Geochronology and radiogenic tracers - Geochemistry: An Introduction - page 86

18

Growth
curves

Sample 4

16

Sample 3

14

Sample 2

Sample 1

12

Isochron

10

Initial

8

8 10 12 14 16 18 20 22

206 Pb/ 204 Pb

Figure 4.11

206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios of samples formed with the same initial isotopic composition
of lead but different U/Pb ratios evolve along growth curves so that the isotopic ratios remain on
an isochron with a slope indicative of the age of formation.

207 Pb
204 Pb

207 Pb
204 Pb

0 =

235 U
204 Pb

e λ 235 U t

1

t −

− (4.31)

t

By dividing (4.31) by ( 4.30 ) , we obtain:
207 Pb

204 Pb t − 207 Pb 204 Pb 0

235 U
238 U

e λ 235 U t / /

− 1

206 Pb

204 Pb t − 206 Pb 204 Pb 0

= (4.32)

e λ 238 U t

/ /

− 1

t

Because the present ratio 235 U/ 238 U is a constant equal to 1/137.88, (4.32) can be recast
as ( y

204 Pb t ( Fig. 4.11 ),
(4.32) describes an isochron straight line going through the point of coordinates ( x 0 , y 0 )
and with slope m . The advantage of this method is that it requires only the isotope ratios
of lead to be determined and not the concentrations, in particular that of uranium which
is commonly strongly affected by water circulation in the water table and by weathering.
For this reason, this method was commonly used for dating all sorts of sedimentary and
magmatic rocks until superseded by zircon geochronology. It will be seen that this was the
first method ever to yield the age of the Solar System and it is still widely used to date
meteorites and planetary samples.

= 206 Pb

204 Pb t , y

= 207 Pb

− y 0 )/( x − x 0 ) = m . In a plot x / /

4.4 Radiogenic tracers

The property that phase separation, such as melting and crystallization, fractionates
parent/daughter ratios has received enormous attention and created the fertile concept of
radiogenic tracers. We have previously discussed the point that phase change does not
fractionate the normalized isotopic ratios themselves, a common source of confusion.