P. Schmoldt, PhD - MTNet - DIAS

5. Earth’s properties observable with magnetotellurics
Xu et al., 2000
EMIS
Egbert and Booker, 1992
Schultz et al., 1993
Lizarralde et al., 1995
Yoshino et al., 2008
Fig. 5.10.: Compilation of resistivity–depth profiles derived by deep-probing electromagnetic (EM) induction studies (MT, GDS) and
laboratory experiments on mantle minerals. The dashed red oval indicates the region related to the upper mantle, for which significant
difference between EM induction studies and laboratory experiments is observable. light-green shaded area: common resistivity range
for deep-reaching EM induction studies in (not tectonically active) regions, cf. e.g. Eaton et al. [2009]; black shaded area: thin
lithosphere and partial melt-bearing mantle region beneath the western US [Egbert and Booker, 1992]; yellow shaded area: stable
Archean craton region in the south-central part of the Canadian Shield (Superior Province) [Schultz et al., 1993]; light-blue shaded
area: oceanic setting in the Northeastern Pacific region [Lizarralde et al., 1995]; light-grey shaded area: range of laboratory results
with most conductive values derived by Xu et al. [2000a] and most resistive values derived by Yoshino et al. [2008] for a relatively dry
mantle (water content ≤ 0.1 wt.%).
resulting in a shift of theoretical curves towards more conductive values. ten Grotenhuis
et al. [2004] derived an inverse relation between bulk conductivity and grain size of a
region, proposing that upper mantle shear zones can exhibit a conductivity increase of
1.5 – 2 orders of magnitude in respect to less deformed lithospheric regions. The relative
motion between lithosphere and sublithospheric-mantle can potentially yield a similar
fine grained region, which facilitates a local conductivity increase.
Hypotheses C: the increased conductivity of the uppermost asthenosphere region is
due to special properties of the layer that are not considered by laboratory studies. If
those special properties are restricted to the related depth range and reduction of conductivity
occurs below the asthenosphere, EM induction studies would meet the results
of laboratory studies for subjacent regions. Potential properties (presuming a significant
amount) that can facilitate high conductivity values at that depth: (i) partial melt (small
fractions of, particularly carbonotite, melts can significantly reduce the resistivity [e.g.
Gaillard et al., 2008; Yoshino et al., 2010]), originating from shear processes along the
LAB, (ii) hydrogen, originating from dehydration of subducting slabs, (iii) a refertilised
mantle. Certainly, such anomalous properties require special settings of related processes
and a global extend is therefore unlikely. Additional long-term EM induction studies are
required to investigate existence of the electric asthenosphere in different regions of the
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