Six Ca-Mg-Fe Carbonate Reference Materials for In-Situ Analysis of Oxygen and Carbon Isotope Ratios

Six Ca-Mg-Fe carbonate reference materials (RMs) were characterized for oxygen and carbon isotope ratios analyses by Secondary Ion Mass Spectrometry (SIMS). This new suite includes four dolomites with Fe# ranging from 0.0004 to 0.3429, one magnesite with Fe# = 0.0099 and one siderite with Fe# = 0.6152. The six samples were split into between 128 to 2048 metrologically identical aliquots, each containing roughly 100 mg of material. We performed a detailed characterization on split materials for their major and minor chemical element composition using EPMA and their mineralogical composition using XRD. The oxygen and carbon isotope reference values were also determined in multiple gas-source isotope ratio mass spectrometry laboratories while the homogeneity of the oxygen and carbon isotope ratios at the sub-nanogram test portion mass were assessed using SIMS. The repeatability of our SIMS measurements varied from ± 0.15 to ± 0.31‰ (1s) for δ¹⁸O and from ± 0.29 to ± 0.71‰ (1s) for δ¹³C. Significant compositional SIMS matrix effects were observed across the dolomite-ankerite and magnesite-siderite solid-solution series in the investigated compositional range. This confirms the need of multiple matrix-matched RMs spanning a wide chemical range in order to get accurate SIMS isotope ratios in minerals that form theses solid-solutions.