Incorporation of U, Pb and Rare Earth Elements in Calcite through Crystallisation from Amorphous Calcium Carbonate: Simple Preparation of Reference Materials for Microanalysis

Uncertainty for elemental and isotopic measurements in calcite by LA-ICP-MS is largely controlled by the homogeneity of the reference materials (RMs) used for calibration and validation. In order to produce calcite RMs with homogeneous elemental and isotopic compositions, we incorporated elements including U, Pb and rare earth elements into calcite through heat- and pressure-induced crystallisation from amorphous calcium carbonate that was precipitated from element-doped reagent solution. X-ray absorption spectra showed that U was present as U(VI) in the synthesised calcite, probably with a different local structure from that of aqueous uranyl ions. The uptake rate of U by our calcite was higher in comparison with synthetic calcite of previous studies. Variations of element mass fractions in the calcite were better than 12% 2RSD, mostly within 7%. The Pb-207/Pb-206 ratio in the calcite showed < 1% variations, while the U-238/Pb-206 ratio showed 3-24% variations depending on element mass fractions. Using the synthetic calcite as primary RMs, we could date a natural calcite RM, WC-1, with analytical uncertainty as low as < 3%. The method presented can be useful to produce calcite with controlled and homogeneous element mass fractions and is a promising alternative to natural calcite RMs for U-Pb geochronology.

Automated summary

The uncertainty in elemental and isotopic measurements in calcite by LA-ICP-MS is largely controlled by the homogeneity of the reference materials (RMs) used for calibration and validation. By incorporating elements like U, Pb, and rare earth elements into calcite through heat- and pressure-induced crystallization, synthetic calcite with controlled and homogeneous element mass fractions can be produced, providing a promising alternative to natural calcite RMs for U-Pb geochronology.