Uranium And Uranium Mineral Development And Research Of Electron Microprobe Dating Dating Techniques Incl

Based on the theory of radioactive decay, and the hypothesis that the initial lead islittle enough to be neglected in the measured mineral and the system is closed, electronprobe micro-analysis (simplized as EPMA) dating method is applied to calculate the ageof the uranium minerals or U-bearing minerals with the contents of U, Th and total Pbwhich are measured by electron probe micro-analyzer. This method was first proposedby Professor Suzuki K. who works in Nagoya University, Japan, and is mainly used inthe micro in-situ dating of monazite and zircon, and rarely in that of uranium mineralsand other U-bearing minerals.In this dissertation, the closure of the U-Th-Pb system is analyzed by the chemicalcomposition and the crystal structure stability of uranium and U-bearing minerals, thebest analyzing condition is optimized by thorough and careful design, the analyticalprocess is constructed from sample preparation to sample analysis, the uncertainties ofthe results are comprehensively evaluated, and finally, the ages of uranium minerals andother U-bearing minerals are calculated by optimizing and combining the agecalculation method. The discriminant criterion is built to distinguish the closure and thecontent of initial lead of uranium mineral system. Meanwhile, two uranium mineral agestandards are recommended for EPMA dating and the age range of EPMA dating foruranium minerals are studied. The main contents are shown as follows:(1) Research on the closure of U-Th-Pb system of uranium and U-bearing mineralsThe ability of keeping radioactive lead and the initial lead content of uraninite andpitchblende are deeply analyzed and investigated, based on the stable occurrence ofuranium, isomorphism displacement, chemical composition and crystal structure ofuraninite and pitchblende. Isomorphism displacements are common in these two kindsof uranium minerals with complex compositions. Lead in this two kind of uraniumminerals loses relatively more easily than that in monazite, especially for the old ageminerals, but much better than other uranium minerals. The initial lead is low enough inuraninite which is more suitable for EPMA dating research than pitchblende. Thorianite,having similar crystal structure as that of uraninite, can also be applied for the study ofEPMA dating. Thorite is usually metamict and easy to lose lead. Therefore, it can not beused for the EPMA dating in most cases.(2) Optimization of measurement conditions ①Operating conditions：20kV accelerating voltage,50nA probe current and1~5μm probe diameter.②Measurement parameters: Mα lines and PETH crystal are used in the U, Th andPb mearsurements. Spectral interferences of Th Mβ on U Mα，Th Mξ on Pb Mα, and YLγ on Pb Mα are corrected on line by the measured correction factors which are0.01251,0.00063and0.00422respectively. The low and high background positions are±4mm away from peak of U Mα and Th Mα, and that are-4.5mm and+5mm away fromthe peak of Pb Mα. The acquisition time is no more than100S for peak and50S forbackground measurement. The measurement modes are “T” for U and Th, and “F” forPb.③Standards: The comparison standards were U metal for U, ThO2for Th and PbSfor Pb.(3) Construction for the measuring procedure①Making the sample as thin section or sanding sheet.②Coating carbon with30nm in thickness on samples and standards at the sametime.③Qualitative analysis by EDS to identify the existing element.④Setting measurement condition.⑤Selecting ZAF correction method, and taking U metal, ThO2and PbS ascalibrating standard for U, Th and Pb.⑥Point analysis.10~20minutes for one point analysis.(4) Uncertainty evaluations for the measurement resultsAccording to the deep study of the measurement process, the uncertainties of themeasurement results of U, Th and Pb are evaluated. Four standard uncertaintycomponents are confirmed and quantizated, of which are the count intensities of sampleand standard, the composition of standards and ZAF correction, and then combined asthe combined standard uncertainty. Finally, the uncertainty of EPMA age is calculatedby error propogation function according to the combined standard uncertainties of U, Th,and Pb.(5) Optimization of age calculation method and reliability evaluation of age resultAll kinds of age calculation methods and softwares are introduced systematically,whose merits and demerits are compared and usability is discussed for the agecalculation of uranium mineral. The suitable age calculation method and software are finally optimized. The steps for age calculation are as follows:①Using Chemage software to calculate the apparent ages for each analysis point.②Distinguishing different age components by Isoplot software and taking themas the final results if there are two or more age components.