Preparation Of U-Pb Zircon Material And Its Application For U-Pb Determination By SIMS

Based on the high sensitivity and spatial resolution of secondary ion mass spectrometry?SIMS?,SIMS zirconium silicate uranium-lead dating has become one of the most important methods for mineral dating.However,due to the difference in ionization energy between uranium and lead itself and the difference in trace element composition and crystal structure,the secondary ionic strength produced by uranium and lead is inconsistent with their content in the sample which may leads to inaccurate U-Pb ages.In order to overcome the influence of the matrix effects,the Standard alignment method is used.In order to investigate the matrix effect,the secondary ion strength of SIMS uranium-lead isotope is discussed in terms of element composition and content by preparing different compositions of zirconium silicate materials.Two types of zirconium silicate uranium and lead materials were prepared by high temperature melting process and sol-gel process:borate zirconium silicate fused uranium lead material and sol-gel zirconium silicate uranium lead material.The optimum process conditions for the high temperature fusion process are:zirconium silicate:borate flux=3:7?m:m?,temperature:1050°C to 1100°C and melting time 30min.The results of LA-ICP-MS show that the homogeneity of U&Pb in RSD in glass flakes is better than 0.5%,and the homogeneity of U&Pb isotope ratio in RSD is better than 0.2%.In sol-gel process,at the temperature 1350°C,the product is domain in the zirconium silicate crystal.The micro-zone morphology and LA-ICP-MS test results show that when the temperature beyond 1200°C,the lead in the product will be significantly decrease by evaporation and also the morphology and element distribution homogeneity deteriorates.The effects of different doping elements on the synthesis of zirconium silicate were investigated.Rare earth elements?Y,La and Lu?have certain auxiliary effects on the synthesis of zirconium silicate,and also have a high stabilizing effect on lead.At 1350°C,lead recovery is approximately in the range of 90 to 95%,elemental homogeneity is better than 5%,and isotopic homogeneity is better than 0.6%.The effect of matrix effect on the secondary ion intensity of U-Pb is represented by?,expressed as ²⁰⁶Pb/²³⁸U=?⁽²⁰⁶Pb⁺/²³⁸U⁺.Based on the study of borate zirconium silicate fusion materials and sol-gel zirconium silicate materials?U:1000¹0000 g/g,Pb:250²500 g/g?,we found that the?value of the series materials is constant under the same matrix and testing conditions,and does not vary with the content of uranium and lead.It provides a theoretical basis for judging the accuracy of uranium lead age of high uranium zircons.Through the study of borate zirconium silicate fused uranium and lead materials and sol-gel zirconium silicate uranium lead materials,the results show that the?value is greatly influenced by the matrix composition and the matrix structure.In a certain range,the preparation temperature and content of doped zirconium silicate lead and lead are positively related to its alpha value,which provides a basis for further preparation of zirconium silicate material with a value of SIMS close to natural zircon.It also provides a more reasonable reference for improving the accuracy of zircon uranium and lead dating and the selection of zircon standard samples.In practical application,the?Liansanguan?uraninite grains and natural uranium product?uranium octaoxide?as the ²³⁸U source and ²⁰⁶Pb source for zirconium silicate synthesis.A series of yttrium-doped zirconium silicate materials of different²⁰⁶Pb/²³⁸U ratio were obtained by sol-gel process.The results of LA-MC-ICP-MS showed that the distribution of uranium and lead in the material was relatively uniform.The homogeneity of U and Pb was checked by the RSD of ²⁰⁸Pb/²⁰⁶Pb,²⁰⁷Pb/²⁰⁶Pb,which were better than 0.6%,and the RSD of Pb/U in content was better than 5.0%.A coefficient C=?^?n?/?^?syn?was obtained by test the standard zircon sample?Plesovice?and the artificial zirconium silicate material under the same SIMS condition.And through this relationship,the value of ²⁰⁶Pb/²³⁸U in content in unknown zircon can be calculated by the coefficient C.The test results of Mud Tank Zircon show that it is theoretically feasible to use zirconium silicate as reference material for SIMS²⁰⁶Pb/²³⁸U determination.