High-precision magnesium isotope analysis of carbonates by laser ablation MC-ICP-MS using wet and dry conditions

Abstract

In situ magnesium isotope compositions of carbonates play an important role in tracing geological and biological processes. Matrix effects of carbonates with distinct chemical and physical properties are the main factors affecting the accurate and precise determination of Mg isotopes by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). This study investigates Mg isotope microanalysis of magnesite, dolomite and siderite under wet and dry conditions. The results demonstrated that the main isobaric interference of ⁴⁸Ca²⁺ on ²⁴Mg⁺ in carbonates with high Ca/Mg ratios (up to 6.79) can be significantly suppressed and downhole isotopic fractionation can be reduced with the addition of water vapor in front of the ablation cell (e.g., under wet conditions). MGS-3, DOL-8, DOL-9 and SD-5 show homogeneous in situ Mg isotope compositions for the investigated pieces (>1 cm²) with intermediate precision better than 0.14‰ and 0.15‰ for δ²⁶Mg and δ²⁵Mg, and are adopted as in-house standards. The obtained in situ δ²⁶Mg and δ²⁵Mg values of matrix-matched samples were consistent with those determined by solution nebulization (SN)-MC-ICP-MS with uncertainties of 0.16–0.27‰ and 0.12–0.25‰ (2SD) under dry conditions and 0.15–0.18‰ and 0.06–0.10‰ (2SD) under wet conditions. This further indicates that wet-laser analyses improved the analytical precision of Mg isotope determinations by 1.1 to 2.5 times. Inaccurate Mg isotope data were obtained under dry conditions when carbonate samples were measured against a physical property and/or chemical composition of a different standard, and such deviations can be largely improved in wet-laser analyses.