Development Of Analytical Method For In Situ U-th Disequilibrium Dating Of Zircons By Secondary Ion Mass Spectrometry

With the improvement of conventional analysis and the rapid development and application of high precision dating techniques,Quaternary geochronology and its dating methods have been greatly improved.The K-Ar and 40Ar/39 Ar dating,14 C dating,U-Th disequilibrium dating,and Fission Track dating have become important and reliable tools to determine the timescales of earth evolution,paleoclimate change and environment change in Quaternary.Compared with other methods,the precision and resolution of 14 C and U-Th disequilibrium dating are much better.However,it's difficult for 14 C dating to measure samples that older than 50 ka.Instead,U-Th disequilibrium dating method can be used to date those samples with ages ranging from few years to 600 ka years.Therefore,U-Th disequilibrium dating method has been widely applied in the timescale research of corals,speleothems,bone and tooth fossils,and young volcanic rocks etc.Traditional U-Th dating method involves bulk dissolution and purification of U and Th,and then measurement by thermal ionisation mass spectrometry(TIMS)or multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS).But a problem could be arose by the bulk analysis that a mixed age or isotope ratio of different samples is yielded.This could be unsuitable for dating of some accessory minerals such as those zircons which have been experienced complex magmatic events.Relative to such analyses,in-situ micro-analysis requires only minor sample preparation while providing high spatially resolved distributions of the isotopes used for U-Th dating,and allows rapid measurement.Such advantages make it be employed widely in several fields of earth science researches.Compared with other in-situ U-Th measure techniques,secondary ion mass spectrometer(SIMS)has the highest spatial resolution.Some foreign laboratories operating large radius magnetic sector SIMS have developed in-situ U-Th disequilibrium dating method for Quaternary zircons,but there wasn't any publications about in-situ U-Th disequilibrium dating by SIMS in China up to now.In this study,detailed research has been carried out on U-Th disequilibrium dating method for quaternary zircons by a CAMECA IMS 1280-HR Secondary Ion Mass Spectrometer installed at the State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences.In order to achieve higher signal intensity,lower instrument background and avoid interference peak,we investigated the machine operating parameters and optimised machine settings for U-Th dating have been achived.In the process of secondary ion formation,U and Th have different secondary ion yields and require relative sensitivity calibration.On account of this,we have also studied the approaches to determine the U-Th relative sensitivity factor(RSF).Three secular equilibrium U-Pb dating standards like 91500,Qinghu and Plesovice were analyzed to check the reproducibility and accuracy of(230Th)/(238U).Quaternary zircon crystals separated from the Dayingshan and Maanshan volcanic rocks collected from Tengchong,Southwest China,were dated by the method we established.The following progresses have been made in this study:1.The reproducibility and accuracy of measured(230Th)/(238U)for standard zircons in this study stand at the level that was reported previously by international laboratories.The measured ratios of(230Th)/(238U)show reproducibility range from 0.90% to 2.30%(2?),and the accuracy is between 98.00% and 99.50%(2?).Both the reproducibility and accuracy are compared with that reported by previous researchers,thus reached the level of other international laboratories.2.The U-Th isochron ages of Dayingshan and Maanshan Quaternary volcanic rocks show an excellent agreement with previous data.Measured zircons separated from sample 13TC12 of Dayingshan yield an isochron age of 85.6±7.6ka(MSWD=2.1,n=18),While those zircons from samples 13TC06 E and 13TC06 H of Maanshan yield ages of 84.1±9.2ka(MSWD=2.9,n=19)and 81.4±4.2ka(MSWD=0.96,n=18)respectively.These results closely match the published data from Dayingshan and Maanshan(Dayingshan:87.5±6.5ka;Maanshan:83.5-9.0+9.8ka)in terms of both precision and accuracy,and indicate that the method is successfully established.