同位素地质年龄测定技术及应用
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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


Isotope Geochronology: Technique and Application
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    摘要:

    同位素地质年代学是地球科学、物理学、化学和技术科学相互交叉发展起来的一门新兴学科,是地球系统科学中一个年轻而充满活力的分支学科。它根据放射性同位素衰变规律确定地质体形成和地质事件发生的时代,以研究地球和行星物质的形成历史和演化规律。本文对几种常用的精度比较高的同位素测年方法从理论、实验技术、应用范围、使用的注意事项等方面予以简要总结和介绍,期望为地质同行们提供有益的参考。所涉及的同位素测年方法主要有U-Pb法、Ar-Ar法、Rb-Sr法、Sm-Nd法、Re-Os法、(U-Th)/He法等。(1)U-Pb法:是最早用来测定地质年龄的放射性方法之一,也是国内目前最重要的同位素测年方法。近10年来,近乎完美的锆石微区U-Pb年龄测定技术的引进对我国的地质科学研究起到了巨大的推动作用,并且其应用领域仍在进一步扩展中。(2)Ar-Ar法:Ar-Ar同位素地质测年方法已经成为同位素地质年代学研究最主要的方法之一。该方法具有以下特点:①测量的时间域较宽,最老可到38亿年(月岩年龄),最年轻可测到千年级(意大利维苏威火山喷发年龄);②测量对象广泛,原则上,所有的含钾矿物、岩石都可以用作Ar-Ar法同位素测年,甚至含有微量钾盐包裹体的非钾矿物如石英、闪锌矿等也有成功测定出Ar-Ar年龄的报道;③独特的分步加热技术和内部组分的Ar同位素相关图处理技术不仅可以获得高精度的年龄,还可以揭示被测定对象所经历的多期地质演化信息;④和激光技术配套可以直接在岩石光片上寻找待测矿物进行微区(几十微米-几百微米)Ar-Ar测年,从而能够获得变质岩P-T-t轨迹研究中最精确的时间信息;⑤应用领域广泛,几乎所有的地质学分支学科中都有应用;⑥是矿床年代学研究的最主要的技术手段;⑦是同位素热年代学研究的支柱技术。Ar-Ar法测年也有其局限性:首先是分析技术复杂导致其成本高、分析周期长。其次中子参数测定的准确性直接影响样品年龄测定的准确性。核反冲效应会导致极细粒的粘土矿物Ar-Ar年龄结果偏高。对于早元古代和太古代古老变质岩样品,由于可能存在K和Ar的自然扩散作用或后期变质、变形等多因素的扰动作用,用Ar-Ar法很难测出早期的变质事件年龄。(3)Rb-Sr法:是一个应用很广泛的方法,利用等时线技术可以测定侵入岩、火山岩、变质岩和某些沉积岩的同位素地质年龄。在用Rb-Sr同位素系统测定中酸性侵入岩和火山岩的年龄时,如果岩石迅速冷却,无论用全岩等时线法或矿物等时线法得到的年龄都可能是岩石的形成年龄。对于变质岩,矿物Rb-Sr等时线年龄一般代表岩石遭受最后一次强变质热事件Sr同位素均一化时间。对于沉积岩,可以利用Rb-Sr法测定成岩自生矿物年龄。对于金属矿床,可以用包裹体的Rb-Sr等时线确定矿床的形成时间。通过断层和韧性剪切带形成的矿物的Rb-Sr年龄测定,可以限定构造形成时间。Rb-Sr法最大缺点是,由于Rb的流动性,极易形成开放系统,从而得到不正确的年龄。此外,还还经常受到假等时线的困扰。(4)Sm-Nd法:由于Sm-Nd体系的保存性能良好,抗蚀变和变质作用的能力较强,因此Sm-Nd法年龄能代表原岩生成的时间和反映成岩物质源区的特性。对于基性岩、超基性岩,对太古宙古老岩石,Sm-Nd等时线法是较好的测年方法。Sm-Nd模式年龄代表地壳岩石从CHUR地幔源中分异出来的时间,利用碎屑沉积岩的模式年龄可以鉴别沉积物的源区,判断岩石形成构造背景,了解其物源区存留地壳的平均年龄,揭示地壳形成和演化历史等。其缺点是由于Sm、Nd地球化学性质的类似,地质作用过程中难以发生相互分离,Sm、Nd在岩石中的比值变化范围就很小,给Sm-Nd等时线法测年带来了困扰,有时不能给出正确可信的年龄。(5)Re-Os法:Re-Os同位素测年法是目前能够直接测定金属矿床矿化年龄的唯一成熟方法。但在实验技术和应用方面还存在不少问题:①近年来发现有些金属矿床辉钼矿的Re-Os年龄高于其赋矿围岩的年龄,原因不明;②黄铁矿等多数硫化物矿物含Re量很低,并含有一定程度的普通Os,对样品化学制备过程中低本底的要求很高,一般实验室难以达到,普通Os也难以准确扣除;③后期的热液活动有时可以使Os同位素发生重置,因此,金属硫化物Re-Os同位素体系封闭温度及其影响因素是一个亟待解决的问题。(6)(U-Th)/He法:(U-Th)/He同位素系统的优势是其封闭温度是已有同位素体系中最低的,能够记录地质体经历较低温度范围的时代与温度信息。该方法在矿床年代学研究中也具有可观应用前景。其不足之处是因为封闭温度很低,在用于地质体定年时要特别关注冷却速率和再加热作用的影响。

