A silica-related matrix effect on NanoSIMS Li isotopic analysis of glasses and its online calibration

Abstract

Accurate Li isotope analysis of silicate glasses using secondary ion mass spectrometry (SIMS) is challenging due to the matrix effect, which causes instrumental mass fractionation (IMF). The characteristics of the matrix effect vary for each element, necessitating empirical correction methods. Here, we present Li isotopic measurements on a set of silicate glasses spanning a broad compositional range from ultramafic to highly siliceous using a Cameca NanoSIMS 50L. A radio-frequency (RF)-O primary beam with an intensity of 1 nA was rastered over a 5 × 5 μm² area on the sample, and the secondary ions ⁶Li, ⁷Li and ³⁰Si were simultaneously detected. We observed a significant matrix effect on δ⁷Li in the silicate glasses, which yielded an IMF of up to 19‰, exceeding the analytical precision of <2.5‰ (1SD). The IMF is strongly correlated with the chemical compositions of the silicate glasses. To correct the matrix effect, we employed various correction schemes, including univariate and multivariable compositions. All results consistently indicated that silica exerted a dominant influence on the IMF. We then propose a straightforward “online” method to correct this silica-related matrix effect by utilizing simultaneously measured ³⁰Si signals. This strategy improves the accuracy of the Li isotope measurements to within 3‰ and can be widely applied to anhydrous subalkaline magmatic glasses with compositions ranging from ultramafic to highly siliceous.