Geoscience Reference
In-Depth Information
0.2845
0.2840
0.2835
0.2830
0.2825
0.2820
0.2815
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
176
Lu/
177
Hf
0.5150
0.5140
0.5130
0.5120
0.5110
0.5100
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
147
Sm/
144
Nd
Figure 4.3
Two examples of isochrons for the
176
Lu
176
Hf (top,
λ
−
1
= 1.865
10
−
11
a
−
1
)andthe
147
Sm
→
×
10
−
11
a
−
1
) systems on the same samples of Proterozoic basalts
and for which the range of isotopic compositions of the daughter element at the time of
emplacement can be considered negligible with respect to the range caused by radiogenic
ingrowth, will lie on a straight line. The slope of the isochron gives the age I of the lava flows and
the intercept the mean isotopic ratio of the daughter element at that particular moment.
143
Nd (bottom,
λ
−
1
= 0.654
→
×
of any sample devoid of parent nuclide is unchanged, alignments simply rotate with time
around the intercept.
Isochrons can alternatively be viewed as mixing lines between two end-members.
The first end-member (
x
D
)
0
is the initial inventory of the daughter ele-
ment, whereas the second end-member (
x
=
=
/
0,
y
(
D
e
λ
t
=∞
,
y
=∞
,
y
/
x
=
−
1) represents pure
radiogenic ingrowth.
Let us now take a quick guided tour of dating methods, beginning with those based
on the measurement of radioactive nuclides; then the systems with high parent/daughter
ratios, in which the initial quantities of radiogenic nuclides can be neglected; and, finally,
systems with low parent/daughter ratios, where the isochron method is applicable. In gen-
eral, a clock can be applied to samples whose age does not exceed five times the half-life.
