MATRIX EFFECT FOR LI ISOTOPE ANALYSES USING NS-LA-Q-ICP-MS

Lithium isotopes are a powerful tracer for many geological processes, such as continental weathering and fluid-mineral interaction. Lithium is emerging as a promising candidate for diffusion geospeedometry, especially in short duration events given its fast diffusion rates in minerals. The large relative mass difference between ⁶Li and ⁷Li causes large isotope fractionation during chemical diffusion, up to 30‰ in nature and 100‰ in diffusion experiments. Typically, in situ Li isotope analyses are performed using SIMS or laser ablation MC-ICPMS. The capability to perform in situ Li isotope analyses using more widely available instruments would be beneficial to the further development and application of Li isotope geospeedometry. Lithium isotope compositions of geological materials have been successfully measured using solution nebulization Q-ICPMS with precision of 2‰ (Liu and Li, 2019). Our goal is to evaluate the capability of laser ablation Q-ICPMS for in-situ Li isotopic analyses, which would make Li isotope geochemistry more accessible to the wider community.

To characterize the matrix effect, we analyzed an array of USGS and MPI-DING silicate glass reference materials with a NWR193 laser ablation system coupled to an Agilent 7900 Q-ICPMS. The measured glass standards range from komatiite to rhyolite in terms of silica content (45.5-75.6 wt%), 8.7-43.8 ppm in terms of Li concentration, and δ⁷Li_L-SVEC of 2.1-31.1‰. Our results show that Li isotopes are strongly fractionated by ns-LA, and the fractionation is dominantly controlled by the silica content. We found that samples containing higher silica content display artificially lighter Li isotope compositions. A potential source of this matrix effect is the kinetic fractionation associated with the preferential mobilization of the light isotope during ablation. This instrumental fractionation can be successfully corrected using a calibration curve defined by matrix-matched standards.