Geoscience Reference
In-Depth Information
both to argon and to potassium since its formation, then the ages determined at
every step should be the same. Loss of argon from some minerals or parts of
minerals and not others results in a disturbed age spectrum which can sometimes
be interpreted to yield the thermal history of the sample. An Ar-Ar isochron can
be constructed by plotting
39
Ar
36
Ar against
40
Ar
36
Ar. The slope of the isochron
/
/
is then (e
(
λ
A
+
λ
C
)
t
J
from Eq. (6.45) and the intercept on the
40
Ar
39
Ar axis
−
1)
/
/
is the
40
Ar
36
Ar ratio of non-radiogenic atmospheric argon, normally 295.5.
A problem with very-fine-grained samples is that
39
Ar can be lost during the
irradiation as it recoils out of the sample during the reaction. This results in a
value for
t
that is greater than the age of the sample.
/
6.8 Samarium-neodymium
Samarium and neodymium are rare-earth elements that occur in low abundance
in most rocks. Although the decay constant for
147
Sm-
143
Nd is very small (see
Table 6.2), which makes the decay most useful for dating very old rocks, the
system can also be used for younger samples. Applying Eq. (6.11)tothis decay
and including an initial concentration of
143
Nd gives
[
143
Nd]
now
=
[
143
Nd]
0
+
[
147
Sm]
now
(e
λ
t
−
1)
(6.49)
144
Nd, is used as a
One of the seven non-radiogenic isotopes of neodymium,
standard:
143
Nd
144
Nd
143
Nd
144
Nd
147
Sm
144
Nd
(e
λ
t
now
=
0
+
−
1)
(6.50)
now
An isochron can therefore give
t
and the initial Nd ratio. Instrumentation of high
precision is required, since neodymium and samarium are frequently present in
abundances of less than 10 ppm. Values of [
147
Sm
144
Nd]
now
are typically 0.1-0.2
in rock samples. In the same manner as was discussed for the rubidium-strontium
method, the [
143
Nd
/
144
Nd]
0
initial ratio provides an indication of the origin of
the samples. The advantage of this method is that samarium and neodymium are
similar in their chemical characteristics and little affected by processes such as
weathering and metamorphism; so, even when a system is not closed to rubid-
ium and strontium, the Sm-Nd method can be successful. Since geochemical
processes do not preferentially separate samarium and neodymium, large differ-
ences in their concentrations are rare. However, Sm-Nd results for some volcanic
rocks can reflect contamination of the igneous melt as it ascended through coun-
try rock; such results can be very difficult to interpret. Very old rocks such as
some meteorites and basalts that contain little K, U or Rb are best dated using
the Sm-Nd method.
The abundance of terrestrial
143
Nd has increased with time since the Earth's
formation because of the decay of
/
144
Nd ratio has
therefore also increased with time. We can model the time dependence of the
143
Nd
147
Sm to
143
Nd. The
143
Nd
/
144
Nd
ratio of the Earth is the same as that of the chondritic meteorites (Section 6.10).
/
144
Nd ratio by using Eq. (6.50) and by assuming that the
147
Sm
/
