Accurate analysis of Li isotopes in tourmalines by LA-MC-ICP-MS under “wet” conditions with non-matrix-matched calibration

Abstract

The Li isotopic compositions of tourmalines can be used as a powerful geochemical tracer and provide valuable information regarding magma–volatile relationships and the magmatic-hydrothermal evolution of pegmatites. Laser ablation-multiple collector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) is the method of choice for in situ Li isotopic analysis. In this study, we investigated the feasibility of Li isotopic analysis in tourmalines by both ns- and fs-LA-MC-ICP-MS with (tourmalines as calibration standards) and without (NIST 610 glass as the calibration standard) matrix-matched calibration. The Li isotopes of tourmalines can be accurately analyzed with matrix-matched calibration by ns- or fs-LA-MC-ICP-MS. However, inaccurate δ⁷Li values were obtained when tourmalines were measured against a non-matrix-matched standard by ns- (∼10.0‰ deviation) and fs-LA-MC-ICP-MS (∼1.5‰ deviation). Wet plasma conditions can significantly reduce the matrix effect in isotopic analysis. Furthermore, the deviation in fs-LA-MC-ICP-MS can be suppressed by water addition after the ablation cell. However, a relatively large deviation (∼4‰) still existed, even though the matrix effect during ns-LA can be largely suppressed by water addition after the ablation cell (∼6‰ deviation was reduced). Fortunately, with the water addition before the ablation cell, an accurate and precise Li isotopic analysis by ns-LA-MC-ICP-MS can be achieved with non-matrix-matched calibration. This study indicated that there was a serious matrix effect in the Li isotopic analysis of tourmalines by ns- and fs-LA-MC-ICP-MS, and water-assisted ns- and fs-LA-MC-ICP-MS were an appealing option for the in situ Li isotope analysis of tourmaline with non-matrix-matched calibration.