年代学-陈福坤-第四节-年代学.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 Principle of Isotope Geochronology Principle of Isotope Geochronology Parent-Daughter System D D0 N eλt-1 87Sr 87Sr0 87Rb eλt-1 87Sr/86Sr 87Sr/86Sr0 87Rb/86Sreλt– 1 t D-D0 N ln λ 1 1 eλt 1 λt λt2 2 λt3 3 ... eλt1 λt when λt 800 OC Monazite 650-740 OC Sphene titanite 500-670 OC Rutile420-380 OC Apatite 350 OC 铷－锶定年方法 Rb-Sr dating 铷－锶定年方法 Rb-Sr dating 84Sr86Sr88Sr87Sr 87Rb85Rb 4648 37 38 4950 Number of neutrons Number of protons Rb – Sr isotope system 87Rb 87Sr β- T1/2 4.88 x 1010year; λ 1.42 x 10-11year-1 85Rb87Rb 72.17 27.83 铷 Rubidium Group 1 alkali metal, trace element in the Earth Atomic abundance Atomic abundance ratio 85Rb/87Rb 2.59265 a constant throughout the Earth, moon and most meteorites due to isotopic homogenization in the solar nebula. 锶 Strontium Group 2 alkaline earth metal, trace element in the Earth Stable isotopes 84Sr, 86Sr and 88Sr Radiogenic isotope 87Sr Atomic abundance ratio 88Sr/86Sr 8.3752 87Sr/86Sr ratio changes with time. 86Sr88Sr 9.87 82.53 Atomic abundance 87Sr84Sr 7.04 0.56 Average This decay system was one of the first to be widely used in geochronology and remains one of the most useful geochemical tracers. An important advantage of the system is relatively large variations of the Rb/Sr ratio in rocks. As difference in geochemical properties of Rb and Sr, Rb/Sr ratio can vary by several orders of magnitude. Accuracy of an age determination depends heavily on the spread of measured Rb/Sr ratios, this makes this system a useful geochronological tool. Rb is a highly soluble, highly incompatible element. Sr is also relatively soluble and is fairly incompatible in maficand, particularly, ultramafic igneous system. Both Rb and Sr are trace elements in the Earth. But, Sr is relatively compatible in silica- rich igneous systems, partitioning preferentially into plagioclase. the mantle has a relatively uni and low 87Sr/86Sr ratio, while the continental crust has a much more variable, and, on average, higher ratio. Rb has an ionic radius of 1.48A. This large ionic radius means it is excluded from many minerals and, hence it is one of the most incompatible elements. 。 Its radius is sufficiently similar to that of potassium 1.33A, therefore, Rb substitutes readily for K in K-bearing minerals, such as mica and K-feldspar. 。 。 The ionic radius of Sr is 1.13A. It is still sufficiently large and it is also excluded from many minerals. It is an incompatible element, but not a highly incompatible one. Sr substitutes for Ca ionic radius 0.99A to varying degrees. It is quite comfortable in the Ca site in plagioclase. 。 In igneous and high-grade metamorphic rocks, most Sr will be in plagioclase. Sr can also substitute for Ca in other minerals, such as calcite, apatite, gypsum, sphene, etc. Sr is also concentrated in the crust relative to the mantle, but not to the degree that Rb is. Sr isotope evolution diagram of the Earth 87Sr/87Sr 87Sr/86Sr0 87Rb/86Srλt basaltic achondrite best initial Continental crust 32-78 ppm Rb 260-333 ppm Sr Depleted Mantle 0.6 ppm Rb 19.9 ppm Sr Time Ma 87Sr/86Sr In the Rb-Sr Sm-Nd as well system, both the initial ratio and the age are virtually always unknown, meaning we must solve for both simultaneously through the isochron . To obtain a meaningful isochron the following conditions must be met Rb-Sr dating 1. 