MC-ICP-MS Two-step Static Method To Measure U-Th Isotope And Its Application In Archaeological Dating Research

This study presents a newly developed multi-static U-Th isotopic measurement method and data-processing protocol.The U and Th isotopic data were both collected in two steps.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 in Faraday cups.Th isotopic measurements is carried out immediately after the U measurements from a same sample.For Th isotopes,the 229Th and 230Th are alternately measured on SEM and 232Th in a Faraday cup.The ratio of 236USEM/233UFC(the ratio of SEM to Faraday cup measurement corrected for mass fractionation)is used to monitor the SEM/Faraday cup relative yield.To test the reliability of this method,the U isotopic standard Harwell uraninite(HU-1)was repeatedly measured and yielded a mean ?234U value of-0.48±1.92‰(±2?a,n=56),which is in agreement with the data previously published by different laboratories.In order to test the reproducibility of U-Th dating results,six in-house secondary carbonate standards(SB-530,SB-8600,SB-108K,SB-240K,SB-335K and SB-435K)were analyzed six times using the newly developed U-Th isotopic measurement method.The repeate measurements suggest that the SB-530,SB-8600,SB-108K,SB-240K,SB-335K and SB-435K have U content of 500?3000 ng/g and Th content of 0.1?1.0 ng/g.Their ?234U 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 ka,8.61±0.01 ka,107.6±0.2 ka,239.4±0.8 ka,332.8±1.9 ka 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.In order to further validate our U/Th isotopic measurements and data processing protocol,an inter-laboratory comparison of 230Th/U dating of stalagmite was carried out between the Nanjing Normal University and National Taiwan University using the LS26 stalagmite,collected from the Luoshui Cave,Hubei province,China.Finally,U-series dating method with the newly developed protocals was used to date the carbonate samples collected from the palaeontological site of Jinyuan Cave,Liaoning province.