Geoscience Reference
In-Depth Information
material has been sweated out of the lower
mantle,thenitwillbelowinCa,Al,U,Thand
K, among many other things. The lower mantle
would then be mainly oxides of Si, Mg and Fe.
The uncertain spin-state and oxidation-state of Fe
introduces a bit of spice into lower mantle min-
eralogy.
The
composition
of the lower mantle is another
story. Most plausible compositions have similar
properties. Candidates for the dominant rock
types in various deep-mantle layers are so sim-
ilar in seismic properties that standard methods
of seismic petrology fail. Small differences in den-
sity, however, can irreversibly stratify the mantle
so it is methods based on density, impedance,
anisotropy, dynamic topography, pattern recog-
nition, scattering and convective style that must
be used, in addition to seismic velocity. Visual
inspection of color tomographic cross-sections
cannot reveal subtle chemical contrasts.
Several methods have been used to estimate
the composition of the lower mantle from seis-
mic data but they are all non-unique and require
assumptions about temperature gradients, tem-
perature and pressure derivatives, equations of
state and homogeneity. Perhaps the most direct
method is to compare shock-wave densities at
high pressure of various silicates and oxides with
seismically determined densities. There is a trade-
off between temperature and composition, so this
exercise is non-unique. Materials of quite differ-
ent compositions, say (Mg,Fe)SiO
3
(
perovskite
)and
(Mg,Fe)O, can have identical densities, and mix-
tures involving different proportions of MgO, FeO
and SiO
2
can satisfy the density constraints. In
addition, the density in the Earth is not as well
determined as such parameters as the compres-
sional and shear velocities. The mineralogy and
composition of the lower mantle are hard to
determine since plausible combinations of per-
ovskite and magnesiowustite ranging from chon-
dritic to pyrolite have similar elastic properties
when FeO and temperature are taken as free
parameters. But they can differ enough in den-
sity to allow chemical stratification that is stable
against overturn. Oxide mixtures, such as MgO
It can be shown that a chondritic composition
for the lower mantle gives satisfactory agreement
between shockwave, equation of state and seis-
mic data, for the most plausible lower mantle
temperature. The SiO
2
content of the lower man-
tle may be closer to chondritic than pyrolitic. If
the lower mantle falls on or above the 1400
◦
C
adiabat, then chondritic or pyroxenitic compo-
sitions are preferred. If temperatures are below
the 1200
◦
C adiabat, then more olivine (
perovskite
plus (MgFe)O) can be accommodated. A variety
of evidence suggests that the higher tempera-
tures are more appropriate. The temperature gra-
dient in the lower mantle can be subadiabatic
or superadiabatic. Attempts to estimate composi-
tion assume chemical and mineralogical homo-
geneity and adiabaticity but the problem is still
indeterminate. A variety of chemical models can
be made consistent with the geophysical data but
the actual chemical composition of the lower
mantle is unknown, except within very broad
limits. Equation-of-state modeling is much too
blunt a tool to 'prove' that the lower mantle has
the same, or different, chemistry as the upper
mantle.
Internal chemical boundaries in the mantle,
in contrast to phase boundaries, and the sur-
face, Moho and core--mantle boundaries, must-
exhibit enormous variations in depth, because
of the low density contrast. This plus the low
predicted seismic impedance means that compo-
sitional boundaries are difficult to detect, even
if they are unbreachable by mantle convection.
They are
stealth boundaries
.
Low-spin Fe
2
+
Fe undergoes a spin-transition at high
pressure
with a large reduction in ionic radius
and a probable increase in the bulk modulus
and seismic velocities. The transition may be
spread out over a large depth interval The major
minerals in the deep mantle are predicted to
be almost Fe-free perovskite [MgSiO
3
] and Fe-
rich magnesiowustite, (Mg,Fe)O. This has several
important geodynamic implications. Over time,
the dense FeO-rich material may accumulate,
irreversibly, at the base of the mantle, and, in
addition, may interact with the core. The lattice
conductivity of this iron-rich layer will be high
+
SiO
2
(stishovite), can have densities, at high pres-
sure,
similar
to
compounds
such
as
perovskite
having the same stoichiometry.
