Geoscience Reference
In-Depth Information
mantle. A lithosphere composed primarily of
peridotite does not satisfy the seismic data. The
lithosphere, therefore, is not just cold astheno-
sphere or a pure thermal boundary layer. The
roots of cratons, however, do seem to be com-
posed mainly of cold olivine but they are buoy-
ant in spite of the cold temperatures. They are
therefore unlikely to fall off. They are probably
depleted residual after basalt extraction, and are
probably garnet- and FeO-poor.
It is likely that the upper mantle is lami-
nated, with the volatiles and melt products con-
centrated toward the top. As the lithosphere
cools, underplated basaltic material is incorpo-
ratedontothebaseoftheplate,andasthe
plate thickens it eventually transforms to eclog-
ite, yielding high velocities and increasing the
thickness and mean density of the oceanic plate
(Figure 8.4). Eventually the lower part of the plate
becomes denser than the underlying astheno-
sphere, and conditions become appropriate for
subduction or delamination.
The thickness of the seismic lithosphere, or
high-velocity LID, is about 150--250 km under
the older continental shields. A thin low-velocity
zone (LVZ) at depth, as found from body-wave
studies, however, cannot be well resolved with
long-period surface waves. The velocity rever-
sal between about 150 and 200 km in shield
areas is about the depth inferred for kimber-
lite genesis, and the two phenomena may be
related.
There is very little information about the deep
oceanic lithosphere from body-wave data. Sur-
face waves have been used to infer a thicken-
ing with age of the oceanic lithosphere to depths
greater than 100 km (Figure 8.4). However, when
anisotropy is taken into account, the thickness
may be only about 50 km for old oceanic litho-
sphere. This is about the thickness inferred for
the 'elastic' lithosphere from flexural bending
studies around oceanic islands and at trenches.
This is not the same as the thickness of the ther-
mal boundary layer (TBL) or the thickness of the
plate.
The seismic velocities of some upper-mantle
minerals and rocks are given in Tables 8.9
and 8.10. Garnet and jadeite have the highest
velocities, clinopyroxene and orthopyroxene the
lowest. Mixtures of olivine and orthopyroxene
0
40
80
120
160
200
0
20
350
°
C
40
650
°
C
60
Previous estimates
of seismic thickness
80
100
Seismic LID
Elastic thickness
120
Age of oceanic lithosphere (My)
Fig. 8.4
The thickness of the lithosphere as determined
from flexural loading studies and surface waves.
The upper edges of the open boxes gives the thickness
of the seismic LID (high-velocity layer, or seismic
lithosphere). The lower edge gives the thickness of the
mantle LID plus the oceanic crust (Regan and Anderson,
1984). The LID under continental shields is about
150--250 km thick.
(the peridotite assemblage) can have velocities
similar
to
mixtures
of
garnet--diopside--jadeite
(the
eclogite
assemblage).
Garnet-rich
assem-
blages,
however,
have
velocities
higher
than
orthopyroxene-rich assemblages.
The
V
p
/
V
s
ratio is greater for the eclogite
minerals than for the peridotite minerals. This
ratio plus the anisotropy are useful diagnostics of
mantle mineralogy. High velocities alone do not
necessarily discriminate between garnet-rich and
olivine-rich assemblages. Olivine is very anisotro-
pic, having compressional velocities of 9.89, 8.43
and 7.72 km/s along the principal crystallogra-
phic axes. Orthopyroxene has velocities ranging
from 6.92 to 8.25 km/s, depending on direction.
In natural olivine-rich aggregates (Table 8.11), the
maximum velocities are about 8.7 and 5.0 km/s
for P-waves and S-waves, respectively. With 50%
orthopyroxene the velocities are reduced to 8.2
and 4.85 km/s, and the composite is nearly
isotropic. Eclogites are also nearly isotropic. For a
