Geoscience Reference
In-Depth Information
ductivity have not been investigated. Two obvious
implications are that the lithosphere can sup-
port a higher thermal gradient than generally
supposed, giving higher upper-mantle tempera-
tures, and that the thermal lithosphere grows
less rapidly than previously calculated. For exam-
ple, the thermal lithosphere at 80 Ma can be
100
Table 21.1
Thermal conductivity of
minerals
Thermal Conductivity
Mineral
10
−
3
cal/cm s
◦
C
Albite
4.71
Anorthite
3.67
0.01 cal/cm s
◦
Cand
only 30 km thick for 0.3 cal/cm s
◦
C. The low
lattice conductivity of the oceanic crust is usu-
ally also ignored in these calculations, but this
may be counterbalanced by water circulation in
the crust.
The lattice (phonon) contribution to the ther-
mal conductivity decreases with temperature,
but at high temperature radiative transfer of heat
may become significant, depending on the opaci-
ties of mantle rocks, which depend on grain size
and iron content.
Convection is probably the dominant mode of
heat transport in the Earth's deep interior, but
conduction is not irrelevant to the thermal state
and history of the mantle as heat must be trans-
ported across thermal boundary layers by con-
duction. Thermal boundary layers exist at the
surface of the Earth, at the core--mantle bound-
ary and, possibly, at chemical interfaces internal
to the mantle. Conduction is also the mechanism
by which subducting slabs cool the mantle, and
become heated up. The thicknesses and thermal
time constants of boundary layers are controlled
by the thermal conductivity, and these regulate
the rate at which the mantle cools and the rate at
which the thermal lithosphere grows. The impor-
tance of radiative conductivity in the deep man-
tle is essentially unconstrained.
The thermal conductivity goes through a min-
imum at about 100 km depth in the upper man-
tle. Higher thermal gradients are then needed to
conduct the same amount of heat out, and this
results in a further lowering of the lattice con-
ductivity. The conductivity probably increases by
at least a factor of 3 to 4 from 100 km depth to
the core--mantle boundary (CMB).
The parameters that enter into a theory of
lattice conductivity are fairly obvious; tempera-
ture, specific heat and the coefficient of thermal
expansion, some measure of anharmonicity, a
measure
km
thick
for
K
=
Microcline
5.90
Serpentine
7.05
Diopside
11.79
Forsterite
13.32
Bronzite
9.99
Jadeite
15.92
Grossularite
13.49
Olivine
6.7--13.6
Orthopyroxene
8.16--15.3
Horai (1971), Kobayzshigy (1974).
relative to the mantle, to sustain the same con-
ducted heat flux. The gradient can be higher still
in sediments.
The thermal gradient decreases with depth in
the Earth. If the crustal radioactivity and mantle
heat flow are constant and the effects of temper-
ature are ignored, regions of thick crust should
have relatively high upper-mantle temperatures.
Thermal conductivity is strongly anisotropic,
varying by about a factor of 2 in olivine and
orthopyroxene as a function of direction. The
highly conducting axes are [100] for olivine and
[001] for orthopyroxene. The most conductive axis
for olivine is also the direction of maximum
P-velocity and one of the faster S-wave directions,
whereas the most conductive axis for orthopy-
roxene is an intermediate axis for P-velocity and
a fast axis for S-waves. In mantle rocks the fast
P-axis of olivine tends to line up with the inter-
mediate P-axis of orthopyroxene. These axes, in
turn, tend to line up in the flow direction,
which is in the horizontal plane in ophiolite
sections. The vertical conductivity in such situa-
tions is much less than the average conductivity
computed for mineral aggregates. Conductivity
decreases with temperature and may be only half
this value at the base of the lithosphere. The
implications of this anisotropy in thermal con-
ductivity and the lower than average vertical con-
of
a
mean
free
path
or
a
mean
