地质学报2011，Vol.85Issue(11)：1917-1947,31.DOI:11-1951/P.20111025.0834.002
同位素地质年龄测定技术及应用
Isotope Geochronology: Technique and Application
摘要
Abstract
Isotope geochronology is a newly developed interdisciplinary science which combines geoscience, physics, chemistry and technical sciences, and is a young and energetic branch of earth science. The decay of radioactive isotopes is used to determine the formation age of geological body and the age of geological events, with aiming to study the formation history and evolution of earth and planetary materials. This paper presents brief introduction and summary of relative high-precise isotope dating methods in terms of theory, experiment techniques, application scope, precautions for use, in hope of providing a useful reference to geologist. Main isotope dating methods involved s are U-Pb, Ar-Ar, Rb-Sr, Sm-Nd, Re-Os and (U-Th)/He methods. (l)U-Pb method: It is one of the earliest radioactive methods for determination of geological age as well as the most important isotope dating method in China so far. In the past 10 years, the introduction of zircon U-Pb dating technique has played a significant role in promoting geological research in China, and its application is expanding. (2) Ar-Ar method: Ar-Ar dating has become one of the most important methods of isotope geochronology. The characteristics of this method are as follows: ① Wide time-domain measurements, with the oldest age up to 3. 8Ga (the age of lunar rocks) and the youngest one in a scale of millennium age (the eruption age of Mount Vesuvius, Italy); ② A wide range of measurement objects. In principle, all the k-bearing minerals and rocks can be used for Ar-Ar isotopic dating, even there are reports on the successful determination of Ar-Ar ages from non-potassium minerals such as quartz and sphalerite which contain trace amounts of kainitite inclusions; ③ Unique step heating technique and Ar isotope correlation diagram of internal components can not only obtain high-precision age but unfold multi-stage geological evolution of the object;④ Combined with laser technique, it can be used to determine Ar-Ar age by searching microzonation directly on polished section of minerals to be tested, so as to obtain the most precision time information of the metamorphic rock P-T-t track; ⑤ Wide application area. It has been used almost in every discipline of geology; ⑥ It has been the most important technical tool of deposit chronology; ⑦ It is the backbone technique of isotope thermochronology. But Ar-Ar dating has its limitations. Firstly, the complex analysis leads to high cost and long cycle. Secondly, the accuracy of the determination of the neutron parameters directly affects the accuracy of samples dating. Nuclear recoil effect can result in high Ar-Ar age of very fine-grained clay minerals. For the samples of Early Proterozoic and Archean metamorphic rocks, there may be natural K and Ar diffusion or later metamorphism, deformation and other disturbance, which will make it difficult to determine the age of early metamorphic events. (3)Rb-Sr method. It is a popular method. The isotope geological ages of intrusive, volcanic, metamorphic, and some sedimentary rocks can be determined by using isochron technique. When Rb-Sr isotopic system is used to determine the age of intermediate and acid intrusive rocks and volcanic rocks, if these rocks cooled rapidly, the ages determined by both the whole rock isochron and mineral isochron age may be the formation ages of the rocks. For metamorphic rocks, Rb-Sr isochron age generally represents the Sr isotope homogenization time when the latest strong thermal event happened. For sedimentary rocks, we can determine the authigenic mineral diagenesis age using Rb-Sr dating. But for metallic deposits, we can determine the formation ages of deposits using inclusion Rb-Sr isochron. Through Rb-Sr mineral dating to determine the formation of faults and ductile shear zone, we can limit the tectonic formation time. The biggest drawback of Rb-Sr dating is that incorrect age can be obtained due to the fact that the mobility of Rb easily produces an open system. In addition, the age determination is often interfered by false isochrons. (4)Sm-Nd method. Due to good preservation, corrosion and metamorphism resistances of Sm-Nd system, Sm-Nd dating time can represent the formation age of primary rocks and and reflect the characteristics of rock-forming material sources. For mafic, ultramafic, and archean rocks, Sm-Nd isochron dating method is a good one. Sm-Nd model age represents the time of crustal rocks which differentiated from the CHUR mantle, and model ages of clastic sedimentary rocks can be sued to identify the rock-forming source, so as to judge the tectonic background of rock formation, understand the average ages of material source detained within the crust, and finally reveal the history of formation and evolution of the crust formation. The disadvantage is that Sm and Nd are difficult to separate from each other due to their similar geochemical properties during the geological process, resulting in a narrow range of Sm to Nd ratios in rocks, giving trouble for SM-Nd isochron dating method, even without reliable age obtained. (5) Re-Os method. Re-Os isotope dating method is the only mature method so far that is directly used to determine mineralization ages of metal deposits. However, there are many problems in experiment techniques and applications. ① It is found in recent years that the Re-Os ages of molybdenite in some metal deposits are higher than the ages of ore-bearing rocks, and the reasons are unclear. ② Most sulfide minerals such as pyrite contains very low content of Re but a certain amount of common Os. This imposes high requirements on the preparation of samples, which general laboratories fail to meet. In addition, common Os can not be precisely deducted. ③ Sometimes post-hydrothermal activities may reset Os isotope, therefore, the closure temperature and influencing factors of the metal sulfide Re-Os isotope system is a key problem to be solved urgently. (6) (U-Th) / He method. The advantage of (U-Th) / He isotope system is that its closure temperature is the lowest among the existing isotopic systems, and can record temperature information and the time when geologic body underwent a lower temperature range. This method also has considerable application potential in deposit chronology. But its disadvantage is very low closure temperatures, and much attention should be paid on cooling rate and re-heating effect during dating of geological bodies.关键词
同位素地质年代学/分析测试技术/地质应用/U-Pb法/Ar-Ar法/Rb-Sr法/Sm-Nd法/Re-Os法/(U-Th)/He法/静态顶空进样/吹扫-捕集进样/凝胶渗透色谱/索氏抽提/快速溶剂萃取/微波萃取/超临界流体萃取/超声萃取/气相色谱Key words
isotope geochronology/ analysis technology/ application/ U-Pb/ Ar-Ar/ Rb-Sr/ Sm-Nd/ Re-Os/ (U-Th)/He引用本文复制引用
陈文,万渝生,李华芹,张宗清,戴檀谟,施泽恩,孙敬博..同位素地质年龄测定技术及应用[J].地质学报,2011,85(11):1917-1947,31.基金项目
本文为国家重点基础研究发展计划项目(编号2007CB411306 （）
2009CB421001)、中国地质调查局项目(编号1212010660212 （）
1212011120293)和公益性行业专项经费(编号200911043-13)资助成果. （编号200911043-13）
