Geoscience Reference
In-Depth Information
Kelvin's assumed initial conditions are probably
close to being correct. The energetics of accre-
tion, core formation, giant impacts and forma-
tion of the Moon result in a hot, partially molten
initial condition; gravitational stratification was
unavoidable. Subsequent refinements of Kelvin's
thermal history incorporated radioactivity and
convection. The acceptance of convection and the
rough equality between the heat output of chon-
dritic meteorites and early estimates of the total
terrestrial heat flow led to the view that there is
essentially a steady state in which heat produc-
tion and heat loss are balanced. However, this
violates the
first law of thermodynamics;
it also cannot be the case since radioactive heat-
ing of necessity decreases with time. The sec-
ular cooling of the Earth -- the Kelvin effect
-- is important but has often been neglected
or dismissed in recent years. A perceived 'miss-
ing energy source,' controversies about 'correc-
tions' to observed surface heat flow, origin of
komatiite, and various
Archean paradoxes
and catastrophes
are prominent in current
discussions. The fact that there are
no thermal
or heat flow anomalies associated with
hotspots
is a paradox for the plume hypothesis
[
mantleplumes
]. Current convection and geo-
chemical models assume uniform radioactivity
throughout the mantle, or a depleted upper man-
tle and a primordial or enriched lower mantle --
these also cannot be correct.
Global heat
-
flow estimates range
from 30 to 44 TW
. Estimates of the radioac-
tive contribution from the mantle, based on cos-
mochemical considerations, vary from 19--31 TW.
About 5--10 TW enters the mantle from the core.
Thus, there is either a good balance between
input and output, as once believed, or there is a
serious missing heat-source problem, ranging up
to a deficit of 25 TW. Attempts to solve the per-
ceived deficit problem by invoking secular cool-
ing, or deeply-hidden heat sources --
stealth
layers in the mantle
-- have run into prob-
lems, some real, some perceived. Survival of
ancient cratonic roots,
komatiitic tempera-
tures
,an
Archean heat flow catastrophe
,
overheating of the lower mantle, and
helium
heat flow paradoxes
have been cited as
potential problems. These concerns have replaced
the
chondritic coincidence
that preoccupied geo-
chemists when the first estimates of global heat
flow came in, which suggested a steady-state
Earth with an instantaneous balance of deep heat
production with surface heat flow.
Initial conditions
A type of radial zone refining process accompa-
nies the accretional process. This sweats out the
crustal and radioactive elements and keeps them
near the surface and drains the dense metal-
lic melts and refractory crystals toward the inte-
rior. This chemical stratification, plus radioactive
heating, stretches out the subsequent cooling of
the mantle. Convection and the subduction part
of plate tectonics accelerate the cooling but the
surface and internal thermal-boundary layers are
still conduction bottlenecks. In the limit of mul-
tiple layers the Earth approaches the conduc-
tion cooling condition calculated by Lord Kelvin.
At the high pressures and temperatures in the
deep interior both lattice conductivity and radia-
tive conductivity may be much higher than in
the shallow mantle, again making the mantle
approach Kelvin's assumptions. Under some con-
ditions the Earth can forget its initial thermal
state but chemical stratification and redistribu-
tion of radioactive elements cannot be forgotten.
A chemically stratified mantle has a low effec-
tive Rayleigh number; vigorous convection and
chaotic mixing are not expected.
In models of geodynamics and geochemistry
that were popular in the last century, and are still
in the textbooks, the lower mantle was assumed
to have escaped accretional differentiation and to
have retained primordial values of radioactivity
andnob e sor entoh eund -
tectable radioactively enriched layers. The crust
was derived entirely from the upper mantle,
making the latter extraordinarily depleted in
the radioactive and volatile elements. However,
mass balance calculations and the
40
Ar content
of the atmosphere show that
most, if not
all, of the mantle must have been pro-
cessed and degassed
in order to explain
the concentrations of incompatible and volatile
elements in the outer layers of the Earth. The
