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)(2)CO3 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)(2)SO4 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)(2)CO3 exchange reaction method and the diluted HCl digestion method over six months. The long-term average of delta U-234 and Th-230/U-238 activity ratio of FGD-1 gypsum was -12.6 +/- 3.0 parts per thousand 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 Th-230 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.