MC-ICP-MS Standard Sample-sample Crossover Test Method To Analyze The ²³⁰Th/U Age Of Stalagmite Samples

The ²³⁰Th/U dating method can be used to precisely date geological samples?e.g.,speleothems and corals?from-600 thousand years ago to present day and has been widely used in various disciplines of Earth sciences,such as paleoclimate,paleoceanography and archeology.Over the past decades,U/Th isotopic measurements have evolved from the usage of a spectrometry to thermal ionization mass spectrometry and Multi-Collector Inductively Coupled Plasma Mass spectrometry?MC-ICP-MS?.As a result,the analytical precision of U-Th isotopic ratios has been improved from several percent to a few permil,and even to sub-permil level.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.The U/Th separation and purification can be realized by a single UTEVA resin,which is more convenient and faster than the traditional procedures using anion exchange resins.The recovery rate of both U and Th can be up to 75-100%,which satisfy the high accuracy and precision MC-ICP-MS measurement.U/Th isotopes are measured by a standard-sample bracketing method using Harwell uraninite-1?HU-1?as the bracketing standard,that is,before and after the measurement of the unknown samples,the isotopes of the standard samples are measured respectively.The instrumental correction factors,such as mass discrimination and secondary electron multiplier?SEM?and Faraday cup gain,are estimated by the measurements of standard sampmes and then are applied to the unknown samples.The precision and accuracy of this method were tested by the repeated measurements of HU-1 as an unknown sample,the averaged ?234U is?-1.27±2.14?‰?±2?,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 8234U 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 stalagmite samples with known ages?N=47?to test the accuracy of our method.In the range of 0-300ka,the results of this study are in good agreement with the published data within ±2a analytical uncertainties.In the range of 300-400ka,the results of this study were slightly older than the published ages,this may be due to the differences in sampling locations and heterogeneity of stalagmite powder samples.It may also be the result of the uncertainty of the SEM and Faraday cup gain correction.The measured 234U/238U and 230Th/238U activity ratios have averaged uncertainties?±2??of 0.6‰ and 2.4‰,respectively.The uncertainties?±2a?of the ²³⁰Th/U correction age increases with the increase of the age?except for the Holocene samples?.The samples with ages of 0-10ka 10?100ka,100?200ka,200?300ka and 300?400ka have age uncertainties of 3?10%,2?3‰,3?6‰,4?8‰ and 8?15‰,respectively.In this study,six in-house U/Th dating reference samples within 0-500ka were prepared??50 g for each?.According to the age,the in-house secondary carbonate standards were named as SB-520,SB-8600,SB-108k,SB-240k,SB-335k and SB-435k.These in-house U/Th dating reference samples were analyzed using the standard-sample bracketing method.The repeated measurements suggest that the SB-520,SB-8600,SB-108k,SB-240k,SB-335k and SB-435k have averaged age of 0.524±0.017ka,8.611±0.026ka,107.928±0.472ka,239.965± 1.416ka,336.600±3.743ka and 434.757±7.101ka,respectively.These U/Th dating reference samples can be used to monitor our long-term reproducibility of U/Th dating,and also can be used for inter-laboratory comparative studies.However,this study shows that the reliability of standard-sample bracketing method is largely influenced by the stability of SEM/Faraday cup yield during the measurement?with RPQ-on?,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.Therefore,this study further attempted a multi-static measurement method to realize the internal correction of the SEM and Faraday cup gain.The 78 sets of trial measurements show that the multi-static method has good repeatability,but the reliability of the method will be further explored.