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

Abstract

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, δ²³⁴U (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 (2σ) of these two gypsum RMs, ²³⁸U content = 1844 ± 39 ng g⁻¹, δ²³⁴U = 229.3 ± 3.9‰, ²³⁰Th/²³⁸U_activity = 0.5741 ± 0.0036, ²³⁰Th ages (corrected) = 66.97 ± 0.31 ka BP for PXCG-1, and ²³⁸U content = 1636 ± 50 ng g⁻¹, δ²³⁴U = 61.0 ± 2.4‰, ²³⁰Th/²³⁸U_activity = 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 δ²³⁴U_initial 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.