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and a residual depleted peridotite or an unal-
tered abyssal peridotite. It has a simple one-stage
history.
In some models of upper-mantle chemistry
only the most depleted materials are used in its
construction; hence
Depleted Upper Mantle
.
fundamental conflicts between isotope
geochemistry and geophysics
, and confu-
sion about whether slabs penetrate into 'the
lower mantle' or not. It appears that the upper
1000 km of the mantle -- about 40% by mass of
themantle--differsfromtherestofthemantle
and this also appears to be the
active layer
for
plate tectonics. The majority of the incompat-
ible trace elements that are not in the crust
may be confined to an even shallower depth
range. Bullen's nomenclature is precise and use-
ful and I will follow it. The terms
upper mantle
and
lower mantle
are now fuzzy concepts because
of usage in the geochemical literature and the
decoupling of this usage from seismological
data. These terms will be used when precise
depths, volumes or masses are not needed. The
term
mesosphere
has also been used for the mid-
mantle, Bullen's Region D
. The terms
shallow
mantle
and
deep mantle
will be used to avoid
the conflict between the precise seismological
definitions of mantle regions and geochemical
usage.
Heterogenous upper mantle
Convective stirring takes large blobs and shears
and stretches them, folds them and stretches
them more, repeatedly, until the dimensions are
very small; for obvious reasons this is known
as the
Baker's transformation
, a funda-
mental result of
chaotic advection the-
ory
. This theory may not be appropriate for the
mantle. It implies high Rayleigh number, stirring
or folding in one direction and low-viscosity pas-
sive particles. We must therefore pay attention
to the materials that enter the mantle instead of
relying on averages of the magmatic products.
We must keep an open mind about the possi-
bility of
large fertile blobs
in the man-
tle, and extensive regions having
high homologous
temperature
.
From a petrological point of view, the man-
tle can be viewed as a multi-component sys-
tem. The known components of the upper man-
tle are recycled continental crust and other
mafic components, ultramafic rocks, MORB and
other basalts, depleted refractory residues and
enriched components (Q, quintessence or fifth
component) such as kimberlite; mixtures of
these satisfy most chemical constraints on the
composition of the mantle or BSE, including
major and trace elements. The
compositions
of basalts
and the
compositions of con-
tinental and abyssal peridotites
are
available in
petrological databases
. These
can be used to reassemble the original petrology
and composition of the mantle, and with a few
other assumptions, the composition of the upper
mantle.
Basalt and peridotite compositions represent
the culmination of melt depletion and enrich-
ment processes over the entire history of the
mantle, including the accretional process. There
are a variety of basalts and peridotites. The com-
positions of many basalts and peridotites appear
to lie along mixing lines and the end-member
Depleted upper mantle; the DUM idea
Geochemists have ideas different from seismo-
logical conventions about what constitutes the
upper mantle. They are based on compositions
of
depleted midocean-ridge basalts
,
DMORB
,and
assumptions about how these form. The defini-
tion of the upper mantle adopted by isotope geo-
chemists is the following.
The upper mantle is that part of the mantle that
provides uniform and depleted midocean ridge
basalts and that formed by removal of the continental
crust; it extends from the Moho to the 670-km mantle
discontinuity. It is also called 'the convecting mantle.'
The upper mantle is usually viewed by
geochemists as homogenous because MORB
are relatively homogenous. The assumption is
that homogenous products require homogenous
sources. The composition of the inferred
MORB
reservoir
has been attributed to the whole upper
mantle. Since MORB is depleted in LIL compared
to other basalts, the above assumptions have
led to the
Depleted Upper Mantle
,or
DUM
, con-
cept. DUM is a two-component system, DMORB
