PXCG-1 and PXCG-2: two reference materials for gypsum U-Th dating

Gypsum is a widespread mineral in various sedimentary deposits. It precipitates from natural aqueous systems with trace amounts of U but almost no Th, rendering it a potentially valuable U-Th geochronometer. However, no gypsum RM for U-Th dating has been available. In this work, two natural gypsum reference materials (PXCG-1, PXCG-2) from the PiXiao cave, southwest China were prepared and characterized for U-Th dating. U-Th contents and isotope ratios were measured on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) using the isotope dilution technique. Data from three different laboratories show good agreement in 238U content, delta 234U (234U/238U relative to secular equilibrium value), 230Th/238U activity ratio, and 230Th ages (corrected) (corrected by assuming the initial 230Th/232Th ratio of the average crust value). The recommended values with the uncertainties (2 sigma) of these two gypsum RMs, 238U content = 1844 +/- 39 ng g-1, delta 234U = 229.3 +/- 3.9 parts per thousand, 230Th/238Uactivity = 0.5741 +/- 0.0036, 230Th ages (corrected) = 66.97 +/- 0.31 ka BP for PXCG-1, and 238U content = 1636 +/- 50 ng g-1, delta 234U = 61.0 +/- 2.4 parts per thousand, 230Th/238Uactivity = 0.9797 +/- 0.0034, 230Th ages (corrected) = 260.82 +/- 3.39 ka BP for PXCG-2, respectively. Information values of 232Th content, 230Th/232Th atomic ratio, 230Th age (uncorrected), and delta 234Uinitial are also provided. PXCG-1 and PXCG-2 are the first reported gypsum age RMs for U-Th dating, making them a useful complement to the widely distributed carbonate age reference materials for U-Th disequilibrium dating in Pleistocene-Holocene deposits. PXCG-1 and PXCG-2 are the first developed gypsum reference materials for U-Th dating.