Geoscience Reference
In-Depth Information
less
than 0.704. Thus, the initial ratio provides an excellent method of determining
the origin of some rocks.
6.4 Uranium-lead
There are three naturally occurring isotopes of uranium:
238
U,
235
U and
234
U. All
are radioactive. They have abundances of 99.3%, 0.7% and 0.006%, respectively.
For the decay
9
of
238
Uto
206
Pb, Eq. (6.11)is
[
206
Pb]
now
=
[
238
U]
now
(e
λ
238
t
−
1)
(6.31)
and from Eq. (6.12) the age
t
is given by
λ
238
log
e
1
+
206
Pb
238
U
1
t
=
(6.32)
now
For the decay
10
of
235
Uto
207
Pb, these equations are
[
207
Pb]
now
=
[
235
U]
now
(e
λ
235
t
−
1)
(6.33)
and
λ
235
log
e
1
+
207
Pb
235
U
1
t
=
(6.34)
now
Therefore, provided that the samples come from closed systems that had no initial
lead, measurement of the lead-uranium ratios will yield an age. An alternative
method of calculating the age is to take the ratio of Eqs. (6.33) and (6.31),
207
Pb
206
Pb
235
U
238
U
e
λ
235
t
−
1
now
=
(6.35)
e
λ
238
t
−
1
now
where [
207
Pb
206
Pb]
now
is the present-day ratio of radiogenic
207
Pb to radiogenic
206
Pb. The present-day universal ratio [
235
U
/
238
U]
now
is 1
137.88 and is inde-
pendent of the age and history of the sample. Equation (6.35) can therefore be
used to estimate
t
from just one sample if the lead ratio is measured. The mineral
zircon is the best candidate for dating by this method because it is often retentive
of uranium and its daughter products, crystallizes with almost no initial lead and
is widely distributed.
/
/
6.4.1 Concordia diagrams
Many samples are not ideal and either have initial lead and/or have lost or
gained lead or uranium and so require alternative dating methods. An ideal
plot of [
206
Pb
238
U]
now
against [
207
Pb
235
U]
now
from Eq. (6.35)isknown as
/
/
9
The decay series from
238
Utostable
206
Pb is the 'uranium series'. It involves a chain of fourteen
decays, with
234
U being one of the intermediate daughters.
10
The decay series from
235
Utostable
207
Pb is the 'actinium series'. It involves a chain of eleven
decays, with actinium being one of the intermediate daughters.
