同位素地质年代学基础知识——同位素地球化学课件PPT

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同位素地球化学6(精选)共35页PPT

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1、不要轻言放弃，否则对不起自己。
2、要冒一次险!整个生命就是一场冒险。走得最远的人，常是愿意去做，并愿意去冒险的人。“稳妥”之船，从未能从岸边走远。-戴尔．卡耐基。
梦境
3、人生就像一杯没有加糖的咖啡，喝起来是苦涩的，回味起来却有久久不会退去的余香。
同位素地球化学6（精选） 4、守业的最好办法就是不断的发展。 5、当爱不能完美，我宁愿选择无悔，不管来生多么美丽，我不愿失去今生对你的记忆，我不求天长ቤተ መጻሕፍቲ ባይዱ久的美景，我只要生生世世的轮回里有你。
61、奢侈是舒适的，否则就不是奢侈。——CocoCha nel 62、少而好学，如日出之阳；壮而好学，如日中之光；志而好学，如炳烛之光。 ——刘向 63、三军可夺帅也，匹夫不可夺志也。 ——孔丘 64、人生就是学校。在那里，与其说好的教师是幸福，不如说好的教师是不幸。 ——海贝尔 65、接受挑战，就可以享受胜利的喜悦。——杰纳勒尔·乔治·S·巴顿

同位素地球化学PPT课件

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1）轻稳定同位素
A. 原子量小，同一元素的各同位素间
的相对质量差异较大（ΔA/A≧5%）；
B. 轻同位素组成变化的主要原因是同
位素分馏作用造成的，其反应是可逆的。
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第五章同位素地球化学Ⅰ
33
2）重稳定同位素
A. 原子量大，同一元素的各同位素间的相
对质量差异小（ΔA/A＝0.7~1.2%），环境的物理和化学条件的变化通常不导致重稳定同位素组成的改变；
526262621放射性同位素衰变定律及同位素地质年代学原理622kar法及40ar39ar法年龄测定623rbsr法年龄测定624smnd法年龄测定625upb法年龄测定53621621同位素地质年代学的基本原理前提及分类541放射性原子释放出粒子和能量的现象即所谓的放2放射性衰变元素的原子核自发地发出粒子和释放能量而变成另一种原子核的过程
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第五章同位素地球化学Ⅰ
11
5. 同位素地球化学发展现状
同位素地球化学发展迅速，已渗透到地球科学的各个研究领域，如：大地构造学、岩石学、矿床学、海洋学、环境科学、空间科学等。
主要表现在以下方面：
♣ 实验测试技术不断完善和提高； ♣ 多元同位素体系的综合研究； ♣ 研究领域不断扩大； ♣ 各种新方法的出现。
28
② 类型
1）放射性同位素(unstable or radioactive isotope)
其原子核是不稳定的，它们能自发地放出粒子并衰变成另一种同位素。
2）稳定同位素(stable isotope)
原子核是稳定的，或者其原子核的变化不能被觉察。元素周期表中，原子序数相同，原子质量不同，化学性

几种年代学方法介绍——同位素地球化学课件PPT

• 近年来迅速发展起来的多接收器双耦合等离子体质谱仪(MC-ICP-MS)综合了等离子体的高电离温度和磁式多接收器质量分析器的优势，使得一些高电离能元素的高精度同位素分析成为现实。
Lu-Hf同位素测年
测试仪器
• 在Re－Os 年代学研究的早期，二次离子质谱、共振离子质谱、加速器质谱、电感耦合等离子体质谱（ICP－MS）都曾用于Re－Os 同位素的测定研究
• 近些年来，随着质谱技术及分析方法的发展，负离子热表面电离质谱（NTIMS）已逐渐成为Re－Os年代学研究尤其是Os 同位素比值测定的主要工具
几种年代学方法介绍
Re－Os法，Sm－Nd法， Lu － Hf法
Re－Os法
铼与锇
• Re，分散元素，不形成独立矿物，与Mo地球化学相似性
• 地幔部分熔融时,中等不相容元素Re趋于进入岩浆,而相容元素Os则趋于保留在地幔中。因此,富集不相容元素的流体对地幔岩石的交代作用通常难以对地幔岩石中Os的同位素组成造成明显的影响。居于此原因,该体系已被广泛地用于研究大陆岩石圈地幔的形成和演化
天然同位素
• Re有两种天然同位素
– 185 -37.398%， – 187 -62.602%
• Os有七种天然同位素
– 184-0.02%, – 186-1.6%, – 187-1.6%, – 188-13.3%, – 189-16.1%, – 190-26.4%, – 192-41%
年龄公式
Re－Os法定年问题讨论
• 有些金属矿床辉钼矿的Re－Os 年龄高于其赋矿围岩，原因不清；
• 黄铁矿等多数硫化物含Re－Os 量明显偏低，并含有普通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；