87Rb/86Sr ratio should be large If yes, variations in Sr isotopic composition will be large relative to our ability to measure them. Under the best of circumstances, isotope ratios can be measured with a precision of 10 ppm or so. If the total amount of radioactively produced 87Sr is small relative to the amount present initially, there is little hope of using the system to produce reasonable “ages”. 2. 87Rb/86Sr ratios should have a large range A large range in 87Rb/86Sr ratio leads to a large range in isotope ratios in the daughter Sr. The error on the regression slope is a function of the range of values used in the computation. So given similar analytical precisions, we will obtain a more precise date with a decay system where the variations in parent-daughter ratio are large than with one where these variations are small. 3. minimaldeviations from closed system Rb tend to be more mobile than Sr and some minerals are less reactive than others. Metamorphism will disturb a system on the scale of mineral grains. An atom created by radioactive decay will generally be a misfit in the lattice site it occupies. The lattice site may be damaged by the decay process. 4. Sr isotopic composition must have been homogeneous at the time of the event Homogenization that takes place through diffusion, is highly temperature dependent. The higher the temperatures obtained during the event, the more homogenized the system will be. Rb-Sr dating using the isochron approach One 87Rb atom decays and produces one 87Sr atom 510 0.71 0.72 0.73 87Rb/86Sr 87Sr/86Sr start later Rb-Sr isochron dating 510 0.71 0.72 0.73 87Rb/86Sr 87Sr/86Sr m eλt-1 Initial 87Sr/87Sr ratio 87Sr/86Sr ratios of seawater through Phanerozoic time Because of its long residence time of Srin seawater, the 87Sr/86Sr ratio of seawater is homogeneous at any given time. The 87Sr/86Sr ratio of seawater is controlled by the relative of Sr from the continents and ridge- crest hydrothermal activity. The ratio of these will vary with mean spreading rate, erosion rates, and plate geometry. Seawater 87Sr/86Sr 0.709 hydrothermal fluids 87Sr/86Sr 0.703 river water 87Sr/86Sr 0.711 carbonate shells 87Sr/86Sr 0.709 Sr isotope composition of the oceans is determined by the relative contributions of Sr from river waters and hydrothermal sources Sr isotope chronostratigraphy is rapidly developing. Since the history of 87Sr/86Sr in seawater is known, an estimate of the age of an authigenic mineral such as calcite can be obtained simply by determining its initial 87Sr/86Sr and comparing that with the seawater evolution curve. 钐－钕Sm-Nd定年方法钐－钕Sm-Nd定年方法 Sm – Nd isotope system 147Sm 143Nd α T1/2 1.06 x 1011year; λ 6.54 x 10-12year-1 82 83 60 62 84 85 Number of neutrons Number of protons 148150149154147144152 143146148145142150144 Sm Nd 86 87 88 89 90 钐 Samarium Group La, rare earth element Atomic abundance 144Sm 3.09 147Sm 14.97 148Sm 11.24 149Sm 13.83 150Sm 7.44 152Sm 26.72 154Sm 22.71 Stable isotopes 144Sm, 148Sm, 149Sm, 150Sm, 152Sm and 154Sm Radioactive isotope 147Sm 钕 Neodym Group La, rare earth element All isotopes are stable; Radiogenic isotope 143Nd Atomic abundance ratio 146Nd/144Nd 0.7219 143Nd/144Nd ratio changes with time. Atomic abundance 142Nd 27.11 143Nd 12.17 144Nd 23.85 145Nd 8.30 146Nd 17.22 148Nd 5.73 150Nd 5.63 compatibleincompatible Rare Earth Element Ionic Radii The primary chemical difference between the rare earth elements is the ionic radius, which shrinks systematically from 1.15Afor La A57 to 0.93Afor Lu A71. Ionic radius is a key factor in their geochemical behavior. Thus, there is a systematic variation in their abundances in rocks, minerals, and solutions. The ionic radii and relatively high charge 3 valence of the rare earths make them fairly unwelcome in many mineral lattices, they can be considered moderately incompatible. 