③Using Isoplot software to calculate the weighted average age when only oneage component exists.The compositions of the pitchblende age reference material of uranium and leadisotopes locating in No.201uranium deposit are analyzed and the weighted average agefor42point is67.8±0.9Ma (95%confidence probability, MSWD=0.76) calculated byIsoplot according to the above measuring procedure and calculation method of age anduncertainty. The weighted average age of the pitchblende reference material iscoincident with its isotopic age (69.8±0.6Ma), which shows that EPMA dating methodcan be used to date the age of pitchblende and the result is reliable and believable, andalso indicates that the uncertainty of EPMA age is equivalent with that of isotopic age.(6) Practical application①The weighted average age for40analyzing points of the uraninite sampledfrom Guangshigou uranium deposit in Shanxi province is398.0±4.2Ma (95%confidence probability, MSWD=1.03), and agree with the age dated by isotope dilutionmethod (ID-TIMS).②The weighted average age for50analyzing points of the uraninite in5sampleswhich locate in Happy Valley zone, Namibia, is500.4±4.7Ma (95%confidenceprobability, MSWD=0.46), and agree with that dated by La-ICP-MS. The weightedaverage age for10points of pitchblende in one of the above samples is35.5±2.6Ma(95%confidence probability, MSWD=1.7), and may be lower than the true age becauseof the radiogenic lead loss.③After deducting the initial lead, the weighted average ages of the pitchblendesampled from Shijiaowei uranium deposit, Xiazhuang uranium ore field, Guangdongprovince, are77.1±5.3Ma (95%confidence probability) and91.7±2.3Ma (95%confidence probability) respectively, and both of them are similar with the isotopic ages.④The weighted average age for7points of the uaninites from Shirenzhangtungsten deposit in Shixing county, Guangdong province, is154.6±3.8Ma (95%confidence probability, MSWD=0.17), and fit the Re-Os age of molybdenite.(7) Establishment of discriminant criterion for distinguishing the closure and the content of initial lead of uranium mineral systemThe system of uranium mineral is relative closed and the age dated by EPMAdating method is more reliable when few compositional elements have obviouscorrelations and the correlation coefficients between Si-Pb, Na-Pb, Na-Al, Na-Ca,Pb-Al, Pb-Ca and Al-Ca are lower than0.5. Vice versa, the system is relative open orthe content of initial lead can not be neglected, and the age is less reliable when manycompositional elements have obvious correlations and most of the correlationcoefficients between Si-Pb, Na-Pb, Na-Al, Na-Ca, Pb-Al, Pb-Ca and Al-Ca are higherthan0.5.(8) Recommendation of two age reference materials for EPMA dating of uraniummineralsAccording to the results of homogeneity and stability, the pitchblende agereference material of uranium and lead isotopes locating in No.201uranium deposit andthe uraninite from Guangshigou uranium deposit, Shanxi province are recommended asthe standards for EPMA dating for the first time. The isotopic ages of these two naturaluranium minerals are used as the calibration ages which are69.8±0.6Ma and402.9±3.9Ma respectively.(9) The range of EPMA dating for uranium mineralsTheoretical calculation indicates that EPMA dating method can date the uraniummineral age of above2Ma. In theory, there is no oldest age limit only if the uraniummineral meets the assumed conditions of EPMA dating. However, the probabilitysuffering late reformation for uranium minerals becomes more possible if the age isolder, and therefore, the possibility of opening system is bigger either. As a result, thesuitability for EPMA dating will be less. The dated EPMA ages (398.0Ma and500.4Ma)of uraninites which are agree with isotope ages from Guangshigou deposit, Shanxiprovince, and Happy Valley zone, Namibia indicate that they have not suffered lategeological reformations.(10) EPMA dating for other U-bearing mineralsThe ages of monazites in Dongnaobao granite locating in Chayouzhongqi, Jiningcity, Inner Mongolia and from Happy Valley zone, Namibia are dated by EPMA datingmethod, and the weighted average ages are292.3±9Ma (95%confidence probability,MSWD=0.78) and509±16Ma (95%confidence probability, MSWD=0.35) respectively,which are coincide with the geology or known age. The EPMA age of thorianite, whichis rarely studied, from Xingshanmiao, Zhenping county, Henan province, is dated and the weighted average age is410.7±4.0Ma (95%confidence probability, MSWD=0.16).The discussed EPMA dating method for uranium and U-bearing minerals in thisdissertation is believable, reliable and feasible, and is a new technique for the dating ofuranium deposits and igneous rock which is one of the complements for traditionaldating method.