同位素讲座ppt-课件

1 同位素的基本概念
同位素的定义同位素定义：核内质子数相同而中子数不同的同
一类原子。
同位素的分类： (1) 放射性同位素：原子核不稳定，能自发进行放射性衰
变或核裂变，而转变为其它一类核素的同位素称为放射性同位素。
(2) 稳定同位素：原子核稳定，其本身不会自发进行放射性衰变或核裂变的同位素。
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1934年诺贝尔化学奖获得者Urey奠定了同位素取代的物理化学性质变化的理论基础，并把它用于地球科学。1946年他在英国皇家学会上发表

同位素地球化学1

I NTRODUCTION AND P HYSICS OF THE N UCLEUSI NTRODUCTIONIsotope 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. It has two broad subdivisions: radiogenic isotope geochemistry and stable isotope geochemistry. These subdivisions reflect the two primary reasons why the isotopes of some elements vary in nature: radioactive decay and chemical fractionation. One might recognize a third subdivision: cosmogenic isotope geochemistry, but this subdivision is perhaps better considered a part of radiogenic isotope geochemistry, since although cosmogenically produced isotopes are not products of radioactive decay, they are products of nuclear reactions.The growth in the importance of isotope geochemistry reflects its remarkable success in attacking fundamental problems of Earth Science, as well as problems in astrophysics and physics. Isotope geo-chemistry has played an important role in transforming geology from a qualitative, observational science to a modern quantitative one. To appreciate the point, consider the Ice Ages, a phenomenon that has fascinated geologist and layman alike for the past 150 years. The idea that much of the northern hemisphere was once covered by glaciers was first advanced by Swiss zoologist Louis Agas-siz in 1837. His theory was based on observations of geomorphology and modern glaciers. Over the next 100 years, this theory advanced very little, other than the discovery that there had been more than one ice advance. Isotopic studies in the last 40 years (and primarily in the past 20) have deter-mined the exact times of these ice ages and the exact extent of temperature change (about 3° or so cooler in temperate latitudes). Knowing the timing of these glaciations has allowed us to conclude that variations in the Earth’s orbital parameters (the Milankovitch parameters) and resulting changes in insolation have been the direct cause of these ice ages. Comparing isotopically deter-mined temperatures with CO2 concentrations in bubbles in carefully dated ice cores leads to the hy-pothesis that atmospheric CO2 plays and important role in amplifying changes in insolation. Care-ful U-Th dating of corals is now revealing the detailed timing of the melting of the ice sheet. Com-paring this with stable isotope geothermometry shows that melting lagged warming (not too surpris-ingly). Other recent isotopic studies have revealed changes in the ocean circulation system as the last ice age ended. Changes in ocean circulation may also be an important feedback mechanism af-fecting climate. Twenty years ago, all this was very interesting, but not very relevant. Today, it pro-vides us with critical insights into how the planet’s climate system works. With the current concern over potential global warming and greenhouse gases, this information is extremely ‘relevant’.Other examples of the impact of isotope geochemistry could be listed. The list would include such diverse topics as ore genesis, mantle dynamics, hydrology, and hydrocarbon migration, monitors of the cosmic ray flux, crustal evolution, volcanology, oceanic circulation, archeology and anthropology, environmental protection and monitoring, and paleontology. Indeed, there are few, if any, areas of geological inquiry where isotopic studies have not had a significant impact.One of the first applications of isotope geochemistry remains one of the most important: geochro-nology and cosmochronology: the determination of the timing of events in the history of the Earth and the Universe. The first ‘date’ was obtained by Boltwood in 1907, who determined the age of a uranium ore sample by measuring the amount of the radiogenic daughter of U, namely lead, present. Other early applications include determining the abundance of isotopes in nature to constrain models of the nature of the nucleus and models of nucleosynthesis (the origin of the elements). Work on the latter problem still proceeds. The usefulness of stable isotope variations as indicators of the condi-tions of natural processes was recognized by Harold Urey in the 1940’s.This course will touch on many, though not all, of these applications. Before discussing applica-tions, however, we must build a firm basis in the physical and chemical fundamentals.P HYSICS OF THE N UCLEUSEarly Development of Atomic and the Nuclear TheoryThat all matter consists of atoms was first proposed by John Dalton, an English school teacher, in 1806. Prout showed in 1815 that atomic weights were integral multiples of the mass of hydrogen.This observation was strong support for the atomic theory, though it was subsequently shown not to hold for all elements. J. J. Thomson developed the first mass spectrograph in 1906 and provided the answer as to why the Law of Constant Proportions did not always hold: those elements not having in-teger weights had several isotopes, each of which had mass that was an integral multiple of the mass of H. In the mean