铷-锶定年方法

• Campbell & Wood (1906) found natural radioactivity of rubidium (Rb) • Hahn et al. (1937) & Mattauch (1937) identified 87Rb as the naturally occurring radioactive isotope • Hahn & Walling (1938) pointed out the feasibility of dating Rb-bearing minerals by the decay of 87Rb to 87Sr • Hahn et al. (1943) reported the first age determination by the Rb-Sr method • Wide use began since 1950’s after the availability of mass spectrometer of the Nier’s design • Gast (1960) first proposed Sr isotopes for geochemical tracer • Nicolaysen (1961) proposed the isochron method for the RbSr dating
Rb has an ionic radius of 1.48Å 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 (1.33Å), therefore, Rb substitutes readily for K in K-bearing minerals, such as mica and K-feldspar
铷 (Rubidium)
Group 1 alkali metal, trace element in the Earth 72.17% Atomic abundance ratio: 27.83%
85Rb/87Rb
= 2.59265
85Rb
87Rb
a constant throughout the Earth, moon and most meteorites due to isotopic homogenization in the solar nebula.
Rb – Sr isotope system
Number of protons
38 37
84Sr
86Sr 85Rb
87Sr
88Sr 87Rb
46
48
49
50
Number of neutrons
87Rb 87Sr
+ β-
T1/2 = 4.88 x 1010 (year); λ = 1.42 x 10-11 (year-1)
(Nicolaysen, 1961)
Y ＝ c + mX
c: initial isotopic ratio Slope of this line m = eλt - 1
increasing rates of 87Sr/86Sr values with different 87Rb/86Sr
Different samples (whole-rocks and minerals) can have variant values of 87Sr/86Sr ratio, and how?
Decay constant
The halflife of 87Rb was difficult to determine as the low maximum energy of the β- particles it emits and as slow rate of its decay Prior to 1977, two different values of decay constant of 87Rb were in use: λ = 1.39 x 10-11 y-1 and λ = 1.47 x 10-11 y-1 In 1977, the Subcommission on Geochronology of the International Union of Geological Sciences recommended a value of 1.42 x 10-11 y-1 (corresponding to halflife time of 4.88 x 109 y) for the decay constant of 87Rb, based on the measurements by Neumann & Huster (1974), Davis et al. (1977) and Steiger & Jaeger (1977)
(λt)2 (λt)3 eλt = 1 + λt + + + ... 2! 3!
when λt <<1,
eλt ~ 1 + λt ~
D = D0 + N (eλt-1) = D0 + Nλt
87Sr/87Sr
= (87Sr/86Sr)0 + (87Rb/86Sr)λt λ
Rb-Sr isochron diagram
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.
Geochemical feature of Rb and Sr
Both Rb and Sr are trace elements in the Earth Rb is a highly soluble, highly incompatible element. Sr is also relatively soluble and is fairly incompatible in mafic and, particularly, ultramafic igneous system
Continental crust: 32-78 ppm Rb 260-333 ppm Sr
Depleted Mantle: 0.6 ppm Rb 19.9 ppm Sr
Average contents (ppm) of Rb, K, Sr, Ca in igneous and sedimentary rocks
Sr is relatively compatible in silica-rich igneous systems, partitioning preferentially into plagioclase The mantle has a relatively uniform and low 87Sr/86Sr ratio, while the continental crust has a much more variable, and, on average, higher ratio.
87Sr/86Sr
0.73 0.72 0.71
later
start 5 10
87Rb/86Sr
One 87Rb atom decays and produces one 87Sr atom
Rb-Sr isochron dating
m = (eλt -1)
87Sr/86Sr
0.73 0.72 0.71 5 10
Atomic abundance
锶ne earth metal, trace element in the Earth
Average
82.53%
Stable isotopes:
84Sr, 86Sr
and 88Sr
Radiogenic isotope: 87Sr
Isochron 等时线
Parent-Daughter System
D = D0 + N (eλt-1)
87Sr
= 87Sr0 + 87Rb (eλt-1)
= (87Sr/86Sr)0 + 87Rb/86Sr(eλt – 1)
87Sr/86Sr
1 ln (D-D0) + 1 t= N λ
How to obtain value of the initial value (D0)?
9.87% 0.56%
84Sr 86Sr 87Sr 88Sr
7.04%
Atomic abundance ratio:
88Sr/86Sr 87Sr/86Sr
= 8.3752 ratio changes with
time.
Atomic abundance
An important advantage of the Rb-Sr isotopic 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 in different rock types or different minerals Accuracy of an age determination depends heavily on the spread of measured Rb/Sr ratios, this makes this system a useful geochronological tool