年代学-陈福坤-第一节.pdf

同位素地质年代学与 放射成因同位素地球化学 同位素地质年代学与 放射成因同位素地球化学 陈福坤 固体同位素地球化学实验室固体同位素地球化学实验室 中国科学院地质与地球物理研究所 课程内容安排 第一节第一节 1. 引言引言 2. 核素物理与同位素核素物理与同位素 3. 同位素衰变原理与概念同位素衰变原理与概念 第二节第二节 4. 实验室介绍实验室介绍 5. 超净化实验室和同位素化学分离纯化超净化实验室和同位素化学分离纯化 6. 质谱计原理和同位素比值测量质谱计原理和同位素比值测量 第三节第三节 7. 同位素地球化学示踪原理同位素地球化学示踪原理 8. 岩石圈和地幔同位素特征岩石圈和地幔同位素特征 第四节第四节 9. 同位素年代学基本原理同位素年代学基本原理 10. Rb-Sr和和Sm-Nd定年方法定年方法 第五节第五节 11. U-Pb定年方法定年方法 第六节第六节 12. Ar-Ar定年方法定年方法 13. 其它定年方法其它定年方法 第七节第七节 14. 应用实例 1 欧洲华力西造山带 2 大别－苏鲁超高压造山带 3 土耳其北部特提斯造山带 应用实例 1 欧洲华力西造山带 2 大别－苏鲁超高压造山带 3 土耳其北部特提斯造山带 第八节第八节 答疑和测验答疑和测验 Introduction to Isotope Geochemistry Isotope geochemistry has grown over the last 40 years to become one of the most important fields in the Earth Sciences as well as in Geochemistry. White 1998 White 1998 The growth in the importance of Isotope Geochemistry reflects its remarkable success in attacking fundamental problems of Earth Science, as well as in astrophysics and physics. Isotope Geochemistry has played an important role in transing geology from a qualitative, observational science to a modern quantitative one. The impact of Isotope Geochemistry on ore genesis, petrogenesis, crustal evolution, mantle dynamics, global tectonics and geodynamics, volcanology, hydrology, oceanic circulation, paleontology, anthropology and archeology, paleoclimate, environmental protection and monitoring, monitors of the cosmic ray flux, etc. Isotopes in Geosciences Stable Isotope Geochemistry Radiogenic Isotope Geochemistry Applications of Radiogenic Isotopes 1. Geochronology 2. Isotope Tracer Radiometric dating - A time machine to the past Isotope GeochronologyIsotope Geochronology Radioactive decay half-lifes, T1/2 if it is possible to determine the ratio of the PARENT and DAUGHTER atoms, it is then possible to determine how long ago the decay process started age determination Radioactive decay Rate of decay is proportional to the number of decaying nuclei Integrate to find the change in N with time dN dt λN− dN N λ−dt⋅ Radioactive decay Integrate Find Nt No N t N 1 N ⌠   ⌡ d 0 t tλ ⌠  ⌡ d− NNoe λ−t ⋅ ⋅ Parent-Daughter system D D0 N eλt-1 Isotope TracerIsotope Tracer How do we know where the rocks came from The Earth’s Interior Goldschmidt’s classification of the elements 亲铁元素亲硫元素亲石元素亲气元素亲铁元素亲硫元素亲石元素亲气元素 Most IncompatibleLess Incompatible Rb La Ce SrSm Hf Y Yb OIB N-MORB IAT DyP TiPb NdUTh NbBa 1 10 Rock/Primitive mantle 100 Schematic profiles of basalts from different tectonic settings MORB OIB Island Arc Basalt Sr and Nd isotopic systematics of the crust and mantle 0.7020.706 143144 Nd/Nd 0.5122 0.5124 0.5126 0.5128 0.5130 0.5132 0.5134 8786 Sr/ Sr 0.7100.714 Continental Crust Bulk Earth OIB Oceanic Sediments Continental Margin Arcs Island Arcs MORB Correlation of Sr-Nd isotopic composition An overview of the tectonic system An overview of the tectonic system Magma DifferentiationMagma Differentiation Fractional CrystallizationFractional Crystallization – Separation of crystals from liquid – Gravitative settling or flotation play a significant role Magma DifferentiationMagma Differentiation Assimilation /Assimilation /- - Fractional CrystallizationFractional Crystallization Magma DifferentiationMagma Differentiation Magma Magma MixingMixing Physics of the Nuleus Early Views of the Atom Dalton′ s atomic theory of matter 1807 All matter consists of atoms – smallest particle of an element All atoms of a given element, such as gold, are identical Atoms of different elements have different masses The Atom Thomsons Atom Diffusive mass and charge Rutherfords Atom Concentrated mass and positive charge at the nucleus Electrons roam through empty space around nucleus Bohr model of the atom Component of an atom for geochemists’ interest Proton Z Neutron N Electron E The Nuclear Symbol A A Z Z N N Atoms of an element have identical number of proton and identical number of electron Atoms of an element have identically chemical feature Weight of an atom number of proton neutron A Z N Atoms of an element can have different number of neutron Atoms of an element can have different weight Such atoms are isotopes For instance Atom1 Weight A1 Z N1 Atom2 Weight A2 Z N2 Isotopes Elements with the same atomic number number of protons or electrons but different mass number number of nucleons are isotopes Isotopes of an element differ only in the number of neutrons Isotopes If we compare different isotopes of the same element The number of protons and electrons are the same, i.e. atomic number, Z, is the same The number of neutrons are different The number of nucleons protons and neutrons