Geoscience Reference
In-Depth Information
The high 3 He/ 4 He ratios found in some basalts
probably reflect long-term evolution in a U-poor
environment. This need not be a large reservoir
but could be single olivine crystals or pieces of
depleted lithosphere.
upper mantle of the Earth may be cooling by 50--
100 C per Ga. This provides about 15--35% of the
global heat flux, but values as high as 50% are
not ruled out. The inferred high mantle temper-
atures in the past probably require a different
style of plate tectonics and heat removal, even
after the continents were formed and stabilized.
Options include the heat-pipe mechanism used
to extract heat from the interior of Io, stagnant
or buoyant lid convection, multiple short-lived
platelets, continuous and widespread volcanism
and magma oceans. Heat productivity at 3.5 Ga
was at least a factor of three higher than today
and a more efficient heat removal mechanism
must have prevailed. Tidal friction and energy
due to mantle and core differentiation may also
have been much higher at that time.
The adiabatic gradient
The horizontally averaged interior temperature of
an actively convecting region of a uniform fluid,
heated from below, at high Rayleigh number, is
adiabatic if one is sufficiently far from thermal
boundary layers. Secular cooling and internal
heating cause the geothermal gradient to be sub-
adiabatic. The seismic velocity gradient in the
deep mantle (
1000 km--2600 km depth) is con-
sistent with an adiabatic gradient, suggesting
that this region may satisfy the conditions of
being mainly heated from below and not experi-
encing substantial secular cooling. However, the
uncertainties are such that a substantial subadi-
abatic gradient is also consistent with the data.
Although there is little heat entering the lower
mantle from the core it may dominate the heat-
ing if the lower mantle is depleted in radioactive
elements.
The upper mantle is radially and laterally
heterogeneous and the seismic properties are
affected by phase changes and partial melting.
There is no reason to believe that it is adiabatic
below the plates. In addition to the surface ther-
mal boundary layer any upward concentration
of buoyant material will extend the depth of
the conduction layer. Upward migration of melts,
underplating and dehydration and melting of the
downgoing slab will concentrate U, Th and K into
the shallow mantle. The bottoming out of slabs in
the upper mantle or transition region contribute
to the secular cooling of the shallow mantle,
and the maintenance of a subadiabatic gradient.
Under these conditions one expects that melting
will be confined to the shallow mantle and per-
haps to the thermal boundary layers where the
thermal gradient is high and positive.
Delayed heat flow
There are several sources of delay in the heat
generation vs. heat-flow cycle. The heat flowing
through the interior represents heat generated
at some time in the past when the heat genera-
tion capacity of radioactive elements was higher.
One does not expect an instantaneous balance
between present heat generation and heat flow.
The Earth is cooling, and this contributes to the
observed heat flow. These secular effects may con-
tribute at least 10 TW to the heat flow compared
to what would be expected from the present level
of radioactivity. This is comparable to the shorter
term fluctuations due to plate and mantle reorga-
nizations. If the crust is still growing the radioac-
tivity in the mantle, in the past, may have been
higher than at the present. If this mechanism
is important the U, Th and K in the mantle
are decreasing with time both by decay and by
removal. These elements are also returned to the
mantle by subduction and the balance between
removal and return may have changed with time.
In the extreme case, all the LIL were stripped out
of the bulk of the mantle during accretion and
the magma ocean stage and only later returned
to the upper mantle when the mantle cooled suf-
ficiently for plate tectonics to operate.
Heat being built up under large plates may be
episodically released upon plate reorganizations,
stress changes and the formation of new plate
Secular cooling
The heat content of the Earth is immense; about
10 38 ergs which is equal to a 10 Ga supply of
the present flux. Calculations suggest that the
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