time, Rutherford had made another important observation: that atoms con-sisted mostly of empty space. This led to Bohr’s model of the atom, proposed in 1910, which stated that the atom consisted of a nucleus, which contained most of the mass, and electrons in orbit about it.It was nevertheless unclear why some atoms had different mass than other atoms of the same element. The answer was provided by W. Bothe and H. Becker of Germany and James Chadwick of England: the neutron. Bothe and Becker discovered the particle, but mistook it for radiation.Chadwick won the Nobel Prize for determining the mass of the neutron in 1932. Various other experiments showed the neutron could be emitted and absorbed by nuclei, so it became clear t h a tdiffering numbers of neutrons caused some atoms to be heavier than other atoms of the same element.This bit of history leads to our first basic observation about the nucleus:it consists of protons and neutrons.Some Definitions and UnitsBefore we consider the nucleus in more detail, let’s set out some defi-nitions:N : the number of neutrons, Z : the number of protons (this is the same as atomic number, since the number of protons dictates the chemical properties of the atom), A : Mass number (N+Z), M : Atomic Mass, I :Neutron excess number (I=N-Z). Iso-topes have the same number of pro-tons, but different numbers of neutrons; isobars have the same mass number (N+Z); isotones have the same number of neutrons but different number of protons.The basic unit of nuclear mass is the dalton (formerly known as the amu, or atomic mass unit), which is based on the mass 12C ≡12, that is,the mass of 12C is 12 daltons. The masses of atomic particles are:proton: 1.007593 daltons (oramu, atomic mass units) =1.6726231 x 10-27 kg neutron 1.008982 daltonsFigure 1.1 Neutron number vs. proton number for stable nu-clides.electron 0.000548756 daltons = 9.10093897 x 10-31 kgNucleons, Nuclei, and Nuclear ForcesFigure 1.1 is a plot of N vs. Z showing which nuclides are stable. A key observation in understand-ing the nucleus is that not all nuclides (combinations of N and Z) are stable. In other words, we cannot simply throw protons and neutrons (collectively termed nucleons) together and expect them to necessarily to form a nucleus. For some combinations of N and Z, a nucleus forms, but is unstable, with half-lives from >1015yrs to <10–12sec.An interesting observation from Figure 1.1 is that N ≈Z for stable nuclei. Thus a significant portion of the nucleus consists of protons, which obviously tend to repel each other by electrostatic force. Notice also that for small A, N=Z, for large A, N>Z. This is another important observation t h a t will lead to the first model of the nucleus.From the observation that nuclei exist at all, it is apparent that another force must exist that is stronger than coulomb repulsion at short distances. It must be negligible at larger distances, otherwise all matter would collapse into a single nucleus. This force, called the nuclear force, is a manifestation of one of the fundamental forces of nature (or a manifestation of the single force in nature if you prefer unifying theories), called the strong force. If this force is assigned a strength of 1, then the strengths of other forces are: electromagnetic 10-2; weak force 10-5; gravity 10-39 (we’ll discuss the weak nuclear force later). Just as electromagnetic forces are mediated by a particle, the photon, the nuclear force is mediated by the pion. The photon carries one quantum of electromagnetic force field; the pion carries one quantum of nuclear force field. A comparison of the relative strengths of the nuclear and electromagnetic forces as a function of distance is shown in Figure 1.2.Atomic Masses and Binding EnergiesThe carbon 12 atom consists of 6 neutrons, 6 protons and 6 electrons. But using the masses listed above, we find that the masses of these 18 particles do not add to 12 daltons, the mass of 12C. There is no mistake, they do not add up. What has happened to the extra mass? The mass has been converted to the energy binding the nucleons.It is a general physical principle that thelowest energy configuration is the most stable. We would expect that if 4He is stable relative to two free neutrons and two free protons, 4He must be a lower energy state compared to the free particles. If this is the case, then we can predict from Einstein's mass-energy equiva-lence:E = mc2 1.1that the 4He nucleus has less mass that 2 free neutrons and protons. It does in fact have less mass. From the principle that the lowest energy configurations are the most stable and the mass-energy equivalence, we should be able to predict the relative stability of various nuclei from their masses alone.We define the mass decrement of an atom as:d = W – M 1.210–410–510–610–710–810–910–10Distance, 10-12 cmV,ergsFigure 1.2. The nuclear and electromagnetic potential of a proton as a function of distance from the proton.where W is the sum of the mass of the constituent particles and M is the actual mass of the atom. For example, W for 4He is W = 2m p +2m n + 2m e = 4.034248 daltons. The mass of 4He is 4.003873 daltons, so d = 0.030375 daltons. Converting this to energy using Equ. 