放射成因同位素在地学领域的应用.pptVIP

Radiometric dating - A time machine to the past 放射性核素衰变规律 衰变速度与放射性核素含量成比例 为什么超净化环境？ － 样品中待分析组分含量低 － 降低环境和人为因素对样品的影响， 即降低实验室本底 为什么分离和纯化样品？ － 除去样品中其它元素的干扰 如：Ca、Fe、Mg离子 － 除去同质异素的叠加 如: 87Rb对87Sr的叠加 144Sm对144Nd的叠加 150Sm对150Nd的叠加 质谱计的基本原理和主要组成 Device used to measure the mass of a given type of atom Mass spectrum: Position of peaks give the atom mass Relative heights of the peaks indicate relative number of atoms of different masses 部分熔融过程微量元素的分馏作用及壳幔的组成特征 Differentiation of the Earth Melts extracted from the mantle rise to the crust, carrying with them their “enrichment” in incompatible elements Continental crust becomes “incompatible element enriched” Mantle becomes “incompatible element depleted” Magma differentiation Fractional Crystallization Assimilation, fractional crystallization Zircon dating methods U-Pb同位素稀释法定年 Pb-Pb蒸发定年 微区原位U-Pb定年 (如：SHRIMP) SiO2 (wt.%) 87Sr/86Sr 50 60 70 80 .704 .706 .708 Fractionation trend Contamination trend Magma mixing 87Sr/86Sr 0.704 0.706 0.708 0.710 -4 0 4 Mixing trend eNd End member 1 End member 2 e.g. depleted mantle e.g. crustal material 等时线原理、类型和基本要求 Y ＝ c + mX Slope of this line m = elt - 1 c: initial isotopic ratio elt = 1 + lt + + (lt)2 2! (lt)3 3! + ... elt 1 + lt ~ ~ when lt 1, D = D0 + N (elt-1) = D0 + Nlt Basis for isochron (等时线） 87Sr/87Sr = (87Sr/86Sr)0 + (87Rb/86Sr)lt Rb-Sr isochron dating 5 10 0.71 0.72 0.73 87Rb/86Sr 87Sr/86Sr m = (elt -1) Initial 87Sr/87Sr ratio The isotope system of interest was at isotopic equilibrium at time t = 0. Isotopic equilibrium in this case means the system had a homogeneous, uniform value of D0. The isotope system as a whole and each analyzed part of it was closed between t = 0 and time t (usually the present time). By “closed system” we mean there has been no transfer of the parent or the daughter element into or out of the system. Type of isochron Internal isochron (内部等时线) All analyzed fractions