Geoscience Reference
In-Depth Information
attributed to crystal orientation and grain bound-
ary melting.
Cold harzburgite, a component of the slab,
averages about 0.1 g/cm
3
denser than warm pyro-
lite between 400 and 600 km depth. At 600 km
it becomes less dense. Some eclogites are denser
than peridotite at the same temperature to
depths of about 500--560 km and, when cold, to
680 km. Subducted oceanic crust is silica-rich and
is denser than other eclogites, at depth, because
of the presence of stishovite (st). Cold eclogite
can be 4--5% denser than warm peridotite above
550 km depth but eclogites have a wide range of
compositions, mineral proportions and densities.
If the 1000-km discontinuity (Repetti Discontinu-
ity) is a chemical boundary, it will be depressed
by the integrated density excess in overlying cold
mantle even if the deeper part of the slab is buoy-
ant. Chemical interfaces in general are expected
to be irregular boundaries in a chemically strati-
fied mantle and to be much deeper under slabs.
The different phase assemblages in the slab rela-
tive to warm mantle will contribute to the den-
sity and seismic velocity contrasts. An increase of
intrinsic density between upper and lower man-
tle and a negative Clapeyron slope will inhibit
slab penetration into the lower mantle (defined
by Bullen to start at 1000 km depth, the top
of his region D
). An increase in viscosity will
also partially support the slab. Some support
is required in order to explain the geoid highs
associated with subduction zones. A chemical
change has a similar effect in holding up the
slab.
The time scale for heating the slab to above
the dehydration and melting points is a small
fraction of the age of the plate upon subduction
since the basalt and volatiles are near the top
of the slab. It is even smaller for delaminated
lower continental crust since this is already hot,
being about midway into the thermal boundary
layer. As far as the slab, or a piece of delam-
inated crust (
delaminate
), is concerned the sur-
rounding mantle is an infinite heat source. The
idea that low-velocity material in the mantle can
be low temperature should not be overlooked
when interpreting seismic images. Seismic veloc-
ity is controlled by composition, mineralogy, and
volatile content as well as by temperature. If
the melts and volatiles completely leave the peri-
dotitic part of a slab then it can become a high
seismic velocity anomaly, at least between depths
of order 60 to 600 km. Deeper than that the eclog-
ite in the slab can become low-density and low-
velocity compared to normal mantle.
The fate of subducted and delaminated mate-
rial has been controversial. Below about 50 km
depth, basalts convert to eclogites with a con-
siderable increase of density. However, eclogite is
not a uniform rock type; it comes in a variety of
flavors and intrinsic densities. NMORB, for exam-
ple, is silica-rich and contains the dense phase
stishovite
at high pressure. Cold NMORB
can probably sink to about 650 km before it
is neutrally buoyant. If it is cold enough so
that both stishovite and
perovskite
are stable
then it can possibly breach the density barrier
at 650 km, if only temporarily. Warmer slab,
and SiO
2
-poor eclogites can thermally equilibrate
at shallower depths. The subduction depth also
depends on the crustal thickness of the plate,
the compositions of lower crustal cumulates,
and whether the crustal part of the slab can
detach from the mantle part. In addition, the
650 km phase change region is not the only
plausible barrier to through-going convection or
subduction.
The evidence from obduction, ophiolites,
flat subduction and
exhumed ultrahigh-
pressure
(UHP) crustal and slab fragments con-
firms the shallow, and temporary, nature of
some recycling. The evidence from tomography
for
Reheated slabs and delaminates
Slabs are cold when they enter the mantle but
they immediately start to thermally equilibrate.
Ambient mantle warms up slabs from both sides.
Slabs, in part, are composed of oceanic crust and
in part of serpentinized peridotite; CO
2
and water
occur in the upper parts. All of these effects serve
to lower the melting point and seismic velocities
of slabs compared to dry refractory peridotite.
After the conversion of basalt to eclogite, the
melting point is still low. Even small amounts
of fluid or melt can drastically lower the seismic
velocity, even if the density remains high.
recumbent
slabs
at
650
km
also
suggests
