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1、,第二讲 新构造学的年代学方法,Outline,Potassium-Argon and Argon-Argon dating (K-Ar and Ar-Ar) Carbon-14 dating Cosmogenic nuclide dating Fission track dating (FT) Uranium series dating Thermoluminescence dating (TL) Optical Stimulated luminescence dating (OSL) Electron Spin Resonance Dating (ESR) Paleomagnetism,B
2、asic theory of isotope dating,其中：t 欲测样品的年龄， 衰变常数， N0 母体原始含量；N 母体残余含量；D 子体t时候的含量，D0 子体t=0时候的含量。,半衰期（T1/2）（half-life）放射性元素衰变一半所需要的时间（单位：年）； 衰变常数（）（decay constant）单位时间内自发衰变的概率（单位：年-1）； 平均寿命（）（mean-life）放射性元素平均生存时间（单位：年）。来衡量。,Isochron diagram,1. K-Ar and 40Ar-39Ar dating,(1) Theory K的三种同位素：39K、40K、41K，其
3、中40K为放射性元素，半衰期为1.31109a.,40K40Ar(K层捕获)(占10.48%)（衰变常数e）； 40K 40Ca(放出负电子)（占89.52%）（衰变常数） e 40Ar+40Ca=40K(et1),其中：40K为衰变后的母体残余量，40Ar+40Ca为子体含量。40K-40Ar测量40K和40Ar的含量，然后计算出年龄。,式中：40Ar*由40K衰变而成,假设条件： t=0时，矿物中不含40Ar。岩石矿物形成是所携带的氩同位素的丰度比，应与现代大气中氩同位素的丰度比相同，即经过大气氩校正之后，样品在形成时的放射成因40Ar应为零。 在t时间里，处于封闭体系，无K、Ar的带进带
4、出。 能够准确地测定衰变常数。 能够准确地测定岩石矿物中40Ar*和40Ar的含量或他们之间的比值。 在岩石矿物形成过程中和形成以后，钾和氩（尤其是氩）从开放体系过度到封闭体系所经历的时间相对与封闭体系维持的时间要短得多，可以忽略不计。否则所得到的年龄值实际上是样品达到封闭温度的时间，否则，可用40Ar-39Ar解决。,40Ar-39Ar法的基本原理： 与K-Ar法不同之处在于，K-Ar法分别测量40Ar和40K的含量；而40Ar-39Ar法不测量40K的含量，而是先用快中子照射，使40K（母体残余量）变为39Ar，这样就可以同时测量39Ar和40Ar ，然后再用39Ar的含量推算出40K的含
5、量。在质谱仪上同时测量39Ar和40Ar ，克服了样品不均匀的影响。 另外，测量过程是逐渐加温，从而可能取得在不同温度阶段的年龄信息（加温过程中可以出现若干平台）(年龄谱，age spectrum)（“坪年龄”, “plateau age”），每一个“坪年龄”代表一次热事件的时间。,(2) 测量对象及年龄范围 火成岩中含K的矿物，如长石、云母等等。 (3) 误差来源及测量误差 被测样品的均匀性；样品年龄（样品越老， 40Ar丢失的可能性越大）和钾含量（是否有后期的蚀变，蚀变能把钾带走）以及大气氩含量（新年龄样品大气氩含量增加，也使误差增大）；实验流程的合理性和测量系统的稳定性及本底水平等。 (
6、4) 取样要求 (5) 应用 火山岩的测年；断层活动年龄及活动期次的研究。,2. Carbon-14 dating,(1) Theory n+N14N15p+C14N14(-decay，emits negative electrons) nneutrons (cosmic ray); pproton; half-live t1/25730a.,The relatively rapid cycling of carbon between the atmosphere and living biosphere allows live organism to maintain a 14C activ
7、ity which is approximately equal to that of the atmosphere. Once an organism dies, it becomes isolated from the 14C source, 14C runs down by radioactive decay by a constant rate. A(t)=A0e-t A(t)parent (after decay); A0parent (before decay)；,(2) Assumption 14C content of the atmosphere has not varied
8、 with time in 50100ka . 14C content of the carbon in organism equaled that of the atmospheric CO2, complete and rapid mixing of 14C. 14C decay rate is constant over time. After isolation, no “new carbon” or “old carbon” mix in. No isotope fractionation between 14C,13C, 12C.,(3) Materials for dating
9、Wood, seeds, pollen, charcoal(木炭), bone, peat, chitin(角质), carbonate shells, (bulk organic sediments, i.e. soil and paleosols) (4) Time range 10 times half-live. (5) Errors origin Content of radiocarbon varied over time, laboratory skill, standard samples, contamination.,(6) Application,3. Cosmogeni
10、c nuclide dating,Theory High energy cosmic rays (nucleons, mostly protons) enter the atmosphere, collide with nuclei, trigger a cascade of secondary nucleons (e.g. protons and neutrons) and mesons, kaons and muons). Secondary nucleons bathe the earths surface, creating new nuclides by spallation, e.
11、g. Be-10, Al-26 (radionuclides). New nuclides, e. g. Be-10, Al-26, are also created by slow negative muon capture, and fast muon reaction, in rocks.,Secondary particle production in the atmosphere and rock.,纬度效应,Atoms of carbon-14 are formed in the upper atmosphere through the interaction between co
12、smic ray neutrons and nitrogen. (Meteoric cosmogenic nuclides) Atoms of Be-10, Al-26, etc. are formed in rocks (in situ) through the interaction between secondary nucleons (including muons) and targets nuclides such as Oxygen, Silicon. (Terrestrial cosmogenic nuclides) (TCN),Origin difference betwee
13、n TCN dating and carbon-14 dating,The concentration of these accumulated nuclides within surficial rock is therefore directly related to the time that the surface has been exposed to cosmic ray activity. The greater the time that has exposed since exposure, the greater will be the abundance of cosmo
14、genic nuclides in rock surface samples.,The concentration of the accumulated nuclides also depend on erosional rate, nuclides lifetime（half-life, mean-life, decay constant）, and nuclides inherited, besides exposure time.,The first term is inherited nuclide concentration. The second term is nuclide c
15、oncentration created by neutron spallation. The last three terms are nuclide concentration due to slow negative muon capture, and fast muon reaction, respectively.,Although the nuclides are produced in very small quantities in rocks, the development of accelerator mass spectrometry (AMS) techniques
16、over the past 20 years has made measurement routine at very low level. 10Be production rate in quartz is 5.10.3 atoms/g.yr.,(2) Application,1) Exposure age 2) Sedimentary age (burial dating) 3) Surface uplift (sensitive to elevation). 4) Erosional rate (outcrop or an entire drainage basin),Various application of TCN dating,(3) Example 1) Surface exposure age. Striated outcrops of glacial landform, landslide, f
