电子探针定年技术在铀及含铀矿物测年中的开发与研究

【作者】 葛祥坤；

【导师】 秦明宽； 范光；

【作者基本信息】 核工业北京地质研究院 ， 矿产普查与勘探， 2013， 博士

【摘要】 电子探针定年法以放射性核素衰变理论为基础，假定待测矿物中的普通铅可以忽略不计，系统封闭，通过电子探针测量含U、Th矿物中的U、Th、Pb含量，最终计算矿物形成年龄。该方法由日本名古屋大学铃木和博（Suzuki K.）教授于1991年正式提出，主要用于独居石和锆石的微区原位定年，对于铀矿物及其它含铀矿物的微区定年研究国外仅有零星报道。本论文从铀及含铀矿物的化学组成、晶体结构稳定性等方面入手，深入分析其U、Th、Pb体系的封闭性，通过详细而周密的实验设计，确定最佳分析测试条件组合，建立从样品制备处理到样品测试的分析测试流程，并对测试结果进行全面的不确定度评定，最后通过优选年龄计算方法，确定铀矿物及其它含铀矿物年龄，并根据化学成分之间的相关性，建立识别铀矿物体系是否封闭或含初始铅的判别标志。同时还推荐了两个用于电子探针定年的铀矿物年龄标样，并对电子探针定年法的测年范围进行了研究。具体包括以下内容：（1）铀及含铀矿物U-Th-Pb体系的封闭性探讨从铀的稳定存在形式、类质同象置换，以及晶质铀矿和沥青铀矿的化学组分和晶体结构等方面，全面分析和探讨了晶质铀矿和沥青铀矿保持放射成因铅的能力和初始铅的含量。两种铀矿物普遍存在类质同象置换，化学成分复杂，比独居石容易发生铅丢失，尤其是年龄比较老时更容易发生，但要比其它铀矿物好很多。晶质铀矿中初始铅含量很低，比沥青铀矿更适合于电子探针定年研究。方钍石与晶质铀矿结构相似，也可以用于电子探针定年研究。钍石常呈变生状态，容易发生铅丢失，多数情况下不能用于电子探针定年。（2）分析测试条件优选研究：①仪器测量条件：加速电压20kV；束流50nA；束斑直径1~5μm；②元素测量参数：U、Th、Pb均选用Mα线，用PETH晶体测量；实测了Th对U Mα和Pb Mα、Y对Pb Mα的干扰系数，分别为0.01251、0.00063和0.00422，并进行在线干扰校正；U和Th的高低背景测量位置选择在±4mm处，Pb Mα选择在-4.5/+5mm处；峰位和背景的测量时间以不大于100S和50S为宜；U和Th选用T方式测量、Pb用F方式测量；③标样：选择金属U、ThO2和方铅矿作为U、Th、Pb的校正标样。（3）建立分析测试流程①将样品制作成光薄片或砂光片；②样品和标样同时喷镀30nm的碳导电膜；③定性分析，确定待分析元素；④设置测量条件；⑤选用ZAF法进行修正计算，选择金属U、ThO2和方铅矿作为U、Th、Pb的标样；⑥定点分析。一个测点的数据获取需要10~20分钟。（4）测试结果的不确定度评定通过对整个测试流程的深入分析，首次对U、Th、Pb测量结果的准确性进行不确定度评定，确定了四种标准不确定度分量，即样品和标样计数统计的标准不确定度分量、标样组分的标准不确定度分量以及ZAF校正引起的标准不确定度分量，对每一个分量进行量化，进而计算合成标准不确定度，最后由U、Th、Pb的合成标准不确定度通过误差传递公式计算年龄的不确定度。（5）年龄计算方法优选及测量结果可靠性分析系统介绍了电子探针年龄的各种计算方法和年龄计算软件，分析比较了其优缺点，并对每种计算方法在铀矿物年龄计算中的适用性进行了讨论，优选出最佳的年龄计算方法和计算软件以及计算步骤，即：首先使用Chemage软件计算单点表观年龄，然后通过Isoplot软件区分年龄域，当存在两个及两个以上年龄域时，直接输出各年龄域的计算结果，当所有单点年龄属于同一个年龄域时，再由Isoplot软件重新计算加权平均年龄。