Geoscience Reference
In-Depth Information
from the improved cleanliness of chemical extractions and
in situ
methods of anal-
ysis (secondary ion mass spectrometry and laser-ablation inductively coupled plasma
mass spectrometry, ICP-MS). The advantage of the method lies in the radioactive (
238
U
and
235
U) and radiogenic nuclides (
206
Pb and
207
Pb) being isotopes of the same ele-
ments: uranium for one and lead for the other. In the absence of any initial radiogenic
lead,
(4.13)
applied to the systems
238
U-
206
Pb (
10
−
9
y
−
1
) and
λ
238
U
= 0.155 125
×
10
−
9
y
−
1
)gives:
235
U
207
Pb (
−
λ
235
U
= 0.984 85
×
206
Pb
238
U
e
λ
238
U
t
t
=
−
1
207
Pb
235
U
e
λ
235
U
t
t
=
−
1
(4.26)
This double clock is routinely applied to the radiogenic lead and uranium of an acces-
sory, but common, zirconium silicate of granite and metamorphic rocks, zircon (ZrSiO
4
).
Uranium U
4
+
substitutes in large quantities for Zr
4
+
; but Pb
2
+
, which is of very different
ionic radius (0.133 nm) and charge from Zr
4
+
(0.084 nm), is essentially excluded at equi-
librium. As with atmospheric argon, there may nonetheless be contamination by lead at
mineral surfaces or in grain fractures. Because of the presence in the atmosphere of tetra-
ethyl lead, used until the 1990s as an anti-knocking agent in fuel, man-made pollution
may also be significant. A very similar technique to that described for argon is employed,
involving subtraction from the total lead content of the contaminated lead, whose isotopic
composition is relatively well known by using a stable isotope
204
Pb.
In situ
isotopic anal-
ysis using modern ion probes also allows the zones for analysis to be selected so that
contamination is almost completely eliminated.
The pair of equations in
(4.26)
defines the locus of points for which the ages indicated by
both methods concur, the locus being traditionally called concordia proposed by Wetherill
rapidly than
238
U: natural uranium today contains only 0.7%
235
U compared with 8%
235
U
three billion years ago. Although methods were developed to attempt to correct the effect
of disagreement related to losses of lead after closure of the system, they are now of little
value because of the improvement in techniques. The ratio of the
x
-axis to the
y
-axis is
proportional to the isotope ratio
206
Pb/
207
Pb:
206
Pb
238
U
t
/
1
1
207
Pb
235
U
t
=
207
Pb
206
Pb
t
×
238
U
235
U
t
(4.27)
/
/
/
The second term on the right-hand side is constant and equal to 1/137.88. The denomina-
tors of the ratios plotted on the
x
- and
y
-axes are therefore proportional and their ratio
238
U/
235
U is constant. It follows from the discussion of ratio behavior during mixing,
alignments in the concordia plot: the intercepts of these alignments with the concordia will
therefore a priori yield interesting ages.
