放射成因同位素

All analyzed fractions come from one sample, e.g., whole rock powder and its minerals
External isochron (外部等时线)
All analyzed fractions come from different samples, which are collected from different parts of a geneticrelated geological body, e.g., a granite pluton
Fractional Crystallization
Assimilation, fractional crystallization
.708
87Sr/86Sr
Contamination trend
.706 Fractionation trend .704
50
60
70
80
SiO2 (wt.%)
Magma mixing
Rb-Sr isochron dating
m = (eλt -1)
87Sr/86Sr
0.73 0.72 0.71 5 10
87Rb/86Sr
Initial 87Sr/87Sr ratio
The isotope system of interest was at isotopic equilibrium at time t = 0.
Continental Crust
La Lu
Rb>Sr Nd>Sm La>Lu
Differentiation of the Earth
Mantle
(After partial melt extraction)
Rb>Sr Nd>Sm La>Lu
Rb<Sr Nd<Sm La<Lu
La
Lu
Melts extracted from the mantle rise to the crust, carrying with them their “enrichment” in incompatible elements
固体放射成因同位素地球化学分析流程
溶解样品
化学分离与纯化
离子色层分离
质谱计同位素比值测定
分析结果计算和评价
超净化实验室环境下同位素分离
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 如何尽可能降低本底？ － 净化实验室空气 － 净化化学试剂和器皿
Enriched Depleted Primitive
Upper Crust Higher Rb/Sr and 87Sr/86Sr ratio Lower Sm/Nd and 143Nd/144Nd ratio Lower Rb/Sr and 87Sr/86Sr ratio Lower Sm/Nd and 143Nd/144Nd ratio Lower Rb/Sr and 87Sr/86Sr ratio Higher Sm/Nd and 143Nd/144Nd ratio
How we can know the concentration?
Pb
204P
Sample 样品
How many?
Spike 稀释剂
Pb
205P
b
206P
b
b
207P
b
208P
b
Red = 24
Red/ Red/blue =4
Blue = 6
Isotope dilution analysis
铷同素为例
– Continental crust becomes “incompatible element enriched” – Mantle becomes “incompatible element depleted”
地壳和地幔Nd和Sr同位素体系特征
Sm-Nd and Rb-Sr Isotope Systematics
地球Sr和Nd同位素演化
87Sr/87Sr
= (87Sr/86Sr)0 + (87Rb/86Sr)λt
basaltic achondrite best initial
87Sr/86Sr
Time (Ma)
143Nd/144Nd
Time (Ga)
同位素初始比值计算
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/86Sr)0 = 87Sr/86Sr - 87Rb/86Sr(eλt – 1) (143Nd/144Nd)0 = 143Nd/144Nd - 147Sm/144Nd(eλt – 1)
放射性核素衰变规律
衰变速度与放射性核素含量成比例
dN dt
− λN
或
dN N
−λ dt
同位素地球化学在地学领域 的影响
同位素地球化学在地学领域研究中的地 位反映在对地学的基础问题研究的显著 性的成功。 同位素地球化学在将地质学由定性观察 向现代的定量化科学的转变发挥重要的 作用。
同位素地球化学在下述领域产生影响： 矿床成因研究、岩石成因研究、地壳演 化、地幔动力学、全球构造演化和动力 学、火山岩石学研究、水文学、古生物 研究、海洋对流循环、人类学和考古学 研究、古气候古环境研究、环境检测和 保护、宇宙射线检测，等等。
部分熔融过程微量元素的分馏作用及壳幔的 组成特征
La Lu
Ni La Region of Partial Melting Nd Sm Co La Lu Melting Residue Rb Sr
Partial melting of rock
Preference of incompatible elements Melt
Rb is more incompatible than Sr; Nd is more incompatible than Sm; During partial melting of mantle and form magma intruding in to crust, Rb-Sr and Sm-Nd will be fractionated. Rb and Nd are easier to go into melt relative Sm and Sr, therefore, mantle will be depleted in Rb and Nd (depleted mantle), while the crust will be enriched in Rb and Nd.
Rbmixture ＝ Rbsample + Rbspike
87Rb 87Rb 87Rb 85Rb
( 85 ) measured = ( 85 ) mix =
Rb Rb
+ 87Rbspi sam + 85Rbspi sam
铅平均原子量的计算
测得某样品的铅同位素组成(原子数比） 如： a= 206Pb/204Pb = 18.8 b= 207Pb/204Pb = 15.6 c= 208Pb/204Pb = 39.2 204Pb/204Pb = 1
Crystallization of melt
Rock Preference of compatible elements
Partial melting
Fractional crystallization
Molten rock Preference of compatible elements
Melt Preference of incompatible elements
该样品的铅平均原子量 （APb）？
铅同位素的原子质量：
204Pb
= 203.97302 206Pb = 205.97444 207Pb = 206.97587 208Pb = 207.97663
(A204) (A206) (A207) (A208)
1xA204 axA206 bxA207 cxA208 + + + APb = a+b+c+1 a+b+c+1 a+b+c+1 a+b+c+1
Lithosphere
Lower Crust
Upper Mantle
Asthenosphere
Low Rb/Sr and 87Sr/86Sr ratio High Sm/Nd and 143Nd/144Nd ratio
Lower Mantle
放射成因同位素地球化学在 岩石成因研究的应用
Magma differentiation
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).
铷－锶同位素分离纯化图例
Ion chromatography
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