根据上述建立的分析测试流程、年龄及不确定度计算方法，对201矿床沥青铀矿铀铅同位素年龄标准物质进行了成分测试，最终得到42个点的加权平均年龄为67.8±0.9Ma（95%置信概率，MSWD=0.76），与标定年龄值（69.8±0.6Ma）吻合较好，验证了电子探针定年法测定沥青铀矿是可靠、可信的，且不确定度与同位素年龄的不确定度相当。（6）实际应用①陕西光石沟矿床晶质铀矿40个测点的加权平均年龄为398.0±4.2Ma（95%置信概率，MSWD=1.03），与同位素稀释法年龄一致。②纳米比亚欢乐谷地区5个样品共50个晶质铀矿测点的加权平均年龄为500.4±4.7Ma（95%置信概率，MSWD=0.46），与激光烧蚀法测得的同位素年龄一致；纳米比亚欢乐谷地区10个沥青铀矿测点的加权平均年龄为35.5±2.6Ma（95%置信概率，MSWD=1.7），年龄结果可能偏低，发生了部分铅丢失所致。③下庄矿田石角围矿床xz12-46号样品根据同位素测量结果，扣除初始铅后区分出两个不同的年龄域，其加权平均年龄分别为77.1±5.3Ma（95%置信概率）和91.7±2.3Ma（95%置信概率），均与同位素年龄一致。④广东始兴石人嶂钨矿床中晶质铀矿7个测点的加权平均年龄为154.6±3.8Ma（95%置信概率，MSWD=0.17），与辉钼矿的Re-Os年龄一致。（7）建立了识别铀矿物体系是否封闭或含初始铅的判别标志当铀矿物化学组成元素之间具有明显相关性的元素对较少，且Si-Pb、Na-Pb、Na-Al、Na-Ca、Pb-Al、Pb-Ca、Al-Ca等元素对的相关系数低于0.5时，铀矿物体系相对封闭，得到的年龄可信度高；反之，当具有明显相关性的元素对较多，且Si-Pb、Na-Pb、Na-Al、Na-Ca、Pb-Al、Pb-Ca、Al-Ca等元素对的相关系数多数都大于0.5时，体系相对开放，或者具有显著的初始铅，年龄结果的可信度低。（8）确定两个用于铀矿物电子探针定年的标准样品经不均匀性和稳定性检验，首次确定201矿床沥青铀矿铀铅同位素年龄标准物质和陕西光石沟矿床晶质铀矿可作为电子探针定年标样，其标定年龄以同位素年龄为准，即分别为69.8±0.6Ma和402.9±3.9Ma。（9）电子探针定年法测定铀矿物年龄的范围通过理论计算，电子探针定年法可以测定大于2Ma以上的铀矿物年龄。从理论上讲，只要铀矿物满足电子探针定年法的假设条件，该方法就没有最老年龄上限。但事实上，年龄越老，铀矿物发生后期改造的几率就越大，体系不封闭的可能性也就越大，作为电子探针定年样品的适用性就越差。本论文测定的陕西光石沟矿床晶质铀矿和纳米比亚欢乐谷地区晶质铀矿的电子探针年龄分别为398.0Ma和500.4Ma，与同位素年龄一致，表明未发生后期地质作用的改造。（10）其它含铀矿物的电子探针定年研究应用电子探针定年法对内蒙古集宁察右中旗东脑包花岗岩体和纳米比亚欢乐谷地区的钍独居石进行了测年研究，得出加权平均年龄分别为292.3±9Ma（95%置信概率，MSWD=0.78）和509±16Ma（95%置信概率，MSWD=0.35），均与地质情况或已知年龄吻合；对鲜为研究的方钍石进行了定年研究，测得河南镇平县行善庙铀方钍石的成矿年龄为410.7±4.0Ma（95%置信概率，MSWD=0.16）。本论文论述了所建立的铀矿物及含铀矿物的电子探针定年法是可信、可靠、可行的，为岩石和铀矿物的定年提供了一种新的技术手段，是传统定年方法的一个补充。更多还原

【Abstract】 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.更多还原