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peridotites and eclogites -- equilibrate at vari-
ous depths and the removed material metasoma-
tizes the shallow mantle (the mantle wedge, the
perisphere and the plate). Young oceanic plates,
delaminated lower crust, subducted seamount
chains and plateaus thermally equilibrate and
melt
660
410
8
6
520
220
260
310
4
800
2
at
depths
different
from
older
thicker
plates.
The 650 km discontinuity, with its nega-
tiveClapyronslope,isatemporarybarrierto
cold sinking material of the same composi-
tion, but such material may eventually break
through. A different material, with higher pres-
sure phase-changes, e.g. eclogite, can be stranded
by phase-changes in peridotite. Eclogite can
density-equilibrate at depths above 600 km (Fig-
ure 9.2). Chemical discontinuities, even those
with very small density jumps, can be a barrier --
or filter -- to through-going convection.
Delaminated continental crust is a particu-
larly potent source of mantle heterogeneity, low-
velocity zones and melting anomalies; it starts
out warmer and equilibrates faster than sub-
ducted oceanic crust. It is also low in SiO
2
, which
means it has more buoyancy below some 400 km
where subducting MORB may have a high dense
stishovite component. Large fertile low melting-
point blobs trapped in the upper mantle may
be responsible for 'melting anomalies' and LVZs.
These recycling, filtering and sampling pro-
cesses can explain many geochemical observa-
tions while avoiding the pitfalls associated with
isolated mantle reservoirs and deep penetration
of all slabs and all components.
High-resolution seismological techniques
involving reflected and converted phase and scat-
tering are starting to reveal the real complexity
of the mantle. Abrupt seismic discontinuities are
not necessarily isotope or reservoir boundaries
and the deeper layers are not necessarily acces-
sible to surface volcanoes. Plate tectonics and
geochemical cycles may be entirely restricted to
the upper
0
200
300
400
500
700
Depth (km)
600
800
900
1000
1100
1200
Fig. 9.4
Robust reflections from the mantle (Deuss and
Woodhouse, 2002).
are about 9% slower. Mixing with normal mantle
will reduce these differences; heating and melt-
ing will increase them. The main point is that
lateral and radial reductions in seismic veloc-
ity of order 2--10% can have a simple petrolog-
ical explanation. Shallow LVZ may be, in part,
due to adiabatic upwelling of displaced astheno-
sphere but this also need not be particularly
hot.
Velocity reversals, or low-velocity zones (LVZ)
have been identified in regional studies at depths
near 100, 185, 380, 410, 460--480, 570--600, 610
and 720 km (Nolet & Zielhuis, 1994; Vinnik
et al.
,
2003). The velocity reduction in these LVZ is gen-
erally between 2--5%. These LVZ are almost invari-
ably attributed to the effects of water, partial
melting or high temperature. These LVZ are in
addition to those that occur in the upper 200 to
350 km in tectonic and volcanic regions such as
Yellowstone, Iceland, western North America and
near oceanic ridges. The LVZ that occur just above
the major phase-change boundaries at 410 and
650 km are particularly instructive since these
are the places where one expects to find barri-
ers to certain kinds of subducted or delaminated
materials.
Tomographic studies suggesting that some
slabs cross the 650 km mantle discontinuity
do not imply that all do. The transition zone
may act as a petrological filter. Recycled mate-
rial can also be trapped at other depths -- deeper
and shallower; thick, cold slabs can sink further
andtakelongertowarmup;youngerslabsor
those with thick crust tend to underplate con-
tinents. The dry and depleted residual phases --
1000 km, where thermal expansion
is high and melting points, viscosity and thermal
conductivity are low.
The seismic velocities of plausible materials in
the mantle differ little from one another, even
if the density contrasts are adequate to perma-
nently stabilize the layering against convective
∼
