P. Schmoldt, PhD - MTNet - DIAS

5. Earth’s properties observable with magnetotellurics
gation zone for rising hot mantle material (plumes) originating at the CMB, as well as
for descending crustal material subducted from the Earth’s surface. Subducting slabs, for
instance, may be horizontally deflected in the MTZ and become dehydrated before continuing
to sink into the lower mantle [Richard and Bercovici, 2009]. This implies reduced
water content in the lower mantle, affecting local composition, density, and temperature
conditions. Permeability of the MTZ, hence characteristics of its interaction with such
vertical transport mechanisms, is highly dependent on its conditions [e.g. Davies, 1995;
Karato et al., 1995; Jin et al., August, 2001; Karato et al., 2001].
The composition of the Earth’s mantle
Two classic models exist for the composition of the mantle: pyrolytic (transformation
of the compounds caused by heat) and piclogitic (picritic eclogite). In pyrolytic mantle
models, the chemical difference between the upper and lower mantle is assumed negligible
and differences within the mantle are attributed to mineral phase changes, whereas
in piclogitic mantle models a more silica-(and iron-)rich lower mantle is proposed. Pyrolytic
models are in good agreement with measured data and are favoured by the majority
of authors [e.g. Ringwood, 1975; Poirier, 2000; Xu et al., 2000a]. Recent support for the
pyrolytic model was provided, for example, by the results of Matas et al. [2007] proposing
that the Earth’s mantle is most likely relatively homogeneous. The authors state that
the lower mantle must have an average Mg–Si ratio lower than 1.3 in order to satisfactorily
fit 1D seismic profiles. Moreover, Matas et al. [2007] claim that when a low value for
the pressure derivative of the shear modulus for perovskite is adopted (µ ′
0 ≈ 1.6 GPa/km),
consistent with the most recent experimental results, the Mg–Si ratio of the bottom part
of the lower mantle reduces to a value close to 1.18, thus denoting a more homogeneous
mantle composition.
The composition of the Earth’s mantle can either be derived directly from rock samples
transported from the lithospheric-mantle to the surface as kimberlitic or volcanic xenoliths,
or indirectly through interpretation of geophysical measurements. As a result of
those measurements it is inferred that olivine, clinopyroxene, orthopyroxene, garnet, and
perovskite are the most common minerals in the Earth’s mantle (cf. Fig. 5.11, and Tabs.
5.3 and 5.4). Olivine and its high-pressure polymorphs (wadsleyite, ringwoodite) form
the most abundant minerals by volume (50 – 60 %) in the upper mantle, thus dominating
its bulk electric conductivity, whereas perovskite and magnesiowüstite account for the
majority of the lower mantle minerals [e.g. Ringwood, 1975; Xu et al., 2000a].
In the absence of areas with well-connected networks of fluids, ion conductors, or partial
melt, the electric conductivity of the mantle is dominated by semiconduction (Sec.
5.1.3), therefore the conductivity of the mantle is mostly controlled by temperature (Eq.
5.9, Sec.5.3). Exact values of conductivity are dependent on specific parameters of the
related materials and their fraction of the local composition. Conductivity of mantle minerals
is primarily controlled by proton (H + ) and small polaron conduction (electron holes
“hopping” between Fe2+ and Fe3+ ) [Xu et al., 1998a; Yoshino et al., 2008; Yoshino, 2010]
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