Geoscience Reference
In-Depth Information
2400
liquid
3.85
100
COLD
MANTLE
cm
3
3.5 g
/
3 pv
+
2 pv
+
2000
mw
400
km
gt
+
mw
cm
3
3.3 g
/
b
+
mj s.s.
+
1600
cpx
NORMAL
MANTLE
4.0
ilm s.s.
+
cm
3
3.6
g
/
gt
+
mw
200
+
Ca
−
pv
ol (a)
+
+
g
2b
+
st
opx
+
cpx
3.65
+
gt s.s.
3.75
+
gt
1200
+
cpx
2g
+
st
670
km
+
gt s.s.
+
cpx
100
150
200
250
Pressure (kbar)
Fig. 22.9
Approximate phase relations in the mantle
system with an ol
>
opx
>
cpx
=
gt mineralogy at low
pressure. The geotherms bracket most estimates of
temperatures in 'normal' or average mantle. Warmer parts of
the mantle may be near the solidus; the interiors of slabs may
be 800
◦
C colder.
300
3.2
3.4
3.6
3.8
4.0
cm
3
)
Density (g
/
Fig. 22.8
Zero-pressure room-temperature density of
FeO-free peridotite (ol
>
opx
>
cpx
∼
gt) using mineral
assemblages appropriate for the temperatures in warm
('normal') mantle and slab ('cold') mantle. This illustrates that
lateral variations due to phase changes are as important as
radial changes, and that tomographic images cannot be
interpreted in terms of temperature variations operating on a
constant mineralogy mantle. The existance of eclogite blobs
in the mantle complicates the matter further.
In peridotite the density in the lower part of
the transition region (Figures 22.9 and 22.10) is
dominated by
3.6 g/cm
3
)or
-spinel
(3.7 g/cm
3
) and majorite (3.52 g/cm
3
) or ilmenite
(3.82 g/cm
3
). In eclogite the mineralogy is garnet
(3.6--3.8 g/cm
3
), calcium--perovskite (
β
-spinel
(
ρ
=
γ
1 g/cm
3
),
stishovite (4.92 g/cm
3
) and an aluminous phase
(
∼
4
.
4 g/cm
3
). Eclogite will have a STP density some-
where between about 3.70 and 3.75 g/cm
3
at the
base of the transition region, which is less than
the uncompressed density of the lower mantle.
Eclogites and peridotites can have the same den-
sity at transition zone pressures, but the silica-
poor eclogites will have lower shear velocities.
The garnet--majorite in cold quartz-rich eclogite
may convert to Ca--pv
∼
Sub-solidus phase relations in MORB
compositions
show
that,
at
this
pressure
MORB--eclogite
is
composed
of
the
phases
majorite
Ca--pv. At transition zone
pressures MORB contains mj
+
stishovite
+
st. Si-poor eclog-
ite, such as lower crustal cumulates or melt
depleted eclogites, will not contain stishovite and
are therefore buoyant at relatively moderate pres-
sures. Garnet solid solutions, including majorite,
arestableoveraverylargepressureandtempera-
ture range. This is important since large amounts
of garnet will decrease both the radial and lateral
variations in physical properties, and decrease
the jumps at the 410 and 650 km phase bound-
aries.
+
Mg--pv at the base of the
TZ and this would allow the eclogite to overpower
the phase change at 650 km.
The high gradient in seismic velocity between
about 400 and 600 km depth implies a gradual
phase change or series of phase changes occur-
ring over this depth interval, or a change in
composition. One cannot rule out an important
role for eclogite in the transition region. The
+
