| The method of zircon radiation damage age dating,one of the methods of low-temperature chronology,has a closure temperature of 230±25°C that has been suggested by previous researchers.Though this method has some applications in geological study,the obtained worse age precisions?the relative standard deviation RSD=10?20%?limited its wide application and popularity.Because the two instruments,Laser Raman Spectrometer?LRM?and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry?LA-ICPMS?,were used in zircon radiation damage age dating,the effect factors of the deviations from instruments and procedures as well as uncertainties from the principle of this dating method on the age precision are examined systematically in this study.The results suggested that:the RSD for LRM to determine the FWHM?full width at half-maximum?of?3?Si O4?vibrational peak(B1gmode)of zircon is about 1.12%.The RSDs for determining the contents of U and Th by using LA-ICPMS depend on their contents.Usually they are less than 3%for normal natural zircon.Another factor affecting the total deviation is the strategy of measuring FWHM??3?by using LRM within the spot where the U and Th contents were determined by using LA-ICPMS.The average FWHM??3?value of 5 positions relative to that of 22 positions measurements by LRM in different locations within 32?m spot,the relative deviation will be less than 4%.The trace element contents in natural zircon may be the major effect factor on the precision as they can increase the FWHM??3?value obviously.When Sri Lanka zircon?Radiation damage age is?375 Ma?was taken as an example,if FWHM??3?increases about 0.5 cm-1caused by trace elements in the zircon,which can lead to its calculated radiation damage age increase of?12.5%?to?422 Ma?.A formula correlating FWHM??3?with element contents:FWHM(cm-1)=0.0006P?ppm?+0.00026?REE+Y??ppm?+0.00036Ti?ppm?+3.28,was linear fitted from the data of synthetic zircon?as Hf<10 wt%,Th<1000 ppm and U<500 ppm?and completely annealed natural zircon.The FWHM??3?calculated from this formula can be taken as a correction term?F0?that can modify the formula of FWHM??3?and radiation damage alpha dosesuggested by previous researchers.The modified formula can be expressed as:FWHM=A[1–exp?–B×D??]+F0.This formula can be used to improve the precision of zircon radiation damage age by deducting the FWHM??3?value caused by trace elements in zircon.In this study,the more information had been obtained from the Sri Lanka zircon?low radiation damage degree?dryly annealed at?1300°C for?96 hours?accumulation time?,the U-Pb age dating of the zircon using Secondary Ion Mass Spectrometry?SIMS?suggested that the zircon had no obvious Pb-loss and the homogeneity was improved,this annealing condition provide the possibility for homogenizing standard zircon for in-situ U-Pb age dating. |
PDF Full Text Request