Secondary Ion Mass Spectrometry Bias on Isotope Ratios in Dolomite–Ankerite, Part I: δ18O Matrix Effects

Abstract

We document the development of a suite of carbonate mineral reference materials for calibrating SIMS determinations of δ18O in samples with compositions along the dolomite–ankerite solid solution series [CaMg(CO3)2–CaFe(CO3)2]. Under routine operating conditions for the analysis of carbonates for δ18O with a CAMECA IMS 1280 instrument (at WiscSIMS, University of Wisconsin-Madison), the magnitude of instrumental bias along the dolomite–ankerite series decreased exponentially by ~ 10‰ with increasing Fe content in the dolomite structure, but appeared insensitive to minor Mn substitution [< 2.6 mol% Mn/(Ca+Mg+Fe+Mn)]. The compositional dependence of bias (i.e., the sample matrix effect) was calibrated using the Hill equation, which relates bias to the Fe# of dolomite–ankerite [i.e., molar Fe/(Mg+Fe)] for thirteen reference materials (Fe# = 0.004–0.789); for calibrations employing either 10 or 3 μm diameter spot size measurements, this yielded residual values ≤ 0.3–0.4‰ relative to CRM NBS 19 for most reference materials in the suite. Analytical precision was ± 0.3‰ (2s, standard deviations) for 10-μm spots and ± 0.7‰ (2s) for 3-μm spots, based on the spot-to-spot repeatability of a drift monitor material that ‘bracketed’ each set of ten sample-spot analyses. Analytical uncertainty for individual sample analyses was approximated by a combination of precision and calibration residual values (propagated in quadrature), suggesting an uncertainty of ± 0.5‰ (2s) for 10-μm spots and ± 1‰ (2s) for 3-μm spots.