Development of in situ Li isotope analysis using laser ablation quadrupole inductively coupled plasma mass spectrometry.

ABSTRACT

RATIONALE Lithium isotope geochemistry is an important tool in the studies of Earth and planetary materials. In situ Li isotope analyses are typically performed using secondary ion mass spectrometry (SIMS) or laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS), but these instruments are not widely accessible. Here, the capability of laser ablation quadrupole ICPMS for conducting Li isotopic analyses is evaluated. METHODS: An array of MPI-DING and USGS silicate glass reference materials was analyzed repeatedly over the course of 6 months. These materials range from komatiite to rhyolite in terms of silica content (45.5-75.6 wt%) with 9-45 ppm Li. Their Li isotope compositions have been previously characterized so that matrix effects could be tested with these reference materials. Analyses were conducted using an NWR193 laser ablation system coupled to an Agilent 7900 ICPMS system. RESULTS: Analytical precision is primarily limited by Li concentration in the samples. For samples with ~9 ppm Li, the internal precision is 6‰ (2 SD, 150 µm spot diameter), whereas that for a sample with ~45 ppm Li is 4‰ (2 SD, 120 µm spot diameter). The technique is somewhat sensitive to sample matrix samples with SiO2 content that deviates from the bracketing standard display fractionated δ7 Li, necessitating correction using a session-specific matrix correction curve. CONCLUSION: Lithium isotope analysis by ns-LA-QICPMS is worthwhile for samples with high Li concentrations and when a matrix-matched standard can be obtained. Although the precision of this method is not as high as those achievable with SIMS and LA-MC-ICPMS, it remains adequate for resolving large isotope fractionations found in natural and laboratory settings.