Geochronology and radiogenic tracers - Geochemistry: An Introduction - page 74

Table 4.1 Decay constants of the major radioactive systems: the daughter nuclide is shown
when it is used for dating

λ (y − 1 ) λ (y − 1 ) λ (y − 1 ) System System System

138 La- 138 Ce

2.24 × 10 − 12

40 K- 40 Ca

4.96 × 10 − 10

26 Al

9.80 × 10 − 7

147 Sm- 143 Nd

6.54 × 10 − 12

235 U- 207 Pb

9.85 × 10 − 10

36 Cl

2.30 × 10 − 6

87 Rb- 87 Sr

1.42 × 10 − 11

146 Sm- 142 Nd

6.73 × 10 − 9

230 Th

9.20 × 10 − 6

187 Re- 187 Os

1.64 × 10 − 11

244 Pu

8.66 × 10 − 9

234 U

2.83 × 10 − 6

176 Lu- 176 Hf

1.865 × 10 − 11

182 Hf- 182 W

7.7 × 10 − 8

231 Pa

2.11 × 10 − 5

232 Th- 208 Pb

4.95 × 10 − 11

129 I- 129 Xe

4.30 × 10 − 8

14 C

1.21 × 10 − 4

40 K- 40 Ar

5.81 × 10 − 11

53 Mn- 53 Cr

1.87 × 10 − 8

226 Ra

4.33 × 10 − 4

238 U- 206 Pb

1.55 × 10 − 10

10 Be

4.62 × 10 − 7

210 Pb

3.11 × 10 − 2

Table 4.1 shows that the clocks spread over a wide range but certain age ranges are not
well covered, especially that at around one million years.
Note that physical time elapsing in the real world is normally given in seconds (s), which
is not a very helpful unit in the Earth sciences, while geological ages, through which we
go back through time, are noted in anni (a), from the Latin annus . Derived units ky and ka
(thousand years), My and Ma (million years), Gy and Ga (billion years) apply to physical
time and time interval (or age), respectively. Appendix D shows the division of geological
time into absolute ages. The geological time scale is the product of the work of literally
thousands of scientists throughout the last century and cannot be credited to one particular
work. An overview of the techniques commonly used for the determination of elemental
concentrations and isotopic ratios is given in Appendix E .

4.1 Dating by radioactive nuclides

This group of methods relates essentially to nuclides produced by cosmic radiation, but we
will see that the approach can be generalized to the descendants of uranium and thorium
with methods based on the surpluses of these nuclides. Here we make an assumption about
the initial isotopic composition of the element to which the radioactive nuclide belongs.

4.1.1 Carbon-14

This method of dating, which is certainly the most familiar to the general public, is not
the oldest historically. However, it has revolutionized archeology and earned its inventor,
Libby (see Arnold and Libby, 1949 ) , the Nobel Prize for Chemistry in 1960. The Earth
is subjected to bombardment from high-energy galactic cosmic rays, mostly protons and
α particles, which react with the Earth's atmosphere. The interaction of these particles

with nitrogen and oxygen produces secondary particles, mostly neutrons. In spite of a