Geoscience Reference
In-Depth Information
1.0
0.8
T
0
3.5
3.0
Zircons
0.6
2.5
0.4
2.0
1.5
1
0.2
0.0
0
10
20
30
40
207
Pb/
235
U
Figure 4.7
The concordia curve is the locus of points for which the
x
-and
y
-axesyieldidenticalagesforboth
the
235
U
206
Pb methods. The numbers 1, 2.0, 2.5, etc., on the curve correspond
to geological ages of 1, 2.0, 2.5, etc., billions of years. Zircons extracted from a single sample, and
shown here by the black ellipses, indicate an age
T
0
of crystallization of 3.5 Ga, with 1.0 Ga old
overgrowth. The dashed lined is a mixing line between the two zircon generations.
207
Pb and
238
U
−
−
4.3 The isochron method
When minerals and rocks form, they already contain some of the radiogenic isotopes used
for dating. The daughter nuclide may be present in large, yet unknown, concentrations.
The isochron method was devised to provide an age, even when the amount of radiogenic
isotope initially present in the system is not negligible with respect to that produced by
radioactive decay after its formation. The key assumption is that the initial isotopic com-
position of the element to which the radiogenic nuclide belongs is unknown, but constant,
in all the samples analyzed. Isotopic homogenization is assumed to be complete at
t
=0,
which may be the case where minerals crystallize from a magma or from seawater within
a time interval that can be considered as very short compared to the age of the rocks. A
large number of geochronological systems are used in this way:
87
Rb-
87
Sr,
147
Sm-
143
Nd,
176
Lu-
176
Hf, and
187
Re-
187
Os are examples (note that chronological systems are denoted
by hyphenating the parent and daughter isotopes in the order
P
-
D
). In addition to the
chronological aspect, the variations in the isotopic abundances of radiogenic isotopes are
useful in studying many geological processes. Let us take as an example the
147
Sm-
143
Nd
with
147
Sm,
D
with
143
Nd, and
D
with
144
Nd. For a closed system, the isochron equation
becomes:
143
Nd
144
Nd
143
Nd
144
Nd
147
Sm
144
Nd
e
λ
147
Sm
t
1
t
=
0
+
−
(4.28)
t
