U-Th dating of gypsum: methodology and reference materials

Gypsum is a common evaporate mineral in a wide variety of geological settings, especially in arid and semi-arid areas. It often precipitated from the natural brine systems with trace amounts of U and almost no Th, rendering it a potentially valuable U-Th geochronometer. However, U-Th dating of gypsum is often challenging, such as how to quickly and completely digest gypsum into solution, and avoid the re-crystallization of gypsum particles during the digestion and chromatography process. Here we present 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-ICPMS. Our developed protocol addresses these conventional challenges by using the chemical reaction between calcium sulfate and carbonate, resulting in the (NH4)2SO4 solution and CaCO3 precipitate. Then the CaCO3 could be easily digested by diluted acid. With the solid-liquid separation, Ca2+ and SO42- ions are also effectively separated, minimizing the recrystallization of gypsum. The (NH4)2CO3 median, the gypsum/carbonate molar ratio of 1: 4, and the exchange reaction duration of 1.5 ~ 2 hours are suggested in this protocol. Since there is no gypsum U-Th dating reference materials to validate the accuracy of different approaches and ensure methods are repeatable across laboratories. We also prepared and characterized two natural gypsum U-Th dating reference materials (PXCG-1, PXCG-2) from PiXiao Cave, southwest China. Data from three different laboratories exhibit good agreement with both 238U content, δ234U, 230Th/238U activity ratio, and the 230Th ages. The 230Th ages of PXCG-1, PXCG-2 RMs proposed are 66.97 ± 0.31 ka, 260.82 ± 3.39 ka, respectively.