The (NH4)2CO3 exchange reaction and double-spike MC-ICP-MS method for gypsum U–Th dating†

This study presents a rapid and practical method for high-precision U–Th dating of gypsum using the (NH₄)₂CO₃ exchange reaction and double-spike method by MC-ICP-MS. The main challenges for gypsum U–Th dating are how to quickly and completely digest gypsum into solution and avoid the re-crystallization of gypsum particles during the digestion and chromatography process. Our developed protocol addresses these challenges by using the chemical reaction between calcium sulfate and carbonate, resulting in the (NH₄)₂SO₄ solution and the CaCO₃ precipitates. Then the calcium carbonate could be easily digested by diluted acid. Furthermore, the solid–liquid separation in this protocol also effectively separates Ca²⁺ and SO₄²⁻ ions, minimizing the re-crystallization of gypsum. In this study, the calcium sulfate–calcium carbonate reaction efficiency was rigorously examined. The precision and accuracy of this method were tested by the measurement of a pure gypsum reference material (FGD-1) using the (NH₄)₂CO₃ exchange reaction method and the diluted HCl digestion method over six months. The long-term average of δ²³⁴U and ²³⁰Th/²³⁸U activity ratio of FGD-1 gypsum was −12.6 ± 3.0‰ and 4.330 ± 0.083 (2SD, n = 49), respectively. The robustness of this method was further assessed by replicated analyses of six gypsum reference materials. The corrected ²³⁰Th ages (BP 1950AD) of GBW03110, GBW03111a, GYP-C, and TIG-1 gypsum standards yielded 229.1 ± 5.5 ka (2SD, n = 3), 324.5 ± 10.6 ka (2SD, n = 4), 235.1 ± 1.5 ka (2SD, n = 3), and 43.2 ± 0.6 ka (2SD, n = 5), respectively.