Mineralogical determination of reef and periplatform carbonates: Calibration and implications for paleoceanography and radiochronology

Detection and precise quantification of the different carbonate minerals is crucial in selecting coral reef samples prior to geochemical and radiochronological measurements, thus helping to interpret changes in the mineralogical composition of geological formations made of reef material and associated sediments such as periplatform oozes. While powder X-ray diffraction (XRD) is a widely used method for mineralogical analysis, precise and accurate quantification of mineral abundance requires a thorough calibration of the instrument and method by means of gravimetric standards. In this study, we optimize a calibration method for the quantitative determination of four widespread Ca and Mg carbonates: calcite, aragonite, magnesian calcite (Mg-calcite) and dolomite. To detect and quantify very low calcite contents, which provide a crucial screening criterion for coral radiochronology, we perform a detailed survey of analytical precision and limits using two different XRD facilities. Detection and quantification limits (≈ 0.18% and ≈ 0.9% calcite in a calcite–aragonite mixture, respectively) are shown to be similar between the two instruments. The reproducibility of measurements clearly demonstrates that it is preferable to use peak area rather than peak height alone to obtain a precise quantification of the abundances of various carbonate minerals.