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plate-tectonic mode removes heat by plate tecton-
ics (plate cooling at the surface and slab cool-
ing in the interior). Io, a moon of Jupiter, may
be an example where continuous volcanism, and
the subsidence of the surface layer -- the heat-
pipe mechanism -- rather than plate tecton-
ics, is the mechanism for coping with the prodi-
gious tidal heating. The current style of plate tec-
tonics also involves massive recycling of water
into the interior. This means that the melting
point of the mantle is low and the viscosity is
low, in spite of relatively low temperatures com-
pared to the Archean. Magmatism is primarily
restricted to plate boundaries and regions of ten-
sile stress; the latent heats of crystallization con-
tribute much less than the heat associated with
cooling from high temperatures. This may not
always have been the case. The cooling rate and
cooling mechanism of the mantle are unlikely to
have been the same throughout time.
The vigor of mantle convection does not
depend only on mantle temperature and viscos-
ity. Plates, slabs and continents affect the style of
mantle convection. Plates dissipate energy by col-
liding, bending, breaking and moving past each
other and the underlying mantle. The buoyancy
that drives convection can be dissipated in the
platesystemaswellasinthemantle.Platesmay
actually regulate mantle convection and over-ride
the buffering effect of mantle viscosity. Thick
plates, and a jammed plate system, can allow the
mantle to over-heat, compared to the viscosity-
regulated effect. Thus, the arguments against
high mantle temperatures in the past, the vis-
cosities associated with such temperatures, the
intuitive relationships between temperature and
heat flow and the self-regulation of mantle tem-
perature must be reconsidered [ fate of fossil
heat ].
reigning paradigm in mantle geochemistry and
geodynamics. The various paradoxes that plague
studies of mantle dynamics and geochemistry
can be traced to the assumptions that underlie
the models. In one class of models the mantle
is divided into large reservoirs that can inter-
change, and exchange or entrain material. The
deeper layers are highly radioactive and usually
gas-rich. Slabs and plumes traverse the whole
mantle, carrying the various components that
are thought to characterize hotspot magmas.
Ocean-island basalts (OIB) are assumed to come
from the lower mantle, or the undegassed man-
tle. The upper-mantle reservoir is called 'the
convecting mantle,' implying that it is well
stirred by vigorous convection and is therefore
homogenous.
Chemical stratification is the natural result
of a planet growing in its own gravity field.
The effect of pressure on thermal properties
makes this stratification irreversible on a large
planet. The standard models of mantle geo-
chemistry and geodynamics, however, are quite
different. The reasons are not hard to find;
unphysical assumptions have been used to rule
out the possibility of chemical stratifica-
tion and irreversible differentiation .
These include the Boussinesq approximation,
assumption of a radioactive-rich lower mantle,
assumed flat chemical interfaces between layers,
and the assumption that the only plausible chem-
ical boundary is the 650-km phase change. The
textbook model of a depleted upper mantle,
an undepleted, undegassed or primordial lower
mantle, and a major isotopic boundary at
650 km, is unsatisfactory on many counts but
this has been used to argue against all stratified
models.
The Kelvin effect, revisited
Chemically stratified mantle?
The production of heat by decay of radioactive
elements and the secular cooling of the man-
tle are the two main contributors to the present
observed mean heat flux Q at the surface of the
Earth. The first term is traditionally written as
the Urey ratio , defined as the ratio between
the heat production by radioactive elements and
We return once again to the possibility of a
chemically stratified mantle . This pos-
sibility has apparently been ruled out by some
workers based on visual inspection of a few
color tomographic cross-sections. One-layer or
whole mantle convection is
the
current
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