1.1 yields 28.28 MeV. This energy is known as the binding energy . Dividing by A,the mass number, or number of nu-cleons, gives the binding energy per nucleon , E b :E b =W –M Ac21.10This is a measure of nuclear stabil-ity: those nuclei with the largestbinding energy per nucleon are the most stable. Figure 1.3 shows E b as a function of mass. Note that the nucleons of intermediate mass tend to be the most stable. This distribution of binding energy is important to the life history of stars, the abundances of the elements, and radioactive decay, as we shall see.Some indication of the relative strength of the nuclear binding force can be obtained by comparing the mass decrement associated with it to that associated with binding an electron to a proton in a hydrogen atom. The mass decrement we calculated above for He is of theorder of 1%, 1 part in 102. The massdecrement associated with bindingan electron to a nucleus of the orderof 1 part in 108. So bonds between nucleons are about 106 times stronger than bonds between electrons and nuclei.The Liquid Drop ModelWhy are some combinations of N and Z more stable than others? The answer has to do with the forces between nucleons and how nucleons are organized within the nucleus. The structure and organi-zation of the nucleus are questions still being actively researched in physics, and full treatment is cer-tainly beyond the scope of this class, but we can gain some valuable insight to nuclear stability by considering two of the simplest models of nuclear structure. The simplest model of the nucleus is the liquid-drop model , proposed by Niels Bohr in 1936. This model assumes all nucleons in a nucleus have equivalent states. As its name suggests, the model treats the binding between nucleons as similar to the binding between molecules in a liquid drop. According to the liquid-drop model, the total bindingFigure 1.3 Binding energy per nucleon vs. mass number.Figure 1.4 Variation of surface, coulomb, and volume energy per nucleon vs. mass number.of nucleons is influenced by 4 effects: a volume energy, a surface energy, an excess neutron energy, and a coulomb energy. The variation of three of these forces with mass number and their total effect is shown in Figure 1.4.In the liquid drop model, the binding energy is given by the equation:B(A,I)= a 1A – a 2A 2/3 – a 3I 2/4A – a 4Z 2/4A 3 + d1.11where:a 1: heat of condensation (volume energy µA) = 14 MeV a 2: surface tension energy = 13 MeV a 3: excess neutron energy = 18.1 MeV a 4: coulomb energy = 0.58 MeVd : even-odd fudge factor. Binding energy greatest for even-even and smallest for odd-odd.Some of the nuclear stability rules above can be deduced from equation 1.11. Solutions for equation 1.11 at constant A, that is for isobars, result in a hyperbolic function of I, as illustrated in Figure 1.5.For odd A, one nuclei will lie at or near the bottom of this function (energy well). For even A,twoFigure 1.5 Graphical illustration of total binding energies of the isobars of mass number A= 81 (left) and A=80 (right). Energy values lie on parabolas, a single parabola for odd A and two parabolas for even A. Binding energies of the 'last'proton and 'last' neutrons are approximated by the straight lines in the lower part of the figure. After Suess (1987).curves result, one for odd-odd, and one for even-even. The even-even curve will be the one with the lower (more stable) one.Odd-Even Effects and Magic NumbersSomething that we have alluded to and which the liquid drop model does not explain well is the even-odd effect. This effect is illustrated in Table 1.1. Clearly, even combinations of nuclides are much more likely to be stable than odd ones. This is the first indication that the liquid drop model does not provide a complete description of nuclear stability. Another observation not explained by the liquid drop model is the so-called Magic Numbers. The Magic Numbers are 2, 8, 20, 28, 50, 82, and 126. Some observations about magic numbers:1. Isotopes and isotones with magic numbers are unusually common (i.e., there are a lot of differentnuclides in cases where N or Z equals a magic number).2. Magic number nuclides are unusually abundant in nature (high concentration of the nuclides).3. Delayed neutron emission in fission occurs in nuclei containing N*+1 (where N* denotes a magicnumber) neutrons.4. Heaviest stable nuclides occur at N=126 (and Z=83).5. Binding energy of last neutron or proton drops for N*+1.6. Neutron-capture cross sections for magic numbers are anomalously low.7. Nuclear properties (spin, magnetic moment, electrical quadrupole moment, metastable isomericstates) change when a magic number is reached.Table 1.1. Numbers of stable nuclei for odd and even Z and NZ N A number of stable nuclei number of very long-lived nuclei(Z + N)odd odd even45odd even odd503even odd odd553even even even16511The Shell Model of the NucleusThe state of the nucleus may be investigated in a number of ways. The electromagnetic spectra emitted by electrons is the principal means of investigating the electronic structure of the atom. B y analogy, we would expect that the electromagnetic spectra of the nucleus should yield clues to its structure, and indeed it