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to the deep partial melting associated with the
Hawaii plume. A high conductivity near the
top of the asthenosphere near ridges,
10 3
10 S/m
10 1 S/m
(Baba, 2005), may require partial melting, but
in most of the asthenosphere partial melting is
not required to explain the inferred electrical
conductivity.
In the transition zone, electrical conductivity
is generally higher than that of the upper man-
tle. The electrical conductivity in the transition
zone cannot be explained by a dry mantle model
as incorrectly claimed by Yoshino et al . (2008a)
(recently Yoshino & Katsura (2012) measured the
nearly dry wadsleyite using the impedance spec-
troscopy, and their new results show considerably
lower conductivity than their earlier results and
this new result leads to the approximately same
conclusion as Karato, 2011). 7 It should be noted
that if one assumes constant water content, then
there should be a drop in the electrical conductiv-
ity at 410 km (the conductivity in the transition
zone would be smaller than that in the upper
mantle) (Figure 5.15). Therefore the increase in
the conductivity at 410 km implies an increase in
water content.
The trade-off between the water content and
temperature for the transition zone is illustrated
in Figure 5.18 (a similar trade-off for the upper
mantle can be seen in Figure 5.17). The average
conductivity of the transition zone (
10 2
10 1
1
10 0
10 -1
10 -1
10 -2
10 -2
10 -3
10 -4
10 -5
1500
1600
1700
1800
1900
2000
temperature, K
Fig. 5.18 The trade-off between water effect and
temperature effect on the electrical conductivity of the
upper transition zone. Both iron-related conduction
and hydrogen-related conduction are considered. A
mixture of 60% wadsleyite and 40% majorite is
assumed. The hatched regions show typical tempera-
ture and conductivity values in the upper transition
zone. For a conductivity of
10 1 S/m (see Figure 5.16),
the water content of
0.1 wt % is inferred.
tectonic history of these regions: subduction of
old and hence cold lithosphere has occurred for
more than
10 1 S/m)
400 Myrs in East Asia that likely
brought a large amount of water to the deep man-
tle (e.g., Maruyama, 1994, 1997; Maruyama &
Okamoto, 2007). In contrast, subducting slabs
around the Europe are generally young and hence
these slabs dehydrate in the shallow regions af-
ter they subduct. Consequently not much water
is transported to the deep mantle in these re-
gions (Maruyama & Okamoto, 2007). Iwamori
et al . (2010) presented a similar model of global
materials circulation based on the analysis of
chemical compositions of the MORB (mid-ocean-
ridge basalt) and OIB (ocean-island basalt) (see
also Chapter 13, this volume).
The high conductivity in the transition zone
may reflect a large amount of eclogite (sub-
ducted oceanic crust). In these cases, eclogite
must be connected, and the regional variation
can be explained by
0.1wt % water and tem-
perature of 1700-1900K. The transition zone in
south-central Europe has a significantly lower
conductivity that may partly be attributed to
the lower water content (as well as the lower
temperature). The transition zone in East Asia
has higher conductivity,
1 S/m. The water con-
tent of
1wt% is needed to explain such a
high conductivity. The inferred regional varia-
tion in the water content is consistent with the
7 A small difference between these two groups still ex-
ists. A possible cause for this difference is the difference
in the sample geometry. In Yoshino and Katsura (2012),
they used electrodes that are larger than the sample
size. This could cause a large contribution from the
surface conduction leading to larger than true bulk
conductivity.
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