SC ‐550: A Sintered Calcite Reference Material for In Situ U‐Pb Geochronology

The accuracy and precision of in situ U‐Pb geochronology of carbonates is limited by the intrinsic isotopic heterogeneity of natural reference materials (RMs). Recent efforts aimed at producing homogenous carbonate RMs through nano‐milling of natural samples or synthesis of amorphous calcium carbonate have shown promising results. Here we present a new, microcrystalline calcite material – SC‐550 (Sintered Calcite 550 °C) – with element mass fractions, Sr and U/Pb isotopic ratios and ablation behaviour comparable to that of a natural carbonate. A microbial limestone is first milled to nano‐sized particles and then sintered at high pressure and temperature using an internally heated pressure vessel (IHPV) apparatus. Sample heterogeneity is removed through milling, while textural coarsening is aimed at generating ablation behaviour like that of natural calcite materials. Ideal synthesis conditions were achieved at 500 MPa, 550 °C, and 24 h, where the material is texturally and compositionally homogeneous at the microscale (tens of μm). Inter‐ and intra‐experimental element mass fraction homogeneity is determined using EPMA (Ca mass fraction) and LA‐ICP‐MS analyses in two laboratories using static spot analyses and area mapping. The results for elements relevant for environmental and palaeoclimate applications show relative standard deviation (RSD) values in the range of 5–10%. The 87 Sr/ 86 Sr ratio of the starting nanoparticulate powder is determined by solution MC‐ICP‐MS as 0.708104 ± 0.000002 (2 s ) and the Sr isotopic micro‐homogeneity in experimental charges is confirmed by LA‐MC‐ICP‐MS. The U–Pb isotopic ratios of the nanoparticulate powder determined by ID‐TIMS are 207 Pb/ 206 Pb = 0.66226 ± 0.00012 (2 s ) and 238 U/ 206 Pb = 4.6244 ± 0.0057 (2 s ). Tests of U/Pb ratio homogeneity in experimental charges by LA‐ICP‐MS show < 0.6% variation (RSD) for 207 Pb/ 206 Pb and < 3% variation for 238 U/ 206 Pb. The suitability of the sintered calcite material as a primary RM for U‐Pb geochronology of carbonates is confirmed by testing ablation and laser induced fractionation behaviour compared with natural carbonates and reproducing published dates within uncertainty limit for a set of natural calcite materials.