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

We document the development of a suite of carbonate mineral reference materials for calibrating SIMS determinations of delta O-18 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 delta O-18 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 similar to 10 parts per thousand 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 mu m diameter spot size measurements, this yielded residual values <= 0.3-0.4 parts per thousand relative to CRM NBS 19 for most reference materials in the suite. Analytical precision was +/- 0.3 parts per thousand (2s, standard deviations) for 10-mu m spots and +/- 0.7 parts per thousand (2s) for 3-mu m 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 parts per thousand (2s) for 10-mu m spots and +/- 1 parts per thousand (2s) for 3-mu m spots.