P. Schmoldt, PhD - MTNet - DIAS

5.3. Parameters controlling the conductivity of the Earth’s mantle
Pressure The effect of pressure on the electric conductivity of the most abundant mantle
materials is smaller than the effects of temperature or fluid content; the σ-dependence
on pressure is small for olivine, its effect over an 800 MPa range is less than a temperature
change of 5 °C at temperatures between 1270 and 1440 °C [Shankland, 1975]. The
conductivity of the MTZ minerals wadsleyite and ringwoodite appears to be virtually
independent of pressure [Xu et al., 2000a]. This finding may be biased, because these
minerals are only stable over a narrow pressure interval, i.e. in the thin layer between top
and bottom of the MTZ. As a first order approximation, used in PREM (Fig. 5.1), pressure
increases in the mantle by around 47.5 MPa/km, yielding an approximate pressure
difference for the MTZ of only 12 GPa. However, this model certainly does not take into
account local pressure changes due to material composition or phase changes within the
mantle.
Water content Yoshino et al. [2008] formulated an equation for the relation of Earth’s
mantle electric conductivity onto different parameters that included the effect of water
content Cw in minerals (Eq. 5.11). The formulation proposes a linear as well as an
exponential dependence of σ on CW for the semiconduction in olivine and its highpressure
polymorphs wadsleyite and ringwoodite. However, due to instrumental limitations,
precise laboratory studies are very difficult at present and results are still under
debate [Yoshino, 2010].
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