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In-Depth Information
seismically observed lateral variations will not
merely reflect temperature derivatives even in a
chemically homogenous mantle.
Akimotoite may dominate in cold parts of
the transition zone, i.e. in areas near subduc-
tion zones. It has higher velocities than garnet
at similar depths in hotter regions of the tran-
sition zone, producing a local velocity high that
may be misinterpreted as pile-up of subducted
materials.
and delaminated continental crust -- are stable at
depth in the mantle, until they warm up to ambi-
ent mantle temperatures. Garnets and eclogites
are nearly elastically isotropic.
Garnets
and
perovskites
are both very accommo-
dating of major elements and incompatible ele-
ments -- they have been called
junk-box minerals
.
They are expected to give diagnostic signatures
to any melts that they interact with.
Clinopyroxene, cpx
Clinopyroxene consists of diopside (CaMgSi
2
O
6
),
hedenbergite (CaFeSi
2
O
6
), and jadeite (NaAlSi
2
O
6
);
and the orthopyroxenes, enstatite (Mg
2
Si
2
O
6
)and
ferrosillite (Fe
2
Si
2
O
6
).
Diopside is a pyroxene mineral that forms
a solid solution series with hedenbergite and
augite (Ca,Na)(Mg,Fe
2
+
,Al,Ti)(Si,Al)
2
O
6
. The ionic
radius of calcium is much greater than alu-
minum, and this is expected to make the tran-
sition pressure to the garnet structure much
higher than for orthopyroxene. The pyroxene gar-
nets form solid solutions with ordinary alumi-
nous garnets, the transition pressure decreasing
with Al content.
The mineralogy in the transition region, at
normal mantle temperatures, is expected to be
the olivine--spinels plus garnet solid solutions. At
colder temperatures, as in slabs and delaminated
continental crust, the mineralogy at the base
of the transition region includes
ilmenite solid-
solution
(Figure 22.4). The cold parts and the warm
parts of the mantle do not necessarily have the
same mineralogy, even at the same depth. This
can be more important that density differences
associated with thermal expansion, the main
buoyancy effect considered in fluid dynamic sim-
ulations of mantle convection. This can also be
important in interpreting tomographic cross sec-
tions. The garnet component of the mantle is sta-
ble to very high pressure, becoming, however, less
aluminous and more siliceous as it dissolves the
pyroxenes. At low temperature and at pressures
equivalent to those in the lower part of the tran-
sition region, the garnet as well as the pyroxenes
are probably in ilmenite solid solutions.
The
ilmenite structure
of orthopyroxene can be
regarded as a substitution of MgSi for 2Al in
the corundum structure. The transformation of
Garnet, gt
Garnets are cubic minerals of various composi-
tions and can incorporate Mg, Fe
2
+
or Ca for the
common garnets or almost any 2
element. Gar-
net exists in two compositional groups, which
form intragroup but not intergroup solid solu-
tions. The calcic group contains
uvarovite, grossu-
lar
and
andradite
. The non-calcic group contains
pyrope, almandine
and
spessartite
.
Some natural garnets have Cr
3
+
or Fe
3
+
instead of Al
3
+
. Garnets are stable over an enor-
mous pressure range, reflecting their close pack-
ing and stable cubic structure. They are proba-
bly present over most of the upper mantle and,
perhaps, below 650 km, at least when they are
colder than ambient mantle. Furthermore, they
dissolve pyroxene at high pressure, so their vol-
ume fraction in the mantle expands with pres-
sure. Garnets are the densest common upper-
mantle mineral, and therefore eclogites and
fertile (undepleted) peridotites are denser than
basalt-depleted peridotites or harzburgites. On
the other hand, they are less dense than other
phases that are stable at the base of the transi-
tion region. Therefore, eclogite can become less
dense than the dominant mantle lithology at
great depth. Most eclogites are less dense then
+
-
spinel and some majorites, so a perched eclogite-
rich layer may form near 500--600 km depth,
in the middle of the transition regions. Silica-
poor eclogites may be trapped by the 400 km
discontinuity. Garnet has a low melting point
and is eliminated from peridotites in the upper
mantle at small degrees of partial melting. The
large density change associated with partial melt-
ing of a garnet-bearing rock is probably one of
the most important sources of buoyancy in the
mantle. Cold eclogite -- subducted oceanic crust
γ
