Measurement of U/Th Isotopic Compositions in Stalagmites for ²³⁰Th/U Geochronology Using MC-ICP-MS by Standard-Sample Bracketing Method

Over the past two decades, multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has gradually become the most commonly used technique for high-precision U/Th dating studies in the world. Based on the previously established methods, this study presents a simplified chemical procedure for U/Th separation and purification using the UTEVA resin columns and a standard-sample bracketing method for U/Th isotope measurements by MC-ICP-MS. In routing measurements, the daily variations of instrumental mass discrimination and SEM/Faraday cup gain are less than 0.1 and 1%, respectively. By the use of rigorous arid washout, potential contribution of instrumental background to U/Th isotopes measurements is less than 0.2‰, or even negligible. The precision and accuracy of this method were tested by the repeated measurements of Harwell uraninite (HU-1) as an unknown sample, the averaged δ²³⁴U in HU-1 is (-1.27±2.14)‰ (±2σ，n=29), which is in agreement with the values that reported previously by different international laboratories with various analytical techniques. The repeatability of this method was assessed by a large number of replicate analyses using the in-house U/Th standard MFT, which has δ²³⁴U of (408.2±3.2)‰, ²³⁰Th/²³⁸U activity ratio of 1.002±0.003 and ²³⁰Th/U age of (124.3±0.5) ka, respectively. We also analyzed 47 stalagmite samples with known ages to test the accuracy of our method. For these stalagmite samples, the measured 234U/²³⁸U and ²³⁰Th/²³⁸U activity ratios and the age have averaged uncertainties (±2σ) of 0.6‰, 2.4‰ and 0.5%, respectively, and they are consistent well with the previously published data. In addition, this study shows that the reliability of standard-sample bracketing method is largely influenced by the stability of SEM/Faraday cup yield, which in turn is sensitive to the stability of room temperature and humidity and is sensitive to the presence of organics or any other matrix materials in the U/Th fractions.