Geology Reference
In-Depth Information
U
39
Stable nuclides
90
Th
Long-lived naturally
occurring radionuclides
40
Pb
Short-lived radionucludes mentioned in the text
(gaps represent other short-lived radionuclides)
80
Os
Re
β
+
decay of
40
K to
40
Ar and
β
-
decay of
40
K to
40
Ca
Hf
Region of short-lived
radionuclides leading
from U and Th to Pb
70
Lu
40
β
-
42
43
20
Ca
60
19
K
39
40
41
β
+
137
Cs
18
Ar
37
131
I
36
38
40
50
18
20
22
α
-decay of
147
Sm to
143
Nd
39
Ar
40
148
149
150
Sm
144
147
152
154
62
α
90
Sr
Pm
30
Isotopes
(
Z
= 60)
150
Nd
60
142
143
144 145146
148
Fe
20
82
84
86
90
β
−
decay of
87
Rb to
87
Sr
Isotones
(
N
= 88)
Isobars
(
A
= 150)
Si
84
86
87
88
Sr
38
10
β
−
O
37
14
C
C
Rb
85
87
He
1
H
46
48
50
0
10
20
30
40
50
60 70
Neutron number (
N
)
80
90
100
110
120
130
140
Figure 10.1.1
The complete 'nuclide chart' of atomic number
Z
versus neutron number
N
showing the stable nuclides as
filled circles, and naturally occurring long-lived (
t
1/2
> 10
8
years) radionuclides as open circles. Also shown for illustration (as
small open squares) are one cosmogenic nuclide (
14
C) and three short-lived anthropogenic fission-products of environmen-
tal concern (
90
S r,
131
I and
137
Cs). Enlarged insets show the decay reactions from
40
K to
40
Ar and
40
Ca, from
87
Rb to
87
Sr, and
from
147
Sm to
143
Nd. Other rectangles identify the Lu-Hf and Re-Os radiogenic isotope systems (Table 11.1).
Cosmogenic radioisotope systems
The naturally occurring radioisotopes upon which
radiogenic isotope systems rely are long-lived
relics of an episode of heavy-element formation
(Chapter 11)
pre-dating
the formation of our Solar
System 4.6 Ga ago. Relatively few radionuclides
possess such long half-lives, which is why the
number of radiogenic isotope systems available in
the geoscientist's toolkit is small (Table 10.1).
Radionuclides are, however, being formed today by
the action of high-energy cosmic rays on atmos-
pheric gases, and these shorter-lived
cosmogenic
radionuclides
, such as the
14
C used in radiocarbon
dating (formed mainly by cosmic ray bombardment
of
14
N nuclei in atmospheric nitrogen), help us to
understand recent geological processes.
Radiogenic isotope systems
Table 10.1 and Figure 10.1.1 (Box 10.1) summarize the
principal radiogenic isotope systems currently used
by geoscientists.
K-Ar geochronology
The potassium isotope
40
K is radioactive, having a
half-life
of 1.25 Ga. The proportion of
40
K in natural
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