Geoscience Reference
In-Depth Information
12
GARNET
PERIDOTITE
a
+
opx
+
cpx
+
gt
stishovite
100
2b
+
st
+
gt s.s.
11
+
cpx
Al
2
O
3
400 km
ilmenite
Δ
T
=
800
°
C
b
+
mj s.s.
2g
+
st
+
gt s.s.
g
perovskite
10
+
cpx
+
gt
+
cpx
1700
°
C
MgO
850
°
b
200
ilm s.s.
+
9
garnet
majorite
2 pv
+
gt
+
mw
gt
+
mw
olivine
+
Ca
−
pv
670 km
orthopyroxene
3 pv
+
mw
8
T
T
−
800
°
C
clinopyroxene
300
3.0
100
200
300
3.2
3.4
3.6
3.8
4.0
4.2
Pressure (kbar)
cm
3
)
Density (g
/
Fig. 22.11
Compressional velocities, at standard
conditions, and stability fields of mantle minerals. The
approximate compressional velocity of garnet peridotite, at
standard (STP) conditions (
P
=
0,
T
=
20
◦
C) for phases
stable at two temperatures (850
◦
C, 1700
◦
C) is also shown.
The small bar gives the approximate change in
V
P
for a 800
◦
C
change in temperature.
Fig. 22.10
Zero-pressure density for peridotite at two
temperatures (a 'normal' geotherm and a temperature
800
◦
C colder). Phase changes and thermal expansion are
included. The density differences would be different for an
eclogitic mantle or for a slab that differs in chemistry from
the surrounding mantle.
melting point of eclogite is so low that eclogite
would likely not be a permanent resident of the
TZ but would yo-yo up and down. Some eclog-
ites may even become neutrally buoyant above
410 km.
The measured or estimated compressional
velocities of the important phases of upper-
mantle minerals are shown in Figure 22.11. Also
shown are the estimated velocities for peridotite
at two temperatures, taking into account the dif-
ferent stable phase assemblages. The major dif-
ferences occur between about 130 and 225 kbar.
The heavy lines show the approximate stability
pressure range for the various phases. Garnet
and clinopyroxene represent less than 20% of
peridotite compositions. Since these minerals are
stable to about 260 and 200 kbar, respectively,
the effect of density and velocity changes asso-
ciated with phase changes as both a function of
temperature and pressure will be lower for an
olivine and orthopyroxene-poor mantle -- eclog-
ite or piclogite.
The small bar in Figure 22.11 shows a typical
change of velocity for an 800
◦
C temperature
change, assuming no phase changes. Note that
the effect of phase changes is to double or triple
the
occur between about 400 and 700 kilometers, the
depth range of deep-focus earthquakes. Note that
clinopyroxene and garnet, minor constituents of
peridotite and pyrolite but major components
of eclogite and piclogite have the largest stabil-
ity fields of the low-pressure minerals. The pres-
ence of clinopyroxene and garnet reduces the size
of the phase-change effects in the upper part of
the transition region, particularly near 400 km.
The smallness of the velocity jump near 400 km
indicates the presence of substantial amounts of
a 'neutral' component -- garnet and clinopyrox-
ene -- near this depth. On the other hand, eclog-
ite experiences major transformations and veloc-
ity increases between 200 and 260 kbar (20 and
26 GPa), which is perhaps related to deep-focus
earthquakes.
Lower mantle mineralogy
Under mid-mantle conditions peridotite crystal-
lizes
into
an
assemblage
of
Mg--perovskite
+
Ca--perovskite
+
magnesiowustite; NMORB and
K-rich
basalt
compositions
crystallize
as
Mg--
perovskite
an alu-
minous phase with a CaFe
2
O
4
-type structure. In
+
Ca--perovskite
+
stishovite
+
effect
of
temperature.
The
largest
effects
