Geoscience Reference
In-Depth Information
Table 26.3
Radioactive elements in meteorites
Meteorite Class
CI
CM
CV
CO
H
L
LL
EH
EL
Radioactivities in whole meteorite
K
ppt (‰)
550
370
360
360
780
920
880
840
700
Th
ppb
29
41
58
80
38
42
47
30
38
U
ppb
8
12
17
18
13
15
15
9.2
7
Radioactivities in volatile-free silicate portion
K
ppt (‰)
775
511
490
498
1102
1211
1128
1315
971
Th
ppb
41
57
79
111
54
55
60
47
53
U
ppb
11
17
23
25
18
20
19
14
10
slab dehydration, result in strong upward concen-
trations of the radioactive elements.
Potassium is a minor contributor to present-
day heat flow. However, in early Earth history,
K and U would have been the major long-lived
radioactive heat sources. Estimates of average
bulk silicate Earth (BSE) abundances (mantle plus
crust) are given in Table 26.2, along with their
heat productivities. Estimates for the heating
potential of BSE range from 12.7--31 TW. Most
analyses give values between 17.6 and 20.4 TW.
These are present-day instantaneous values. Heat
conducted through the surface was generated
some time ago, when the radioactive abundances
were higher, so these are lower bounds on
the contribution of radioactive elements to the
present-day surface heat flow, assuming that the
estimates of U, Th and K are realistic. The allow-
able variation in U and Th contents of the man-
tle is a large fraction of the so-called discrepancy
between production and heatflow. Production of
heat can be much larger if potassium contents
have been underestimated. Because of the short
half-life of
40
K, most of the
40
Ar in the atmo-
sphere would have been generated in early Earth
history. Efficient degassing in early Earth history
may explain, in part, the large fraction of the ter-
restrial
40
Ar that is in the atmosphere, compared
to the reluctance of
4
He to leave the mantle today
(the helium-heatflow paradox). Different solubil-
ities of He and Ar in mantle materials may also
be involved.
The amount of radioactivity in the crust
must be subtracted out in order to obtain man-
tle abundances and heat productivities. Using
8 TW as the best estimate of crustal productiv-
ity gives 9.6--12.4 TW as the mantle heat flow
from radioactivity, or 18.8--24 mW/m
2
. These can
be compared with the basal heat-flow estimates
(25--39 mW/m
2
). Delayed heat flow and other
sources of mantle heating may need to con-
tribute up to about 20 mW/m
2
, more than half of
the mantle heat flow. Heat from the core (about
9 TW), solid Earth tides (1--2 TW) and thermal
contraction (2 TW) are non-radiogenic sources
that may add 12 TW to the mantle heat flow,
about the same as the current (non-delayed) man-
tle radiogenic contribution. The radiogenic con-
tribution can be increased by about 25% if it takes
1 Gyr to reach the base of the lithosphere. On top
of all this is secular cooling of the mantle. In a
chemically stratified mantle, the outer layers cool
much faster than the deeper layers. If cooling is
confined to the upper 1000 km (Bullen's Region
B and C) a temperature drop of 50
◦
C in a bil-
lion years corresponds to a heat flow of 3 TW.
Cooling rates of twice this value have been sug-
gested. Thus, there appears to be no need for any
exotic heat sources or hidden sources of radioac-
tivity in the mantle. This conclusion is indepen-
dent of the uncertain contribution of hydrother-
mal circulation to the surface heat flow. There
are implications, however, for the temperatures
of Archean -- and older -- mantle and the style of
