Geology Reference
In-Depth Information
Table 10.6.
Noble gas evolution, 'Medium gas' case
a
.
(a) Concentrations (atoms/g)
4
He
3
He
21
Ne
22
Ne
40
Ar
36
Ar
(10
13
)
8
)
6
)
8
)
13
)
(10
8
)
(
(
(
(
Initial
23
383
141
43
0.007
718
MORB source, present
25
26
20
2.9
12
48
OIB source (2.5 Ga)
34
76
41
8.5
16
142
OIB source (3.75 Ga)
41
130
63
15
18
244
Production (4.5 Ga)
45
-
21
-
27
-
(b) Ratios
4
He/
3
He
21
Ne/
22
Ne
40
Ar/
36
Ar
MORB source
98 400
0.070
24 400
OIB source (2.5 Ga)
44 800
0.049
10 900
OIB source (3.75 Ga)
31 600
0.043
7 500
a
τ
g
=
10.1 Gyr; early degassing enhanced by 3 for 0.5 Gyr; initial non-nucleogenic
concentrations increased by 4.
To include radiogenic species, the uranium concentration in the MORB source
is taken to be 10 ng/g, as deduced in Section 10.2. Thorium and potassium con-
centrations are obtained by assuming Th/U
10
4
=
3.8 and K/U
=
1.3
×
[184].
10
14
atoms
4
He/g over 4.5 Gyr.
21
Ne
production is obtained using the production ratio
This amount of radioactivity generates 4.5
×
10
7
[230].
Total production of radiogenic species over 4.5 Gyr is included in Table 10.6(a).
4
He/
21
Ne
=
2.2
×
10.8.5 Application of the model
Equation (10.10) can be integrated to calculate concentrations as a function of time.
The strategy is to estimate initial concentrations of the non-radiogenic isotopes, to
integrate forward in time, and then to test whether the radiogenic isotopes reach
appropriate present values. A successful calculation turns out to require a period
of enhanced degassing early in Earth history [227], as we will see, and as has been
inferred previously [233]. The calculation shown here is based on the 'Medium gas'
case of Table 10.5(b). It is meant to be indicative, not definitive, but is nevertheless
sufficient to demonstrate the initial plausibility of the present interpretation.
Preliminary initial concentrations of the non-radiogenic isotopes were estimated
from present values using Eq. (10.9). A low initial value of
40
Ar was assumed. The
initial value of
4
He was obtained by assuming the
4
He/
3
He ratio of 6000 measured