are different, i.e. mass number, A, is different OxygenOxygen- -16 16 8 protons, 8 neutrons8 protons, 8 neutrons OxygenOxygen- -18 18 8 protons, 10 neutrons8 protons, 10 neutrons Isotopes Potassium atomic number 19 Average atomic mass for K 39.10 amu 39K 40K 41K * * * 38.964 amu 36.338amu 39.964 amu 0.0048 amu 40.962 amu 2.757 amu 93.26 0.012 6.73 100 100 100 Average atomic massAverage atomic mass Isobar Different elements have the same weight number A, but different Z example 87Rb, Z37, N50 87Sr, Z38, N49 Proton 1.007593 daltons or amu atomic mass units 1.6726231 x 10-27kg Neutron 1.008982 daltons or amu Electron 0.000548756 daltons or amu 9.10083897 x 10-31kg Neutron number vs. proton number for stable nuclides Binding energy When protons and neutrons combine, mass is lost to energy – Binding energy The binding energy of a nucleus can be found based on E mc2 A more useful version of the equation is ∆E ∆mc2 where ∆E the binding energy ∆m mass difference between the nucleus and the separate nucleons Binding energy B i n d i n g e n e r g y p e r n e u c l e o n Eb Eb W-M*C2/A Mass decrement δ W-M M actual mass of atom W sum of the mass of constituent particles of atom Example of 4He W 2mp 2mn2me 4.034248 daltons M 4.003873 daltons d 0.030375 daltons Eb 7.07 MeV Binding energy per nuleon Eb Eb高的核素，其稳定性高 Chart of the Nuclides Radioactivity A property of some nuclei in which they undergo a transation to a lower more stable energy state This is commonly observed by the spontaneous emission of particles and/or energy from a nucleus First observed by Henry Becquerel in 1896 So why does it happen Reasons for a decay Too many nucleons characterized by α-emission Too many protons βemission or electron capture Too many neutrons β−emission Too much energy emission of a γ-ray. Typically associated with other types of emission Major types of radioactive decay Decay trends radioactiveradioactive parentparent radiogenicradiogenic daughterdaughter nuclearnuclear particleparticle Alpha α Decay radioactiveradioactive mothermother radiogenicradiogenic daughterdaughter Alpha a Decay Decay trends Beta - Decay Decay trends Beta Decay Electron Capture A parent nucleus may capture one of its orbital electrons and emit a neutrino Most commonly, it is a K-shell electron which is captured, and this is referred to as K-capture 707 Be e -- Li ν 4-13 Gamma γ Decay Gamma radioactivity is composed of electromagnetic rays. It is distinguished from x-rays by the fact that it comes from the nucleus. Chart of the Nuclides Concept of Isotope Decay Radioactive decay half-lifes, T1/2 if it is possible to determine the ratio of the PARENT and DAUGHTER atoms, it is then possible to determine how long ago the decay process started age determination, last lecture -λN dN dt Basic equation of radioactive decay λ is the decay constant, which we define as the probability that a given atom would decay in some time dt. It has unit of time-1 where N0is the number of atoms of the radioactive, or parent, isotope present at time t0. -λdt dN N N N0 t 0 -λt N N0 ln e-λt N N0 N0e-λtNor Suppose we want to know the amount of time for the number of parent atoms to decrease to half the original number, i.e. t when N/N0 1/2. Setting N/N0to 1/2 -λt1/2 1 2 ln λt1/22ln OR Half-life time t 1/2 t1/2 ln2 λ Thedecay of the parent produces some daughter, or radiogenic nuclides. The number of daughters produced is simply the difference between the initial number of parents and the number remaining after time t D N0- N D Neλ λt– N Neλ λt- 1 This tells us that the number of daughters produced is a function of the number of parents present and time. In general, there are some atoms of the daughter nuclide around to begin with t 0, a more general expression is D D0 Neλ λt- 1 where D0is the number of daughters originally present 87Sr 87Sr0 87Rbeλt– 1e.g., 87Sr/86Sr 87Sr/86Sr0 87Rb/86Sreλt– 1 FigurePeriodic Table showing the elements having naturally occurring radioactive isotopes and the elements produced by their decay 1. Potassium-40 - Argon-40 2 3. Uranium-235 - Lead-207 4. Uranium-238 - Lead-206 5. Thorium-232 - Lead-208 . Rubidium-87 - Strontium-87 s relying on the decay of naturally occurring radiogenic isotopes Geochronology Parent isotopeDaughter isotope Clocks in the rocks TABLEGeologically Useful Long-Lived Radioactive Decay Schemes s relying on the isotopes produced by cosmic rays s relying on disequilibrium within the U-series decay chain Cosmogenic isotopes Other s 1. Tritium 2. Carbon-14 3. Beryllium-10 4. Aluminium-26 5. Chlorine-36 Lead-210 Clocks in the rocks 谢谢