does. However, the g spectra of nuclei are so complex that not much progress has been made interpreting it. Observations of magnetic moment and spin of the nucleus have been more useful (nuclear magnetic moment is also the basis of the nuclear magnetic resonance, or NMR, technique, used to investigate relations between atoms in lattices and the medical diagnostic technique nuclear magnetic imaging).Nuclei with magic numbers of protons or neutrons are particularly stable or ‘unreactive’. This is clearly analogous to chemical properties of atoms: atoms with filled electronic shells (the noble gases) are particularly unreactive. In addition, just as the chemical properties of an atom are largely dictated by the ‘last’ valence electron, properties such as the nucleus’s angular momentum and mag-netic moment can often be accounted for primarily by the ‘last’ odd nucleon. These observations sug-gest the nucleus may have a shell structure similar to the electronic shell structure of atoms, and leads to the shell model of the nucleus.In the shell model of the nucleus, the same general principles apply as to the shell model of the atom: possible states for particles are given by solutions to the Schrödinger Equation. Solutions to this equation, together with the Pauli Exclusion principle, which states that no two particles can have exactly the same set of quantum numbers, determine how many nucleons may occur in each shell. In the shell model, there are separate systems of shells for neutrons and protons. As do electrons, pro-tons and neutrons have intrinsic angular momentum, called spin, which is equal to 1/2h (h =h/2π, where h is Planck's constant and has units of momentum, h = 6.626 x 10-34 joule-sec). The total nuclear angular momentum, somewhat misleadingly called the nuclear spin, is the sum of (1) the intrinsic an-gular momentum of protons, (2) the intrinsic angular momentum of neutrons,and (3) the orbital angular momentum ofnucleons arising from their motion in thenucleus. Possible values for orbitalangular momentum are given by l , theorbital quantum number, which mayhave integral values. The total angular momentum of a nucleon in the nucleus is thus the sum of its orbital angular momentum plus its intrinsic angular momentum or spin: j = l ± 1/2. The plus or minus results because the spin angular momentum vector can be either in the same direction or opposite direction of the orbital angular momentum vector. Thus nuclear spin is related to the constituent nucleons in the manner shown in Table 1.2.Let’s now return to magic numbers and see how they relate to the shell model. The magic numbers belong to two different arithmetic series:N = 2, 8, 20, 40, 70, 112...N = 2, 6, 14, 28, 50, 82, 126...The lower magic numbers are part of the first series, the higher ones part of the second. The numbers in each series are related by their third differences (the differences between the differences between the differences). For example, for the first of the above series:28204070112Difference 612203042Difference 681012Difference 222This series turns out to be solutions to the Schrödinger equation for a three-dimensional harmonic os-cillator (Table 1.3). (This solution is different from the solution for particles in an isotropic Coulomb field, which describes electron shells).Table 1.3. Particles in a Three-Dimensional Harmonic Oscillator (Solution of Schrödinger Equation)Nl 010213j1/21/23/21/23/25/21/23/25/27/2State s +p -p +s +d -d +p -p +f -f +No.2242462468S 261220Total(2)(8)(20) (40)N is the shell number; No. gives the number of particles in the orbit, which is equal to 2j +1; S gives the number ofparticles in the shell or state, and total is the total of particles in all shells filled. Magic number fail to follow the progression of the first series because only the f state is available in the fourth shell.Magnetic MomentA rotating charged particle produces a magnetic field. A magnetic field also arises from the orbital motion of charged particles. Thus electrons in orbit around the nucleus, and also spinning about an in-ternal axis, produce magnetic fields, much as a bar magnet. The strength of a bar magnet may be mea-sured by its magnetic moment, which is defined as the energy needed to turn the magnet from a posi-tion parallel to an external magnetic field to a perpendicular position. For the electron, the spin magnetic moment is equal to 1 Bohr magneton (µe ) = 5.8 ¥ 10-9 ev/gauss. The spin magnetic moment of the proton is 2.79 nuclear magnetons, which is about three orders of magnitude less than the Bohr magneton (hence nuclear magnetic fields do not contribute significantly to atomic ones). Surprisingly,in 1936 the neutron was also found to have an intrinsic magnetic moment, equal to -1.91 nuclear magne-tons. Because magnetism always involves motion of charges, this result suggested there is a non-uni-Table 1.2. Nuclear Spin and Odd-Even Nuclides Number of Nucleons Nuclear Spin Even-Even 0Even-Odd 1/2, 3/2, 5/2, 7/2 ...Odd-Odd 1,3form distribution of charge on the neutron, which was an early hint that neutrons, and protons, werecomposite particles rather than elementary ones.Total angular momentum andmagnetic moment of pairs of protons