    Abstract:

    Isotope geochronology is a newly developed science which combines geoscience, Physics, Chemistry and technical sciences, and it is a young and energetic branch of the earth system science. Based on the decay of radioactive isotopes, this science can determine the formation age of geological body and the age of geological events, 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 aspects of theory、experiment techniques、application scope、precautions for use, expecting to provide a useful reference to geologist. Involved isotope dating methods are U-Pb method、Ar-Ar method、 Rb-Sr method、Sm-Nd method、Re-Os method and(U-Th)/He method. (1)U-Pb method: It is one of the earliest radioactive methods for geological age determination as well as the most important isotope dating method in China so far. In the past 10 years, the introduction of insitu 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, the oldest age is 3.8Ga (the age of lunar rocks), millennium age can be measured for the youngest age (eruption age of Mount Vesuvius, Italy); ② wide range of measurement object, in principle, all the k-bearing minerals and rocks can be used as Ar-Ar isotopic dating, even there are reports on successful determination of Ar-Ar ages from non-potassium minerals such as quartz、sphalerite which contain trace amounts of kainitite inclusion; ③Unique step heating technique and the management of Ar isotope correlation diagram of internal components can not only obtain high-precision age but unfold multi-stage geological evolution of the object; ④Joining with laser technique, it can be used to determine microzonation Ar-Ar age by search the minerals to be tested in polished section, so that we can obtain the priciest time information of the metamorphic rock P-T-t research; ⑤ wide application area, it is used in almost all the branches of geology; ⑥ the most important technique of deposit chronology; ⑦ the backbone technique of isotope thermochronology. 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 dating samples. Nuclear recoil effect can result in high Ar-Ar age of very fine particles of clay minerals. As 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 for us to determine the age of early metamorphic events. (3)Rb-Sr method: It is a broadly applicated method. We can determine the isotope geological ages of intrusive rocks、volcanic rocks、metamorphic rocks and certain sedimentary rocks by using isochron technique. When determining the age of intermediate and acid intrusive rocks and volcanic rocks by using Rb-Sr isotopic system, if the rocks cooled rapidly, 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 represent 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. The metal 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, we may get wrong age due to the mobility of Rb which will result in an open system. In addition, we often face false isochrons problems. (4)Sm-Nd method: For Sm-Nd system is good at preservation and alteration and metamorphism resistance, ages from Sm-Nd dating can represent the original time when primary rocks formed and reflect the characteristics of diagenesis source material. For basic rocks、 ultrabasic rocks and for ancient rocks of Archean, Sm-Nd isochron dating method is a good method. Sm-Nd model age represent the time when the crustal rocks differentiated from CHUR mantle, using model ages of clastic sedimentary rocks, we can identify the source, the background of tectonic, the mean crustal residence age, and the formation and evolution history of crust. The disadvantage is the similarity of Sm, Nd geochemical properties, it will be difficult to separate from each other in the geologic process, and will result in narrow range of Sm, Nd ratio, which will make the Sm-Nd isochron dating problematic and even can not give reliable age. (5) Re-Os method: Re-Os isotope dating is the only mature method to determine mineralization ages of metal deposits directly. However, there are many problems in experiment techniques and applications: ① In recent years, some scientists find that the Re-Os ages of molybdenite in some metal deposits were higher than the ages of ore-bearing rocks, the reasons are unclear; ② the amount of Re in most sulfide mineral including pyrite is significantly low, and contains certain amount of common Os, this requires very low background in the process of chemical preparation of samples, which makes it difficult for general laboratory to achieve, and the common Os is difficult to deduct; ③ sometimes, post hydrothermal activities may reset Os isotope, therefore, the closure temperature and influencting factors of the metal sulfide Re-Os isotope system is a pressing problem to be solved. (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 it can record temperature information and the time when geologic body went through a lower temperature process. The method also has considerable application in the deposits chronology. The disadvantage is that as the closure temperature is very low, we should pay special attention to the cooling rate and effects of re-heating in geological dating.

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陈文,万渝生,李华芹,张宗清,戴橦谟,施泽恩,孙敬博.2011.同位素地质年龄测定技术及应用[J].地质学报,85(11):1917-1947.
chenwen, WAN Yusheng, LI Huaqin, ZHANG Zongqin, DAI Tongmo, SHI Zeen, SUN Jingbo.2011. Isotope Geochronology: Technique and Application[J]. Acta Geologica Sinica,85(11):1917-1947.

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  • 收稿日期:2011-10-25
  • 最后修改日期:2011-10-25
  • 录用日期:2011-10-27
  • 在线发布日期: 2011-11-11
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