Geoscience Reference
In-Depth Information
center
surface
depth, km
1
1000
500
0
1500
50% oliivne
50% orthopyroxene
1500
10 -5
water effect
1000
500
10 -10
0
0.5
1.0
0.5
1.0
0
R (radius)/R o
R (radius)/R o
(a)
(b)
Fig. 5.20 (a) Electrical conductivity profile of the Moon inferred from the observed electromagnetic induction and
(b) its interpretation in terms of temperature (from Hood et al ., 1982b). The temperature-depth model shown here is
based on the electrical conductivity of dry (water-free) olivine and orthopyroxene assuming all aluminium is in
orthopyroxene. If the influence of water is included, the inferred temperature will be reduced. A gray region shows
the temperature-depth profile inferred from seismic wave velocities and gravity data by Khan et al . (2007).
(Lambeck & Pullan (1980) obtained similar results from the analysis of the gravity anomalies.) Reproduced with
permission of Elsevier.
interior (lower crust, upper mantle and transi-
tion zone), electrical conductivity is dominated
by hydrogen. However, the original idea by Karato
(1990) needs to be modified. In many cases, the
enhancement of electrical conductivity by hydro-
gen is not the direct effect of all the dissolved
hydrogen. Not all hydrogen atoms in a mineral
have high mobility. The mobility of hydrogen is
different among various minerals. A hybrid model
is needed to explain the experimental results in
which the role of minor hydrogen-bearing defects
is emphasized. In addition to ionic defects, elec-
tronic defects created by hydrogen-related defects
may play an important role. Also, dehydration
of hydrous minerals (in the mid- or lower crust)
enhances conductivity through the change in the
oxidation state of iron.
Some differences were noted between the ex-
perimental results from Yoshino's group (for a
review see Yoshino (2010) and Karato's group
(Karato, 2011) and a summary presented here).
Reasons for these differences were discussed in
the previous papers (see e.g., Karato, 2011; Karato
& Dai, 2009) but reiterated in this chapter. Most
of our conclusions are also supported by stud-
ies by Yang et al . (2011, 2012) who use the
impedance spectroscopy and determine the wa-
ter content through the careful FTIR spectroscopy
(their ''dry'' samples contain very little water < 10
ppm wt). The basic features of the results from
Yang's group and those from the Karato's group
agree well. In addition to the use of inappro-
priate, single low frequency in many studies by
Yoshino's group, the most serious problem in the
studies by Yoshino's group is the poor character-
ization of water content in their ''dry'' samples. 9
9 Recently Yoshino's group started using the impedance
spectroscopy and in some studies they use nearly dry
samples (e.g., Yoshino and Katsura (2012)). In these
cases, the difference between these two groups is
smaller.
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