Keywords:Amorphous calcium carbonate, LA-ICP-MS, Reference material, U–Pb geochronology
Calcium carbonates are ubiquitously present throughout the Earth history as animal shells, speleothems, fault-related vein fillings, and hydrothermal or cold-seep precipitates. Direct dating of carbonates provides valuable information on paleoenvironmental change, tectonics, and fluid and material cycling. Uranium–lead (U–Pb) dating using high spatial-resolution laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a key technique to date natural carbonates such as fault-hosted calcite that cannot be dated by biostratigraphy or Sr isotope stratigraphy (Roberts and Walker, 2016; Beaudoin et al., 2018). In situ U–Pb dating by LA-ICP-MS needs matrix-matched reference materials to correct matrix-dependent elemental fractionation in LA-ICP-MS. Roberts et al. (2017) demonstrated that a natural calcite cement WC-1 (254.4 ± 6.4 Ma) is suitable as a calcite reference material. However, the WC-1 calcite has an inhomogeneous distribution of U and Pb and lacks concordance in the U–Pb system (Roberts et al., 2017). In this study, we synthesized novel calcite reference materials with homogeneous U and Pb concentrations and isotope ratios. Incompatible elements such as Sr can be incorporated into calcite through pressure- or heat-induced crystallization from amorphous calcium carbonate (ACC) precipitated from element-doped reagent solution (Matsunuma et al., 2014; Saito et al., 2017). We incorporated U and Pb into calcite through heat-induced crystallization from U, Pb-doped ACC. The homogeneity of the U/Ca and Pb/Ca ratios in the synthetic calcite examined with LA-quadrupole (Q)-ICP-MS was generally better than 8% and 13%, respectively, in relative standard deviation (2SD, n = 10). The 207Pb/206Pb ratio of the synthetic calcite measured with LA-multiple-collector (MC)-ICP-MS was homogeneous within ~1% errors, whereas the 238U/206Pb ratio was less homogeneous (3%–11% errors). The inhomogeneity in the U/Pb isotope ratio would be propagated into the analytical precision for unknown samples. In order to test the usability of the synthetic calcite, we dated WC-1 using the synthetic calcite for correction of elemental fractionation. We calculated the nominal 238U/206Pb ratio in the synthetic calcite from its U and Pb concentrations measured by LA-Q-ICP-MS using Ca as an internal standard. We then obtained the fractionation factor to correct bias between the isotope ratios measured by LA-MC-ICP-MS and the nominal value. Using the fractionation factor, we could accurately date WC-1 with an ~3% precision (246.6 ± 7.3 Ma). If the isotopic compositions of the synthetic calcite are certified by isotope-dilution technique, we could date natural carbonates with <10% precisions using the synthetic reference calcite.