cancel because the vectors of each member of the pair are aligned in opposite directions. The same holds true for neutrons. Hence even-even nuclei have 0 angular momentum and magnetic moment.Angular momentum, or nuclear spin,of odd-even nuclides can havevalues of 1/2, 3/2, 5/2, and non-zero magnetic moment (Table 1.2). Odd-odd nuclei have integer value of angular momentum or 'nuclear spin'.From this we can see that the angu-lar momentum and magnetic mo-ment of a nuclear are determined by the last nucleon added to the nu-cleus. For example, 18O has eight protons and 10 neutrons, and hence 0angular momentum and magnetic moment. Adding one proton to this nucleus transforms it to 19F, which has angular momentum of 1/2 and magnetic moment of ~2.79. For thisreason, the shell model is also sometimes called the single-parti-cle model, since the structure can be recognized from the quantum-mechanical state of the “last”particle (usually). This is a little surprising since particles are assumed to interact.The three-dimensional harmonic oscillator solution explains only the first three magic numbers;magic numbers above that belong to another series. This difference may be explained by assuming there is a strong spin-orbit interaction, resulting from the orbital magnetic field acting upon the spin magnetic moment. This effect is called the Mayer-Jensen coupling. The concept is that the energy state of the nucleon depends strongly on the orientation of the spin of the particle relative to the orbit, and that parallel spin-orbit orientations are energetically favored, i.e., states with higher values of j tend to be the lowest energy states. This leads to filling of the orbits in a somewhat differ-ent order; i.e., such that high spin values are energetically favored. Spin-orbit interaction also occurs in the electron structure, but it is less important.Pairing EffectsIn the liquid-drop model, it was necessary to add a term d , the even-odd effect. This arises from a 'pairing energy' that exists between two nucleons of the same kind. When proton-proton and neutron-neutron pairing energies are equal, the binding energy defines a single hyperbola as a function of I (e.g., Figure 1.4). When they are not, as is often the case in the vicinity of magic numbers, the hyper-bola for odd A splits into two curves, one for even Z, the other for even N. An example is shown in Figure 1.6. The empirical rule is: Whenever the number of one kind of nucleon is somewhat larger than a magic number, the pairing energy of this kind of nucleon will be smaller than the other kind.neutron excess number in the vicinity of N=50.Capture Cross-SectionsInformation about the structure and stability of nuclei can also be obtained from observations of the probability that a nucleus will capture an additional nucleon. This probability is termed the cap-ture-cross section, and has units of area. Neutron capture cross sections are generally of greater use than proton capture cross sections, mainly because they are much larger. The reason for this is simply that a proton must overcome the repulsive coulomb forces to be captured, whereas a neutron, being neu-tral, does not feel the electrostatic forces. Neutron-capture cross sections are measured in barns, which have units if 10-24 cm2, and are denoted by s. The physical cross-section of a typical nucleus (e.g., Ca) is of the order of 5 x 10-25 cm2, and increases somewhat with mass number (more precisely, R = r0A1/3, where A is mass number and r0 is the nuclear force radius, 1.4 x 10-13 cm). While many neutron capture cross sections are of the order of 1 barn, they vary from 0 (for 4He) to 105 for 157Gd, and are not simple functions of nuclear mass (or size). They depend on nuclear structure, being for example, gen-erally low at magic numbers of N. Capture cross-sections also dependent on the energy of the neutron, the dependence varying from nuclide to nuclide.Collective ModelA slightly more complex model is called the collective model. It is intermediate between the liq-uid-drop and the shell models. It emphasizes the collective motion of nuclear matter, particularly the vibrations and rotations, both quantized in energy, in which large groups of nucleons can partici-pate. Even-even nuclides with Z or N close to magic numbers are particularly stable with nearly per-fect spherical symmetry. Spherical nuclides cannot rotate because of a dictum of quantum mechanics that a rotation about an axis of symmetry is undetectable, and in a sphere every axis is a symmetry axis. The excitation of such nuclei (that is, when their energy rises to some quantum level above the ground state) may be ascribed to the vibration of the nucleus as a whole. On the other hand, even-even nuclides far from magic numbers depart substantially from spherical symmetry and the excita-tion energies of their excited states may be ascribed to rotation of the nucleus as a whole.R EFERENCES AND S UGGESTIONS FOR F URTHER R EADINGDickin, A. 1995. Radiogenic Isotope Geochemistry. Cambridge: Cambridge University Press.Faure, G. 1986. Principles of Isotope Geology. New York: Wiley & Sons.Suess, H. E. 1987. Chemistry of the Solar System. New York: John Wiley and Sons.。

