P. Schmoldt, PhD - MTNet - DIAS

5. Earth’s properties observable with magnetotellurics
1000/T (K -1 )
1.1
1
0.9
0.8
0.7
0.6
0 2 4
Log (ρ) (Ωm)
10
6
olivine
orthopyroxene
clinopyroxene
ilmenite plus garnet
aluminium-free perovskite
aluminium-bearing perovskite
magnesiumwuestite
Fig. 5.14.: Conductivity – temperature profiles for typical mantle minerals using an Arrhenius-like relationship (Eq. 5.9 with the
assumption that contribution of proton conduction is negligible); see Table 5.7 for references regarding mineral parameters.
the effect of water onto the small polaron conductivity (Eq. 5.11, Fig. 5.15). Through
their experiment Yoshino et al. [2008] could show that the mantle transition zone (MTZ,
Sec. 5.2.2) has to be essentially dry and that changes of conductivity in the MTZ can
be adequately described by transformation of olivine into its high-pressure polymorphs
wadsleyite and ringwoodite. However, they admit that the MTZ cannot be proven to be
entirely dry, since water content of less than 0.1 wt% cannot be adequately resolved for
the normal geotherm. This is due to fact that at MTZ temperatures the contribution of
proton conduction is masked by small polaron conduction owing to the higher activation
enthalpy of the latter [Yoshino et al., 2008]. Whereas exact parameters of pre-exponential
factors and activation energies are still under debate, wide agreement exists regarding
general relations between mantle material conductivity and the dominant controlling parameters,
temperature, pressure, and water content. A short summary about the relations
is given in the following paragraphs.
Temperature Temperature is the dominating factor of electric conductivity within the
Earth’s mantle due to the exponential relationship between semiconduction and temperature
(Eq. 5.9 and 5.11). The σ–T relation is dependent on the temperature, which determines
whether the intrinsic or extrinsic component of semiconduction is the dominant
mechanism in charge transport (cf. Fig. 5.4). The transition from extrinsic to intrinsic
transport is material specific, e.g. for wadsleyite it is derived to takes place at approximately
1500 °C [Yoshino et al., 2008], whereas for olivine it does not occur before
1400 °C [Constable et al., 1992].
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