Geoscience Reference
In-Depth Information
effects similar to increasing pressure -- lateral
temperature changes are similar to vertical pres-
sure
180 kb
3000
a
+
opx
+
cpx
+
gt
200 kb
α
T
is the
approximate effect of thermal expansion alone
on density. The bar labeled 800
◦
C shows the
expected change in temperature across a sub-
ducted slab and is approximately half the max-
imum expected lateral temperature changes in
the mantle. Note that a temperature change of
800
◦
C placed anywhere in the field of tempera-
tures expected in the mantle will cross one, two
or even three phase boundaries, each of which
contributes a density change in addition to the
term from thermal expansion. Changes in elas-
tic properties are associated with these phase
changes.
Figure 22.8 shows the approximate zero-
pressure room-temperature density for
phase
relations in the CMAS system
with a
low-pressure mineralogy appropriate for garnet
peridotite
changes.
The
curve
labeled
a
+
Mj
+
cpx
2500
b
+
Mj
+
cpx
2000
g
+
Mj
+
cpx
a
Δ
T
g
+
pv
+
Ca
−
pv
1500
g
+
mj
+
cpx
g
+
g
+
ilm
+
gt
ilm
+
di
−
mj
+
Ca
−
pv
1000
800
°
+
gt
ilm
MgO
+
+
ilm
+
Mgo
+
Ca
−
pv
+
gt
ilm
+
MgO
500
Ca
−
pv
+
gt
+
Ca
−
pv
+
Al
2
O
3
0
3.0
3.5
4.0
cm
3
)
Density (g
/
Fig. 22.7
Approximate variation of zero-pressure density
with temperature taking into account thermal expansion
(curve to left) and phase changes at two pressures. The effect
of pressure on density is not included. The inclusion of
density jumps associated with phase changes increases the
average effect of temperature by a factor of 3 to 4. There are
seismic velocity changes associated with these phase changes
but these are usually ignored in visual interpretations of
tomographic cross-sections.
with
olivine
>
orthopyroxene
>
clinopyroxene
garnet. Note that the low-
temperature assemblages are denser than high-
temperature assemblages (normal mantle) until
about 600 km depth, and that the differences are
particularly pronounced between about 300 and
550 km. The density change associated with a
temperature change of 800 K ranges from 7--17%
in the temperature interval 1000 to 2300 K at
pressures near 600 km depth. This includes
thermal expansion and isobaric phase changes.
Thermal expansion alone gives 2--3%. Note that
the density anomaly associated with a slab is
far from constant with depth. Furthermore,
the density anomaly of a slab with respect to
the adjacent mantle is quite different from the
density contrast between the surface plate and
the underlying mantle, as estimated from the
bathymetry-age relation for oceanic plates.
The important phase changes in the mantle
mostly have Clapeyron slopes that correspond
to depth variations of 30--100 km per 1000
◦
C.
The figures in this section show that several
phase boundaries are crossed in covering the
normal expected range of mantle temperature,
at constant pressure. At 230 kbar (23 GPa)
Ca--
perovskite
, Mg--
perovskite
and magnesiowustite
is the normal assemblage in peridotite;
ilme-
nite
replaces Mg--
perovskite
at cold temperature.
∼
an important role for upper-mantle circula-
tion. The low-velocity zones above slabs suggest
that volatile and low-melting point components
leave the slabs at shallow depths. The inference
that some slabs break through 650 km, even
if a valid interpretation of some tomographic
cross-sections, does not imply that they all do.
Most of the material entering subduction zones
apparently
never
makes
it
to
650
km
depth,
but
1000 km.
This is based on cross-correlation and
plate
reconstruction studies
, not on
visual
evidence for occasional slab penetra-
tion
or
some
apparently
makes
it
to
∼
qualitative
analysis
of
tomographic
cross-sections.
Thermal phase changes
The approximate zero-pressure density for a peri-
dotitic assemblage as a function of temperature,
at two pressures, is shown in Figure 22.7. Tem-
perature is plotted increasing upward to empha-
size the fact that decreasing temperature has
