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
陈福坤
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
Parent-Daughter System
λt
D = D0 + N (e -1)
87 87 87 λt
Sr = Sr0 + Rb (e -1)
87 86 87 86 87 86 λt
Sr/ Sr = ( Sr/ Sr)0 + Rb/ Sr(e –1)
(D-D )
t = 1 ln 0 + 1
λ N
Basis for isochron (等时线）
(λt)2 (λt)3
eλt = 1 + λt+ + + ...
2! 3!
when λt <<1, eλt ~ 1 + λt
λt
D = D0 + N (e -1)
= D0 + Nλt
87 87 87 86 87 86
Sr/ Sr = ( Sr/ Sr)0 + ( Rb/ Sr)λt
Rb-Sr isochron diagram
(Nicolaysen, 1961)
Y ＝ c + mX
c: initial isotopic ratio
Slope of this line m = eλt -1
Type of isochron **
Internal isochron (内部等时线)
All analyzed fractions come from one sample, .,
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 genetic-
related geological body, ., a granite pluton
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.