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300--400 km even if melt--solid separation does
not occur until shallower depths. Low-melting
point materials may be introduced into the shal-
low mantle from above, and heat up by conduc-
tion from ambient mantle. Upwellings do not
have to initiate in thermal boundary layers.
Although a strong case can be made for local-
ized or regional partial melting in the shallow
mantle, the average seismic velocity over long
paths, such as are used in most tomographic
studies, may imply subsolidus conditions. If the
partial melt zones are of the order of tens to hun-
dreds of kilometers in lateral dimensions, and
tens of kilometers thick, then they will serve to
lower the average velocity, and perhaps to intro-
duce anisotropy, in global tomographic models.
There are also compositional effects to be consid-
ered. Eclogite, for example, has lower shear veloc-
ities than peridotite at depths between about
100 and 600 km. Eclogite, however, also has a
melting point that is lower than ambient mantle
temperature.
8.5
8.0
Rise-Tectonic
No. Atlantic
Shield
W. Pacific
7.5
4.7
4.5
4.3
Absorption and the LVZ
Elastic-wave velocities are independent of fre-
quency only for a nondissipative medium. In
a real solid dispersion must accompany absorp-
tion. The effect is small when the seismic quality
factor
Q
is large or unimportant if only a small
range of frequencies is being considered. Even in
these cases, however, the measured velocities or
inferred elastic constants are not the true elas-
tic properties but lie between the high-frequency
and low-frequency limits or the so-called 'unre-
laxed' and 'relaxed' moduli.
The magnitude of the effect depends on the
nature of the absorption band and the value of
Q
. When comparing data taken over a wide fre-
quency band, the effect of absorption can be con-
siderable, especially considering the accuracy of
present body-wave and free-oscillation data. The
presence of physical dispersion complicates the
problem of inferring temperature, chemistry and
mineralogy by comparing seismic data with high-
frequency ultrasonic data. Anelasticity as well
as anharmonicity is involved in the temperature
dependence of seismic velocity.
Figures 8.8 and 8.9 show calculations for seis-
mic velocities for different mineral assemblages.
0
100
200
300
400
Depth (km)
Fig. 8.7
Compressional and shear velocities for two
petrological models, pyrolite and piclogite, along various
adiabats. The temperatures (
◦
C) are for zero pressure.
The portions of the adiabats below the solidus curves are
in the partial melt field. The seismic profiles are for two
shields (Given and Helmberger, 1981; Walck, 1984), a
tectonic-rise area (Grand and Helmberger, 1984a; Walck,
1984), and the North Atlantic region (Grand and
HeImberger, 1984b).
The melting that is inferred for the lower
velocity regions of the upper mantle may be ini-
tiated by adiabatic ascent from deeper levels. The
high compressibility and high iron content of
melts means that the density difference between
melts and residual crystals decreases with depth.
High temperatures and partial melting tend to
decrease the garnet content and thus to lower
the density of the mantle. Buoyant diapirs from
depths greater than 200 km will extensively melt
on their way to the shallow mantle. Therefore,
partially molten material as well as melts can
be delivered to the shallow mantle. The ulti-
mate source of some basaltic melts may be below
