Determination of SIMS matrix effects on oxygen isotopic compositions in carbonates

Carbonates have variable chemical composition and crystal structure, which can affect the accuracy of in situ (SIMS) isotopic measurements. Carbonate reference materials (calcite, dolomite, ankerite, magnesite, siderite, and rhodochrosite) were characterised for delta O-18 by IRMS, and for the chemical composition by electron microprobe, and then used to evaluate the SIMS instrumental mass fractionation (IMF) of oxygen isotopes. A clear relationship between the IMF and the MgO (wt%) content was observed during two different sessions for low Fe- and Mn-carbonates. For Fe- and Mn-bearing carbonates (i.e. more than 1 wt%), Fe and Mn contents have to be taken into account for IMF correction. Due to the highly variable Mg content in carbonates, calibration using suitable carbonate reference materials with different Mg contents (i.e. calcite, dolomite, and magnesite) seems to be the best solution for performing accurate delta O-18 measurements by SIMS. This matrix correction is not universally applied. Therefore the overall significance of uncorrected published delta O-18 values from high Mg-carbonates has to be taken with caution. There are numerous applications of this method covering domains as varied as environmental studies (stromatolites, high-Mg calcite corals), geothermometry involving calcite co-existing with dolomite, or siderite co-existing with ankerite, and cosmochemical studies of carbonate phases in meteorites.