Geology Reference
In-Depth Information
flow must come from cooling of the Earth over
the geological time. It is important to note that
the average rate of production of radiogenic heat
in the crust (including the oceanic crust), which
is
H
C
D
2.9
10
-10
Wkg
-1
, is much greater than
that of the mantle, which is estimated to be
H
m
D
5.1
10
-12
Wkg
-1
. This larger productivity
is overcome by a mantle to crust mass ratio
that is
143, thereby the total crustal radioac-
tivity results to be 8.2
10
12
W, with respect
to a total mantle production of 20.0
10
12
W
(Stacey and Davis
2008
). As mentioned above,
radiogenic decay is not the unique source of heat
in the Earth's interior. Another important source
is represented by the basal heating of the litho-
sphere along the LAB, associated with mantle
convection, which also contributes to the heat
flux
q
m
through the Moho (Fig.
12.2
). Turcotte
and Schubert (
2002
) estimated that
75-80 %
of the present-day surface heat flow should be
attributed to decay of radioactive isotopes, while
about 20-25 % would originate from the
secular
cooling
of the Earth. This is the continuous loss
of primordial heat stored in the Earth's mantle
and core studied by Kelvin (
1864
), which drives
mantle convection. The relative contribution of
secular cooling and radioactive decay to the total
heat budget estimated by Turcotte and Schubert
(
2002
) is controversial and other authors give
very different values of the relative importance of
these sources of heat (e.g., Korenaga
2003
,
2008
).
The
Urey ratio
is a quantity that is commonly
used to measure the relative importance of the
radiogenic heat generated in the Earth's crust
and mantle. It is defined as the ratio of internal
heat production to surface heat flux. This is a
key parameter characterizing the global thermal
budget and a strong constraint for both thermal
history and mantle convection models. Calling
q
(
H
CC
),
q
(
H
m
),
q
(
S
m
), and
q
(
S
c
) the components
of surface heat flux associated with each of the
sources illustrated in Fig.
12.2
,wehavethefol-
lowing budget relations:
8
<
Fig. 12.2
Sketch illustrating the sources of heat in the
Earth and the components of the global heat flow.
q
CC
and
q
OC
are respectively the surface heat flux from continental
and oceanic regions,
q
m
is the heat flux through the Moho,
and
q
c
is the heat flux through the CMB. Four important
heat sources exist in the Earth: radiogenic decay in the
continental crust (
H
CC
), radiogenic decay in the mantle
(
H
m
), secular cooling of the mantle (
S
m
), and secular
cooling of the core (
S
c
)
Then,
q
60 mWm
-2
for older lithosphere and
decreases smoothly to about 50 mWm
-2
in the
oldest lithosphere.
The typical thermal conductivity of near-
surface rocks depends from rock type, com-
position, grain size, grain orientation, density,
porosity, composition of pore fluid, and
temperature. In the case of sedimentary and
volcanic rocks, the main controlling factor is
the porosity, while the thermal conductivity
of plutonic and metamorphic rocks depends
from the dominant mineral phase (Clauser and
Huenges
1995
). In general, most sedimentary
rocks have values of
k
between 0.5 and 2.5 Wm
1
K
1
, while for the majority of volcanic and
plutonic rocks
k
ranges from 1.5 to 3.5 Wm
1
K
1
. Finally, in the case of metamorphic rocks,
the thermal conductivity is generally between
2and4Wm
1
K
1
when the quartz content
is low, and between 5 and 6 Wm
1
K
1
in
the case of quartzite. Assuming
k
D
2.5 Wm
1
K
1
, we have an average vertical temperature
gradient at the Earth's surface of
16 Kkm
-1
when
q
D
40 mWm
-2
and
24 Kkm
-1
when
q
D
60 mWm
-2
.
A consistent part of the heat flow at the Earth's
surface undoubtedly originates by the radioactive
decay of
40
K,
235
U,
238
U, and
232
Th in the mantle
and, to a lesser extent, in the continental crust
(Fig.
12.2
). The remaining part of the surface heat
q
CC
D
q
m
C
q.H
CC
/
q
OC
D
q
m
q
m
D
q.H
m
/
C
q.S
m
/
C
q
c
q
c
D
q.S
c
/
(12.4)
:
