Let’s take it from the top: the crust and upper mantle - New Theory of the Earth - page 105

(MgFe) 2 SiO 4 from the olivine to the -spinel --

wadsleyite -- structure (Table 8.12). The best fitting
mineralogy at this depth contains less than 50%
olivine. The velocity jump at 410 km is too small
to accommodate all the olivine (ol) and ortho-
pyroxene (opx) in a pyrolite mantle converting to
spinel and majorite; there must be substantial gt
and cpx.
In some places there is a shear-wave
velocity drop of about 5% on top of
the 410 km discontinuity . This low-velocity
zone has a variable thickness ranging up to per-
haps 90 km. This may be due to a dense partial-
melt layer, in which the solidus has been reduced
by the presence of eclogite or CO 2 . Eclogite itself,
at depth, has a lower shear velocity than peri-
dotite and even cold eclogite can be a low-shear
velocity zone at 400 km.

β

50

40

30

1.82

Gt

1.78

Cpx

1.74

OL

No. Atlantic
Rise

Fo
Jd

1.70

− Tectonic

Shield
W. Pacific

Opx

1.66

1.62

0 100 200 300 400

Depth (km)

Anisotropy

Fig. 8.8 Seismic parameters for two petrological models
and various seismic models. Symbols and sources are the
same as in Figure 8.7. V p / V s ratios for various minerals are
shown in the lower panel. The high V p / V s ratio for the
rise-tectonic mantle is consistent with partial melting in the
upper mantle under these regions.

Rayleigh and Love wave data are often 'incon-
sistent' in the sense that they cannot be fit
simultaneously using a simple isotropic model.
This has been called the Love-wave--Rayleigh-
wave discrepancy, and attributed to anisotropy.
There are now many studies of this effect
which has also been called polarization
or radial anisotropy, or transverse
isotropy . Independent evidence for anisotropy
in the upper mantle is now strong (e.g. from
receiver-functions amplitudes and shear-wave
splitting). Radial anisotropy is very strong in the
central Pacific. Anisotropy can be caused by crys-
tal orientation or by a fabric of the mantle caused
by slabs, dikes and sills. Seismic waves have such
long wavelengths that it is immaterial whether
the effect is due to centimeter- or tens-of-km-size
features. Similarly, anelasticity may be due to km-
size scatters, rather than to cm-sized dislocations.

Pyrolite is a garnet peridotite composed mainly
of olivine and orthopyroxene. Piclogite is a
clinopyroxene- and garnet-rich aggregate with
some olivine. Note the similarity in the
calculated velocities. Below 200 km the seismic
velocities under shields lie near the 1400 ◦ C adi-
abat. Above 150 km depth the shield lithosphere
is most consistent with cool olivine-rich mate-
rial. The lower velocity regions have velocities
so low that partial melting or some other high-
temperature relaxation mechanism is implied.
The adiabats falling below the solidus curves
are predicted to fall in the partial-melt field. At
depth, eclogite and piclogite have lower shear
velocities and lower melting temperatures than
peridotites. Low shear-velocity regions may be
subducted or delaminated crust.

The transition region (TR)

The mantle transition region , Bullen's
region C is defined as that part of the man-
tle between 410 km and 1000 km (the Repetti
discontinuity). The lower mantle was defined
by Bullen as the mantle below 1000 km depth

The 410 km discontinuity

The seismic discontinuity at 410 km depth is
generally attributed to the phase transition of