Geoscience Reference
In-Depth Information
accretional zone-refining process results in an
outer shell that contains most of the U, Th and
K, at levels about three times chondritic (if the
outer shell is equated with the volume of the
present upper mantle), from which the proto-
crust and basaltic reservoirs were formed. The
residual (current) upper mantle retains radioac-
tive abundances greater than chondritic while
the bulk of the mantle, including the lower
mantle, is essentially barren. Previous chapters
explored this possibility. The outer shells of Earth
probably also contain, or contained, the bulk of
the terrestrial inventory of noble gases. This is
the reverse of the classical models and those used
in convection simulations.
In more recent models of geodynamics the
entire mantle is assumed to have escaped chem-
ical differentiation, except for crust extraction,
and to convect as a unit, with material circulat-
ing freely from top to bottom. This is
one-layer
or whole mantle convection
. Recycled
material is quickly stirred back into the whole
mantle. In these models, (1) chemical hetero-
geneities are embedded in a depleted matrix,
(2) the whole mantle is uniformly heterogenous
and (3) the mantle is relatively cold and the hot
thermal-boundary layer above the core plays an
essential role in bringing heat to the surface.
Convection is assumed to be an effective homog-
enizer. An amendment to this idea is that there
is a radioactive-rich layer deep in the mantle,
but this is based on unlikely assumptions about
upper-mantle radioactivity. Ancient radioactive-
rich regions in the deep mantle (if they survive
accretional differentiation) will overheat, over-
turn and deliver their heat and heat-producing
elements to the shallow mantle.
be conducted readily into the base of the mantle
and become part of the general background flux
of the mantle. In any case the heat flow from
the core and the heat generation of a depleted
lower mantle are minor compared to the heat
generation, secular cooling and plate insulation
effects, and lateral temperature gradients in the
upper mantle. Plate tectonics, recycling and mag-
matism are forms of convection but the active
layer may not extend deep into the mantle where
the effects of pressure, low-heat sources and more
uniform thermal gradients suppress the impor-
tance of convection. The plate tectonic style of
convection and heat removal may not extend too
far back in time.
At upper-mantle conditions, viscosity, conduc-
tivity and thermal expansion are strongly depen-
dent on temperature. When this is so, mantle
convective vigor adjusts itself to remove more
heat as the temperature rises, unless the plates
at the surface do not allow this. At high pressure,
temperature is less effective in changing ther-
mal and physical properties that depend on vol-
ume and there is less negative feedback. Viscosity
also depends on water content. Recycling serves
to
reintroduce
water
into
the
upper
mantle,
which
promotes
melting
and
a
lowering
of
viscosity.
One might think that a hotter Archean Earth
convected more vigorously than the current man-
tle. But if melt and volatile removal are more
important than recycling then a hot dry early
mantle may have convected more slowly than the
current mantle. If so, present-day secular cool-
ing can contribute to the observed heat flow
with no
Archean thermal catastrophe
. The
ability of continents to divert heat to the ocean
basins, and to move toward cold mantle, means
that the stability of continental crust and man-
tle does not imply the absence of secular cool-
ing for the mantle as a whole. The mantle may
have cooled more slowly during the Archean than
now because of the combined roles of subduction
depth, recycling and mantle viscosity on mantle
heat loss. The fact that the core is still mostly
molten and the upper mantle is still near or
above the melting point implies a long drawn-out
cooling process. This favors a chemically stratified
mantle but also one with most of the radioactive
Style of mantle convection
The deep mantle convects sluggishly because of
the effects of pressure on thermal expansion,
thermal conductivity and viscosity. Large grain
size, high temperature and partitioning of iron
may increase the ability of the lower mantle to
transmit heat by conduction and radiation. The
buoyancy flux of hotspots is often equated with
the heat flux from the core but this heat may also
