Geoscience Reference
In-Depth Information
gishly (because of the
h
3
term in the Rayleigh
number) but its presence slows down the cool-
ing of the mantle and the core. The overlying
FeO-poor layer may have high radiative conduc-
tivity, because of high
T
and transparency, and
have high viscosity and low thermal expansivity,
because of
P
effects on volume. This part of the
mantle will also convect sluggishly. If it repre-
sents about one-third of the mantle (by depth) it
will have a Rayleigh number about 30 times less
than Rayleigh numbers based on whole mantle
convection and orders of magnitude less than Ra
based on
P
mantle minerals. Fe is more-or-less uniformly par-
titioned among the major minerals. This and low
temperatures suppress the role of radiative trans-
port of heat.
Two electronic configurations,
high-spin and low-spin, are possible
for Fe
2
+
in the lower mantle
. The high-spin
(HS) state is usually stable in silicates and oxides
at normal pressures. The ionic radius of the
low-spin (LS) state is much smaller than the
high-spin state, and a spin-pairing transition is
induced by increased pressure. A large increase
in density accompanies this phase transfor-
mation. For example, the volume change accom-
panying a phase change in FeO due to the
high-spin--low-spin transition, is expected to be
11--15%. This far exceeds other phase changes in
the deep mantle. Partial transformation is also
possible so smaller volume changes may also
occur.
The small ionic radius of Fe
2
+
(LS) probably
means that Fe
2
+
will not readily substitute for
Mg
2
+
under lower-mantle conditions. Additional
Fe
2
+
(LS)O-bearing phases will form with high
densities and bulk modulus and this means that
the
lower mantle could be enriched in
FeO and SiO
2
relative to the upper man-
tle
. The magnesium-rich phases of the lower
mantle may be relatively iron free:
0 properties.
The LS iron in the deep mantle would behave
likeadifferentelementthantheHSironat
shallow depths. There may be phase separation
between Fe-rich and Mg-rich phases. The melt-
ing temperature of the Fe-rich end member may
be higher than the Mg-rich end members. The
HS Fe
2
+
ions in the lower mantle may hinder
blackbody radiation in the near-infrared, allow-
ing more efficient radiative heat transfer.
=
Phase equilibria in mantle systems
at high-pressure
The lateral and radial variations of seismic veloc-
ity and density in the mantle depend, to first
order, on the stable mineral assemblages and,
to second order, on the variation of the veloci-
ties with temperature, pressure and composition.
Temperature, pressure and composition dictate
the compositions and proportions of the vari-
ous phases. In order to interpret observed seis-
mic velocity profiles, or to predict the veloci-
ties for starting composition, one must know
both the expected equilibrium assemblage and
the properties of the phases. Olivine, ortho-
pyroxene, clinopyroxene and an aluminous phase
(feldspar, spinel, garnet) are stable in the shal-
low mantle.
MgFeSiO
4
→
MgSiO
3
(perovskite)
+
FeO(LS)
which
would
facilitate
the
entry
of
FeO
into
molten iron and removal to the core.
The major minerals in the deep mantle may
be almost Fe-free perovskite, Mg--pv and Fe-rich
magnesiowustite [(Mg,Fe)O] and post-
perovskite
(ppv) phases. This has several important geody-
namic implications.
Perovskite
, being the major
phase, will control the conductivity and vis-
cosity. Radiative conductivity and viscosity may
be high in Fe-poor minerals. Both effects will
tend to stabilize the mantle against convection
and decrease the Rayleigh number. Over time,
the dense FeO-rich phases may accumulate, irre-
versibly, at the base of the mantle, and, in addi-
tion, may interact with the core. The lattice con-
ductivity of this iron-rich layer will be high and
the radiative term should be low but the trade-
offs are unknown. A thin layer convects slug-
β
-spinel, majorite, garnet and clino-
pyroxene are stable in the vicinity of 400 km,
near the top of the transition region.
γ
-spinel,
majorite or
-spinel plus stishovite,
Ca--perovskite
,
garnet
and
ilmenite
are stable between about 500
and 650 km.
Garnet, ilmenite, Mg--perovskite, Ca--
perovskite
and magnesiowustite are stable near
the top of the upper mantle, and
perovskites
and
γ
