Geology Reference
In-Depth Information
Table 10.1. Uranium and heat generation budgets a .
Mass
Mass
U conc.
U mass
Heat gen. b
Heat gen.
(10 22 kg)
(10 15 kg)
Reservoir
(%)
(ng/g)
(pW/kg)
(TW)
20 c
Average silicate Earth
400
100
80
5
20
1400 d
Continental crust
2.6
0.65
36
350
9
(900-1800)
(23-47)
(225-450)
(6-12)
D
8.5
2.1
50 e
4
10.5
1
(50-80)
(4-7)
(10-15)
MORB source,
389
97
10
40
2.5
10
i.e. rest of mantle
(6.5-13)
(26-53)
(1.5-3)
(7-13)
a (Plausible ranges in parentheses.) Bold numbers inferred from U mass balance.
1pW
10 12 W.
b Heat generation productivity from Stacey [141], assuming Th/U
10 -12 W. 1 T W
=
1picowatt
=
=
1 terawatt
=
13 000.
c McDonough and Sun [140], O'Neill and Palme [182] and Lyubetskaya and Korenaga
[187].
d Rudnick and Fountain [142].
e Assumed similar to oceanic crust; Sun and McDonough [188] and Donnelly et al. [189].
=
3.8 and K/U
=
crust and mantle rather than the metallic core), and the trend to lower abundance
with higher volatility is evident.
It is useful to follow the arguments for uranium, as it is one of the refractory
elements whose total abundance in the Earth is reasonably well known, it is one
of the incompatible elements that is partitioned strongly into melt phases in the
mantle, and it is a heat source element. Table 10.1 summarises estimates of uranium
concentration in the Earth and in its main reservoirs, as they have been inferred
in preceding chapters. The average U content of the primitive mantle is inferred
from meteorites to be 20
10 15 kg of U.
±
4 ng/g, which implies a total mass of 80
×
10 15 kg is estimated to be in the continental crust, i.e. 30-60%.
About (23-47)
×
10 15 kg or 40-70% is inferred to be in the mantle.
Some of this mantle uranium we can plausibly locate in the seismic D zone
at the bottom of the mantle. D is interpreted here as including the accumulations
of subducted oceanic crust hypothesised by Hofmann and White [74] and demon-
strated by models like those of Figures 9.5 and 9.10. The accumulation is taken,
from the numerical models, to be equivalent to a layer about 100 km thick [118].
If its U content is similar to that of oceanic crust, then it would contain only about
5% of the total U budget.
According to the mantle picture inferred in previous chapters, the only remaining
mantle reservoir is the rest of the mantle, in other words excluding the crust and the
D layer. The balance of the Earth's uranium is then required to be in this reservoir,
Thus (33-57)
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