地震地质 ›› 2016, Vol. 38 ›› Issue (4): 1107-1123.DOI: 10.3969/j.issn.0253-4967.2016.04.024

• 专题综述 • 上一篇    下一篇

磷灰石(U-Th)/He年龄准确度的影响因素

张炜斌1,2, 吴林1, 王非1   

  1. 1. 中国科学院地质与地球物理研究所, 岩石圈演化国家重点实验室, 北京 100029;
    2. 中国科学院大学, 北京 100049
  • 收稿日期:2015-10-08 修回日期:2016-04-30 出版日期:2016-12-20 发布日期:2017-01-20
  • 作者简介:张炜斌,男,1987年生,中国科学院地质与地球物理研究所在读博士研究生,主要研究方向为同位素热年代学,电话:010-82998560,E-mail:fossilchina@gmail.com。
  • 基金资助:

    国家杰出青年基金项目（41025010）与中国科学院战略性先导科技专项（XDB03020203）共同资助

FACTORS IMPACTING THE ACCURACY OF APATITE(U-TH)/HE DATING

ZHANG Wei-bin1,2, WU Lin1, WANG Fei1   

  1. 1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
    2. University of Chinese Academy of Sciences, Beijing 100049, China
  • Received:2015-10-08 Revised:2016-04-30 Online:2016-12-20 Published:2017-01-20

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摘要：

磷灰石（U-Th）/He体系因具有已知最低的放射性同位素体系封闭温度（70℃，约3km深度），而被广泛应用于浅地表构造变形及地貌演化研究，是近20a来发展较为迅速的1种热年代学方法。但由于定年原理、测试方法和4He扩散特点等诸多因素的限制，该同位素年龄体系的准确度受到了一定程度的制约。文中归纳了现有文献中常见的10类磷灰石（U-Th）/He定年准确度的影响因素，即粒径、包裹体、α粒子射出效应、α粒子植入效应、U-Th成分环带、辐射损伤、磷灰石化学成分、钐含量、多期热事件和铀系不平衡等，并初步探讨了造成偏差的原因和解决方法，用以抛砖引玉，引起相关研究者的考虑和重视。

关键词: 磷灰石, (U-Th)/He年代学, 准确度, 影响因素

Abstract:

Apatite (U-Th)/He dating has gained popularity since its rejuvenation as geochronometry and thermochronometry applied in the deduction of the geological processes of the upper-three-km crust.However,this irreplaceable method,which has the lowest known closure temperature (~70℃),sometimes is suffering from large dispersion and deviation because of its dating theory,its analytical method,and its diffusion process.In this paper,we summarized ten factors impacting the accuracy of (U-Th)/He dating.They are grain size,fluid and mineral inclusions,α-particle ejection,α-particle implantation,U-Th zonation,radiation damage,chemical composition,samarium concentration,multiple thermal events,and U-series disequilibrium.We discussed how these ten factors would affect the (U-Th)/He ages and how to reduce and/or avoid the deviation caused by them.The factors of grain size (different size,different diffusion domain) and inclusions (parentless 4He) can be suppressed in the procedure of grain selection under binocular.It is the precise measurement of a homogenous crystal that endows the (U-Th)/He dating method credible due to the correction of α-particle ejection based on the dimension of crystals.The possible implantation of α-particles can be evaluated by the negative correlation of age and eU (eU=[U]+0.235[Th]).U-Th zonation,a heterogeneous distribution of parent nuclides,makes the correction of α-particle ejection inaccurate;besides,this factor also amplifies the side effects of radiation damage and grain size.LA-ICP-MS can detect this phenomenon.Radiation damage outstands when the samples experience reheating or long residence time in partial retention zone (40~70℃),indicated by the positive correlation between age and eU/[4He].Apatite (U-Th)/He age can be entangled by higher Cl content,with which the crystal accumulates more radiation damage,leading to a larger age,meanwhile this factor needs more investigation.Having larger half-life than U-series nuclides,the 147Sm is not a big problem in the dating of a large timescale,while more precise event needs the data of 147Sm in the age calculation.The multiple thermal events should be evaluated to exclude thermal perturbation,when applying the (U-Th)/He geo/thermochronology to the deduction of a particular thermal event.U-series disequilibrium has an impact on the ages smaller than 1Ma,making the age results larger than the true one.U-series disequilibrium method and mineral couples can solve this problem.Inclusion,α-particle ejection and implantation,and U-Th zonation are at the mercy of analytical methods.The factors stemming from helium diffusion in the crystals are grain size,radiation damage,Cl content,multiple thermal events.The dating theory of (U-Th)/He method renders U-series disequilibrium and 147Sm outstand as side effects in some specific conditions.It would be our pleasure if this paper could provide some useful information for the works relevant to this dating method.

Key words: apatite, (U-Th)/He dating, accuracy, effects

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