。 。 Both Sm and Nd are rare earth elements and have a 3 valence. Chemical features of both are similar; they are insoluble, so the Sm/Nd ratio is so little affected by weathering and alternation. The ionic radii of Sm and Nd differ by only 0.04ANd1.08A, Sm1.04A. Nd is slightly more incompatible than Sm. 。。。 Sm-Nd isochron dating 0.51.0 0.511 0.512 0.513 147Sm/144Nd 143Nd/144Nd m eλt-1 Initial 143Nd/144Nd ratio 1. 147Sm/144Nd ratio should be large 2. 147Sm/144Nd ratios should have a large range 3. minimal deviations from closed system 4. Nd isotopic composition must have been homogeneous at the time of the event Meaningful Sm-Nd isochron dating The similarity of the chemical behavior in Sm and Ndmeans that the range in Sm/Nd and 143Nd/144Nd ratios is generally quite limited. The limited variation in Sm/Nd is certainly the major drawback of this system for geochronology. The rare earths are refractory elements, it can be assumed that their relative abundances in the Earth are the same as in chondritic meteorites. The 147Sm/144Nd ratio of the Earth is believed to be the same as the chondritic value of 0.1967, Sm/Nd 0.32. The solar system is assumed to be isotopically homogeneous when the Earth ed. The initial 143Nd/144Nd ratio of the Earth should be identical to the initial 143Nd/144Nd ratio of other bodies ed 4.55 Ga ago, including meteorite parent bodies. 143Nd/144Nd Time Ga 同位素初始比值计算 ** Calculation of initial isotopic ratio examples of Rb-Sr and Sm-Nd 测得样品的现在87Sr/86Sr和143Nd/144Nd比值 测得样品的现在Rb、Sr、Sm、Nd含量 已知样品的形成时代 87Sr/86Sr0 87Sr/86Sr - 87Rb/86Sreλt – 1 143Nd/144Nd0 143Nd/144Nd - 147Sm/144Ndeλt – 1 为什么计算同位素初始比值 计算εNd值 ** Calculation of εNdvalue 现代εNd值 143Nd/144Ndsample 143Nd/144NdCHUR εNd -1x 10000 CHUR 143Nd/144Nd 0.512638 147Sm/144Nd 0.1967 初始εNd值 143Nd/144Nd0, sample 143Nd/144Nd0, CHUR εNdt-1 x 10000 CHUR 143Nd/144Nd 0.512638 147Sm/144Nd 0.1967 143Nd/144Nd0, sample 143Nd/144Nd - 147Sm/144Ndeλt – 1 143Nd/144Nd0, CHUR 143Nd/144Nd - 147Sm/144Ndeλt – 1 Sm-Nd模式年龄（TCHUR和TDM）计算 143Nd/144Ndsam 143Nd/144Nd0 147Sm/144Ndsameλt– 1 TCHUR 143Nd/144NdCHUR 143Nd/144Nd0 147Sm/144NdCHUReλt– 1 143Nd/144Ndsam- 143Nd/144NdCHUR 147Sm/144Nd - 147Sm/144NdCHUR x eλt– 1 TCHUR 1 λ 143Nd/144Ndsam- 143Nd/144NdCHUR 147Sm/144Ndsam- 147Sm/144NdCHUR 1ln 143Nd/144Ndsam 143Nd/144Nd0 147Sm/144Ndsameλt– 1 TDM 143Nd/144NdDM 143Nd/144Nd0 147Sm/144NdDMeλt– 1 143Nd/144Ndsam- 143Nd/144NdDM 147Sm/144Ndsam- 147Sm/144NdDM x eλt– 1 TDM 1 λ 143Nd/144Ndsam- 143Nd/144NdDM 147Sm/144Ndsam- 147Sm/144NdDM 1ln Ch e mi c a l p r o c e d u r e f o r R b - Sr a n d Sm- Nds e p a r a t i o n a n d p u r i f i c a t i o n Sample digestion 1.Weight sample 2.Weight spike solution 87Rb, 84Sr, 150Nd, 149Sm 3.Add concentrated HF acid and a small amount of HClO4acid 4.Heat at high temperature and/or high pressure 5.After digestion, re-dissolution of the fluorides into chloride salts Rb/Sr and Sm/Nd separation 铷、锶和稀土元素分离 5-ml resin bed of AG 50W-X12, 200-400 mesh 铷－锶同位素分离纯化图例 Ion chromatography REE 钐－钕位素分离纯化 Quartz column using 1.7 ml Teflon powder coated with HDEHP as exchange medium 5-ml resin bed of AG 50W-X12, 200-400 mesh Rb, Sr and REE from others Sm and Nd from other REEs 钐－钕位素分离纯化图例 Time 0.3 N HCl0.7 N HCl LaCe PrNdSm Richard et al. 1976 Measurements of Rb, Sr, Sm, Ndisotope ratios using thermal ionization mass spectrometer TIMS uation of measured Rb, Sr, Sm, Nd isotopic data 谢谢