Geoscience Reference
In-Depth Information
into heat-flow measurements. These effects all
change the conducted heat-flow and imply that
one should not replace the measurements with a
theoretical cooling curve in order to estimate the
total heat loss of the mantle. Variations in perme-
ability at the top of the plate cause variations
in the hydrothermal component of heat flow.
This component of heat flow must be allowed for
separately. The important conclusion for present
purposes is that there are uncertainties and tem-
poral changes in surface heat flux that must be
considered before one declares an 'energy' crisis.
Even if there is a mismatch between time aver-
aged heat flow and the energy sources in the inte-
rior, the location of the missing energy source, if
there is one, cannot be determined from heat-
flow data.
Table
26.2
Distribution
of
radioactive
heating
TW
Continental crust
5.8--8.7
Continental lithosphere
1
Oceanic crust
1
Upper 410 km if made of:
NMORB
13.4
Peridotites
4.6
oceanic
2.1
continental
9.2
Upper 650 km if made of:
Pacific MORB or eclogite
28
Oceanic peridotites
4
Continental peridotites
18
Picrite
6
Plausible range
10--22
Depleted MORB
1--2
Heat sources
Crust
lithosphere
upper 650-km
18--30
+
+
Upper 1000-km if it has all
the U, Th, K minus crust
Radioactivity
All estimates of terrestrial abundances of the
heat-producing elements depend, one way or
another, on meteorite compositions. Carbona-
ceous chondrites are the usual choice of building
material but enstatite achondrites and meteorite
mixes are also used. In detail, the Earth is
unlikely to match any given class of meteorite
since it condensed and accreted over a range of
temperature from a range of starting materials.
The refractory elements are likely to occur in
the Earth in cosmic ratios but there is evidence
that the volatile elements are depleted. The large
metallic core indicates that the Earth, as a whole,
is a differentiated and chemically reduced body,
although at least the crust and the outer shells
of the mantle are oxidized. Enstatite achondrites
(EH) match the Earth in the amount of reduced
iron (oxidation state) and in oxygen isotopic com-
position and have been used to estimate terres-
trial abundances. Since the terrestrial planets are
mainly oxygen, by volume, this is a non-trivial
consideration. The U and Th concentrations in
various meteorite classes are given in Table 26.3,
both for the bulk meteorites and calculated on a
volatile and iron-free basic, to approximate man-
tle or BSE concentrations.
12--24
Upper 1000 km if heat generation
21
is 12.5 fW/g
Lower mantle
3--14
Composition of Earth
Estimates of the U, Th and K contents of the bulk
silicate earth (BSE) are given in Table 26.2.
The trickiest element to estimate is K since it
is not refractory. Estimates of the K content of
BSE range from 151--258 ppm (chondrites fall in
the 200--550 ppm range, EH are 840 ppm). On a
H-, C-, S- and Fe-free basis meteorites range from
490--1315 ppm (Table 26.3). From
40
Ar abundances
in the atmosphere the minimum K in the Earth
is inferred to be 116 ppm. If the degassing effi-
ciency of the mantle is comparable to the frac-
tionation efficiency of LIL into the crust then the
K content of BSE may be about 230 to 350 ppm.
Most of the K, U and Th may be in the outer
shells of Earth -- crust, recycling crust, shallow
mantle, kimberlites and the MORB-source region.
The original processes of accretion and differenti-
ation, and the ongoing processes of recycling and
