U-Th Isotopic Measurements by MC-ICP-MS with Multi-Static Method

This study presented a newly developed multi-static U-Th isotopic measurement method. For U isotopes, firstly, the ²³⁴U was measured on the SEM and simultaneously, the signals of ²³³U, ²³⁵U, ²³⁶U and ²³⁸U were measured in Faraday cups. Secondly, ²³⁶U was collected on the SEM, and the ²³³U, ²³⁴U, ²³⁵U and ²³⁸U were measured in Faraday cups. The ratio of ²³⁶U_SEM/²³³U_FC (the ratio of SEM to Faraday cup measurement corrected for mass fractionation) was used to monitor the SEM/Faraday cup relative yield. The ²³⁸U/²³⁵U ratio was used for instrumental mass fractionation correction by comparing the measured value with the natural value. For Th isotopes, the ²²⁹Th and ²³⁰Th were alternately measured on SEM and ²³²Th in a Faraday cup. Th isotopic measurements were carried out immediately after the U measurements from a same sample. The correction factors of SEM/Faraday cup relative yield and instrumental mass fractionation determined in U measurements were used for Th isotopic data corrections. The ideal ion beam intensities for measurements are 3-6 mV for ²³⁶U, 5-15 V for ²³⁸U (equivalent to 10¹¹ Ω amplifier), <10⁵ cps for ²²⁹Th and ²³⁰Th. The U and Th isotope data are usually acquired 200-400 cycles taking 20-30 min. To test the reliability of this method, the U isotopic standard Harwell uraninite (HU-1) was repeatedly measured and yielded a mean δ²³⁴U value of (-0.48±1.92)‰ (±2σ, n =55), which is in agreement with the data previously published by different laboratories. In this study, the multi-static method was applied to six in-house secondary carbonate standards, SB-530,SB-8600, SB-108K, SB-240K, SB-335K and SB-435K. The repeated measurements suggest that the six standards have U content of 500-3 000 ng/g and Th content of 0.1-1.0 ng/g. Their δ²³⁴U values range from 95 to 1030, and ²³⁰Th/²³⁸U ratios range from 0.01 to 1.33. Their error weighted mean ages are (0.53±0.01), (8.61±0.01), (107.6±0.2), (239.4±0.8), (332.8±1.9) and (437±5) ka, respectively. These results were confirmed by the comparative study using the SEM method carried out in the Institute of Geology and Geophysics, Chinese Academy of Sciences. This study demonstrates that the newly developed U-Th isotopic measurement method can routinely measure the ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U ratios at the precision of 1‰-3‰, but its reliability needs to be further validated by more international standards.