P. Schmoldt, PhD - MTNet - DIAS

5.2. Variation of electric conductivity with depth
Fig. 5.11.: Mineral proportions and phase transitions in the Earth’s mantle assuming pyrolitic composition, with the shaded areas
indicating the mantle transition zone between the 410 and 660 km discontinuities; from Yoshino [2010]. PX: pyroxene, OPX: orthopyroxene,
CPX: clinopyroxene, GRT: garnet, MJ: majorite garnet, OL: olivine, WD: wadsleyite, RW: ringwoodite, FP: ferro-periclase
(magnesiowüstite), PV: silicate perovskite, Ca-PV: Ca-perovskite.
world. Such studies would enable investigators to evaluate the hypotheses of a laterally
confined anomalous region by comparing the EM induction results with findings from
other methods, and in the long run to aid the merging of data from induction and laboratory
studies in the depth range of the LAB. Until then, exact electrical properties of the
upper asthenosphere will remain controversial.
Below the LAB, Xu et al. [2000a] infer an increase in resistivity at around 300 km
depth, coinciding with the disappearance of orthopyroxene (opx) in favour of clinopyroxene
(cpx) proposed for a pyrolitic bulk composition [Ringwood, 1975; Irifune and
Ringwood, 1987] (cf. Fig. 5.11), due to the relatively higher resistivity of cpx derived
in laboratory experiments [Xu and Shankland, 1999]. However, in global seismic models
no discontinuity is inferred for this depth, raising the question about sharpness of this
boundary, and its lateral extent. This finding supports the idea that the replacement of opx
by cpx takes place within a broader zone, and potentially, regionally at different depth
ranges.
The MTZ is located in the depth region between approximately 410 km and 670 km
with its boundaries electromagnetically defined by increases of conductivity, commonly
attributed to phase changes of olivine to its high-pressure polymorph wadsleyite, and
from ringwoodite to perovskite. Today, the existence of an additional phase change from
wadsleyite to ringwoodite is widely accepted and considered to occur at a depth of around
510 km. Exact composition of the MTZ and nature of its boundaries are still the subject
of ongoing debate (cf. Secs. 5.2.2 and 5.3).
MTZ conditions are of particular interest for geophysical studies as the MTZ plays an
important role in Earth convection models. The MTZ can provide a restraint and aggre-
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