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and the radiative term should be low. A thin
layer convects sluggishly (because of the
h
3
term
in the Rayleigh number) but its presence slows
down the cooling of the mantle and the core. The
overlying FeO-poor layer may have high radiative
conductivity, because of high T and transparency,
and have high viscosity and low thermal expan-
sivity, because of P effects on volume. This part of
the mantle will also convect sluggishly. If it repre-
sents about one-third of the mantle (by depth) it
will have a Rayleigh number about 30 times less
than Rayleigh numbers based on whole mantle
convection and orders of magnitude less than Ra
based on P
gradient may even be negative. Regions of neg-
ative shear velocity gradient in D
are probably
regions of high temperature gradient and high
heat loss from the core. It is plausible that the
layers at the base of the mantle interact with
the core and therefore differ in composition from
the rest of the mantle.
Lateral heterogeneity
D
may represent a chemically distinct region of
the mantle. If so it will vary laterally, and the dis-
continuity in D
will vary considerably in radius,
the hot regions being elevated with respect to the
cold regions. A chemically distinct layer at the
base of the mantle that is only marginally denser
than the overlying mantle would be able to rise
into the lower mantle when it is hot and sink
back when it cools off. The mantle--core bound-
ary, being a chemical interface, is a region of high
thermal gradient, at least in the colder parts of
the lower mantle.
Seismic observations suggest the presence of
broad seismic velocity anomalies in the deep
mantle. The nature of these anomalies is incon-
sistent with purely thermal convection and
suggests the existence of large-scale chemical
heterogeneities in the lower mantle. The anti-
correlation between bulk sound speed and shear
wave velocity anomalies in the lowermost mantle
suggests
0 properties.
It is likely than some of the Fe in the lower
mantle is low-spin and some is high-spin with
the proportions changing with depth. The oxi-
dation state of Fe in the lower mantle is also
likely to be different than in the shallow man-
tle. These considerations complicate the inter-
pretation of lower mantle properties and the
geodynamics and melting point of the deep man-
tle. It is certainly dangerous to fit a single equa-
tion of state to the whole lower mantle or to
argue that seismic data requires the lower man-
tle to be homogenous, or the same as the upper
mantle.
=
Region D
the
presence
of
chemical
density
heterogeneities.
The lowermost mantle, Bullen's Region D
is a
region of generally low seismic gradient and
increased scatter in seismic travel times and
amplitudes. Lay and Heimberger (1983) found a
shear-velocity jump of 2.8% in this region that
may vary in depth by up to 40 km. They con-
cluded that a large shear-velocity discontinuity
exists about 280 km above the core, in a region
of otherwise low velocity gradient. There appears
to be a lateral variation in the velocity increase
and sharpness of the structure, but the basic
character of the discontinuity seems to be well
established.
Because the core is a good conductor and
has low viscosity, it is nearly isothermal. Lateral
temperature variations can be maintained in the
mantle, but they must converge at the base of D
.
This means that temperature gradients are vari-
able in D
. In some places, in hotter mantle, the
The core
In the first place please bear in mind that I do not
expect you to believe this story. Nor could you wonder
had you witnessed a recent experience of mine when,
in the armor of blissful and stupendous ignorance, I
gaily narrated the gist of it to a Fellow of the Royal
Geological Society. . . . The erudite gentleman in
whom I confided congealed before I was half
through! -- it is all that saved him from exploding --
and my dreams of an Honorary Fellowship, gold
medals, and a niche in the Hall of Fame faded into
the thin, cold air of his arctic atmosphere.
But I believe the story, and so would you, and so
would the learned Fellow of the Royal Geological
Society, had you and he heard it from the lips of the
man who told it to me
Edgar Rice Burroughs
