Six Ca‐Mg‐Fe Carbonate Reference Materials for <i>In Situ</i> Measurement of Oxygen and Carbon Isotope Ratios

Six potential Ca‐Mg‐Fe carbonate reference materials were assessed for oxygen and carbon isotope ratios homogeneity by secondary ion mass spectrometry (SIMS). This new suite includes three dolomites (Tucka‐22, M9548.2, Eugui) and one ankerite (WPA‐177‐2) with Fe# ranging from 0.0004 to 0.3429, one magnesite (M21350.2) with Fe# = 0.0099 and one siderite (Vizille‐1) with Fe# = 0.6152. The six materials were split into 128 to 2048 metrologically identical aliquots, each containing roughly 100 mg of sample. In this study, we report a detailed characterisation of the split materials for their major and minor chemical element compositions using electron probe microanalysis (EPMA), mineralogical compositions using X‐ray diffraction (XRD), oxygen and carbon isotope values determined in three independent gas‐source isotope ratio mass spectrometry (GS‐IRMS) laboratories. The repeatability of our SIMS measurements varied from ± 0.15 to ± 0.31‰ (1 standard deviation, 1 s ) for δ 18 O and from ± 0.29 to ± 0.71‰ (1 s ) for δ 13 C. These six carbonate samples span a wide range in oxygen (δ 18 O VSMOW from 12.98 ± 0.09‰ to 26.92 ± 0.11‰, 1 standard error of the mean, 1SE) and carbon (δ 13 C VPDB from ‐11.74 ± 0.20‰ to 2.43 ± 0.06‰, 1SE) isotope ratios. Significant SIMS compositional matrix effects of +6.7‰ (for δ 18 O) and ‐3.2‰ (for δ 13 C) were observed along the dolomite‐ankerite solid‐solution series in the compositional range Fe# = 0.0004 to 0.3429. Similarly, large compositional matrix effects of +4.2‰ and ‐9.5‰ were observed for δ 18 O and δ 13 C, respectively, along the magnesite‐siderite solid‐solution series. This confirms the need for multiple matrix‐matched reference materials spanning across a wide range in chemical compositions in order to get accurate SIMS isotope ratios in carbonate minerals that form solid‐solutions. The Ca‐Mg‐Fe carbonate reference materials described in this study were produced in sufficient amounts to be available to the global geochemical community for years to come.