Dolomite reference material synthesized by pressureless sintering for laser ablation MC-ICP-MS carbon and magnesium isotope analysis

Abstract

Microanalysis of carbon and magnesium isotopes in dolomite is gaining increasing attention as a valuable tool in various fields of earth sciences. However, the matrix effect poses a major challenge for determining C and Mg isotopes using laser ablation multi-collector ICP-MS, particularly due to the wide range of chemical compositions found in natural dolomite. A preferred solution is to use a matrix-matched reference material with homogeneous isotope compositions at the micron scale. In this contribution, we established a new protocol for preparing a dolomite reference material for isotopic microanalysis through pressureless sintering. The effects of particle size and sintering temperature on the isotopic homogeneity of sintered dolomite powder pellets (SDPPs) were carefully evaluated. Our results indicate that a smaller d₉₀ particle size improves isotope homogeneity, and SDPPs with a d₉₀ of 3.17 μm show the best laser ablation repeatabilities of δ²⁶Mg and δ²⁵Mg (0.11‰ and 0.09‰) that are 2–4 times better compared to the repeatability of those with a d₉₀ of 10.37 μm. Higher sintering temperature enhances particle cohesion and pellet hardness, which are crucial for laser ablation performance; Vickers hardness (HV) is improved to 41.58 kgf mm⁻² when SDPPs are heated up to 400 °C below decomposition temperature. These SDPPs show the best δ¹³C, δ²⁶Mg and δ²⁵Mg homogeneity with a twice standard deviation value of 0.37‰, 0.11‰ and 0.06‰ (n = 15), respectively. Of note, in situ carbon and magnesium isotope characterization of SDPPs using LA-MC-ICP-MS shows negligible isotope fractionation against natural dolomites. This further confirms the reliability and robustness of the proposed protocol for dolomite isotopic reference preparation via pressureless sintering.