第七讲同位素地球化学Re-Os同位素体系

0.02 1.59 1.96 13.24 16.15 26.26 40.78
其中187Os和186Os分别为187Re和190Pt的衰变子体。
Re-Os同位素地球化学特征
187Re的半衰期为约42 Ga(= 1.666 10-11 a-1)，且Re的地球化学性质与钼元素十分相近，因此 Re-Os体系作为定年(如辉钼矿)和示踪研究方法较早得到了地质应用；
Table 1 Osmium isotope ratios and isotope percentage for the University of Maryland at College Park (UMCP) Johnson-Matthey Os standarda
Isotopic ratio Measured value Isotope
Percentage composition
184Os/188Os 0.0013313 ± 13 184Os
0.0177
186Os/188Osb 0.119848 ± 8
186Os
1.593
187Os/188Osb 0.113791 ± 15 187Os
1.513
189Os/188Os 1.21967 ± 14
0.075
Picrite, Basalt
Olivine
20
5
Picrite, Basalt
Sulfide
2000000
Synth MORB Olivine
Experimental Experimental phenocryst-matrix phenocryst-matrix 0.001 Experimental
Vanwestrenen et al. 2000

U-Pb同位素年代学(含作业)ppt课件

但是，由于在低级变质作用和表生风化作用中 Pb、Th特别是U的活动性较大，因此硅酸盐岩石的U-Pb和Th-Pb体系很少保持封闭：
例如美国怀何明州的Granite Mountain岩基的全岩Th-Pb分析给出大致的等时线年龄为2.8Ga，但U-Pb等时线图显示U已大量丢失而无法给出年龄结果（Rosholt and Bartel, 1969）
9. U-Th-Pb同位素
9.1、U、Th、Pb的地球化学
U、Th是锕系元素，
U、Th在自然界多以四价氧化态出现，其离子半径相近（U4+=0.105nm, Th4+=0.11nm），两者可以相互置换
在氧化条件下，U形成可溶于水的铀酰离子（UO22＋）而成为活动性元素 Th只呈四价氧化态，且一般不溶于水
晶质铀矿和独居石最初被用来定年，但其有限的分布限制了它们的应用。
锆石在中-酸岩中是一个广泛分布的富铀矿物，因此被广泛地应用于U-Pb定年。
基性岩中的斜锆石也被用来定年（Krogh et al., 1987）。
2 2 P U * 0 3 ( e 2 t - 6 1 3 8 ) b 8
2 P * 0 7 b 式中*号代表放射成因铅
232Th
100.00
14.010× 109
4.9475× 10-11
238U/235U=137.88
由于238U、235U、232Th的半衰期比它们的子体的半衰期长得多，即其衰变常数比子体的衰变常数小得多，符合建立长期平衡的条件（见“衰变定律”一节）
经过数百万年以上的地质时代，并且矿物保持封闭体系，就能达到长期平衡状态:
Isotope Abundance(% ) Half-life(years) Decayconstant(y-1)

同位素基础获奖课件

大，垂直层理方向变化较大
■从矿床底部到顶部，δS34具有增大趋势 ■在共生矿物中： δS34黄铁矿＞δS34闪锌矿＞δS34方铅矿
