track the evolution of continental silicate weathering, which has an important role in maintaining Earth's long-term climate stability. At present, complex separation procedures and analytical mass bias restrict the accurate determination of Li isotopic compositions in various geological samples with different Li concentrations and matrix/Li ratios. Here, an optimization of dual-column chromatography achieves high Li recovery, low procedural blank, and low Na/Li ratio (< 1) after purification, allowing the acquisition of high precision Li isotope ratios by a Nu Plasma 3 multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). In order to evaluate the accuracy of new method, the 87Li values of BHVO-2, BCR-2, AGV-2, JG-2, RGM-2, SGR-1b, GSR-6, GSR-12, and seawater, are repeatedly analyzed more than 3 times each and are consistent with the published data within the uncertainty. The long-term external reproducibility of 87Li based on L-SVEC and IRMM-016 pure solutions is about +/- 0.2%0 (2SD), and is suitable for analysis of geological samples. These results suggest that new dual-column method is reproducible and reliable. We also report ten new reference materials including two USGS rocks (SDC-1: 1.8 +/- 0.4%0; W-2a: 2.9 +/- 0.3%0), four soils (GSS-3: 1.3 +/- 0.2%0; GSS-4: -5.1 +/- 0.4%0; GSS-6: -1.9 +/- 0.3%0; GSS-8: 0.4 +/- 0.3%0), and four stream sediments (GSD-6: -1.0 +/- 0.4%0; GSD-11: 1.1 +/- 0.3%0; GSD-12: 1.8 +/- 0.2%0; GSD-14: 1.6 +/- 0.4%0), all previously analyzed for K isotopes. There is significantly fractionation of Li and K isotopes during silicate weathering. Compared to the K isotopes, Li isotopes have more promising as an isotope tool for weathering due to its larger fractionation and its abiotic property.

Automated summary

Lithium isotopes are useful for tracking the evolution of continental silicate weathering and maintaining Earth's long-term climate stability. A new optimization of dual-column chromatography allows for high precision determination of lithium isotope ratios in various geological samples. The method has been shown to be reproducible and reliable.