Lithium Isotopic Analysis of Spodumene by Secondary Ion Mass Spectrometry

RATIONALE: Lithium (Li) stable isotopes can provide insight into fluid-rock interaction and magmatic differentiation. Therefore, it is essential to develop an accurate and precise method to determine the Li isotope signature in spodumene. Five spodumene reference materials were developed for in situ measurement of Li isotopic ratios by secondary ion mass spectrometry (SIMS). METHODS: In situ Li isotopic measurements were performed on eight spodumene samples (UM1-8) with differing chemical compositions using a CAMECA IMS 7f SIMS instrument. Analyses were performed using different parameters to test the effects of ion velocity on precision, matrix, and orientation effects. Sessions using a sample voltage offset of -40V and detection in an electron multiplier were compared to those using a sample voltage offset of 0V and detection in a Faraday cup. RESULTS: The two analytical protocols produced similar precision (~0.60‰; 2SD) but markedly different results with respect to matrix effects. Analysis using a -40V offset displayed matrix effects that strongly correlate to the Fe content in the spodumene, whereas analysis using a 0V sample offset shows no matrix effects. Crystallographic orientation effects were not observed in this study under either protocol. CONCLUSIONS: Using SIMS, a precision of ±0.60‰ (2SD) can be achieved when determining the Li isotopic composition of spodumene. This precision is limited by in situ homogeneity of available reference material, rather than instrumental setup. Matrix effects can be avoided by using a 0V sample offset, and crystallographic orientation is not a factor.