Geoscience Reference
In-Depth Information
8
6
1000
4
500
500
300
100
0.1
2
0
0
400
800
1200
o
Temperature ( C)
Figure 9.13.
Effects of temperature and pressure on P-wave-velocities. Ultrasonic
P-wave-velocity measurements were made on dry basaltic (lower four curves) and
peridotitic (upper two curves) samples in a laboratory. The numbers beside each
curve denote the pressure (MPa) at which the measurements were made. Note the
sudden decrease in velocity at the solidus temperature and the steady decrease over
the solidus-liquidus interval (approximately 1000-1200
◦
C for basaltic samples and
1200-1300
◦
C for peridotitic samples). These measurements were made on dry
samples. The presence of water would reduce the solidus and liquidus temperatures
but would not greatly change the shape of curves: they would merely be shifted to
the left and slightly downwards. (After Khitarov
et al
.(1983).)
velocity gradients in the low-velocity zone (1 km s
−
1
over distances of 15-25 km)
are the result of partial melting of mantle material. The S-wave-velocities pre-
dicted by this model are in broad agreement with all the surface-wave-dispersion
results. Were the mantle drier than was assumed for these models, the melting
temperatures and hence the velocities would be somewhat higher.
All of the seismic-velocity models are dependent on the elastic properties
of partially molten systems. The effects of partial melting on seismic velocity
and attenuation are significant, as illustrated by an NaCl-ice system: for exper-
iments on a sample with 1% concentration of NaCl, reductions in the P- and
S-wave velocities of 9.5% and 13.5% were measured at the onset of melting, and
attenuation increased by 37% and 48%, respectively. These measurements were
made for a 3.3% melt. Partial melting can have a large effect on seismic velocity
and attenuation even at very low melt percentages. Results of experiments on
basaltic and peridotitic samples indicate that a large decrease in velocity occurs
over the liquidus-solidus temperature range (Fig. 9.13). Basalt P-wave velocities