三、硫同位素旳地质应用
1、鉴别成岩物质起源
■在地质作用过程中，因为多种硫化物旳形成条件不同，相应旳硫同位素构成也不同，所以硫同位素构成也就能够用来鉴别成岩物质起源。
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17
每个测定样品旳δ(‰)值可正可负，正值表达与原则相比所测样品中重同位素有一定旳富集，而负值则表达重同位素有一定旳贫化，亦即轻同位素有所富集。
不同相（不同矿物、液体、气体）中同位素构成不同，即产生了同位素分馏，两相间同位素比值之商称为同位素分馏系数
R / R， RA 、RB分别为A相及B相中重同位素
(
D H
)标准
1000
(
D H
)标准
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同位素分析资料要能够进行世界范围内旳比较，就必须建立世界性旳原则样品。世界各国所采用旳原则样品已基本统一。国际原则样品旳名称及其同位素绝对比值见下：
氢、碳、氧、硫同位素原则样品
元
标
准
素
H 平均大洋水标准（Standard Mean Ocean Water）
24
2、花岗岩旳硫化物 ■因为花岗岩成因复杂、多样，故其硫化物旳
δS34值也不相同 ■一般由幔源衍生而来旳花岗岩，其硫化物中
旳δS34值在-3~+8‰之间，且单个岩体中δS34 值变化范围窄，阐明成岩物质比较均匀
■ S花岗岩δS34值为-9.4~+7.6 ‰ ■ I花岗岩δS34值为-3.6~+5.0 ‰
与轻同位素A旳比值B。
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18-同位素地球化学PPT课件

235U 231 Th ...... 227 Th 223 Ra ...... 211 Bi ...... 207 Pb
235U → 207Pb + 7 + 4 - + E
t1/2 = 704 106yr 2= 9.8485 10-10yr-1
232Th 228 Ra ...... 224 Ra 220 Rn ...... 212 Pb ...... 208 Pb
Pb Pb
207 204
Pb Pb
i
235U 204 Pb
(e235t
1)
208 Pb 204 Pb
208 204
Pb Pb
i
232Th (e232t 204 Pb
1)
11
U-Th-Pb同位素体系
由于U-Th-Pb体系可构成相互独立的衰变体系，原理上对同一地质体进行同位素分析，可获得三个独立的年龄值。如所分析样品符合等时线前提，238U-206Pb、235U207Pb、232Th-208Pb体系应该给出一致的年龄（相对误差小于或等于10%）；然而，由于U、Pb的易活动性，在风化作用和低级变质作用，常可导致体系U-Pb同位素组成不能保持封闭，往往得不到一致年龄，因此用等时线法成功获得有意义年龄的实例相对有限。
20
谐和曲线
假如矿物形成于3.0Ga，则其具谐和年龄的207Pb/235U、
206Pb/238U比值将分别为 18.1902和0.59249
21
如果样品铅丢失或铀获得，则坐标点落在一致曲线下方;
如果样品铅获得或铀丢失，则坐标点落在一致曲线上方.
若样品形成后丢失部分放射成因铅，随后又封闭，那么一组经历相同的样品，在207Pb*/ 235U和 206Pb*/ 238U为横纵坐标的图上的数据点就构成了一条直线，即不一致线。

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