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

Abstract

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, 1s) for δ¹⁸O and from ± 0.29 to ± 0.71‰ (1s) for δ¹³C. These six carbonate samples span a wide range in oxygen (δ¹⁸O_VSMOW from 12.98 ± 0.09‰ to 26.92 ± 0.11‰, 1 standard error of the mean, 1SE) and carbon (δ¹³C_VPDB from -11.74 ± 0.20‰ to 2.43 ± 0.06‰, 1SE) isotope ratios. Significant SIMS compositional matrix effects of +6.7‰ (for δ¹⁸O) and -3.2‰ (for δ¹³C) 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 δ¹⁸O and δ¹³C, 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.