Geoscience Reference
In-Depth Information
contains a large amount (
0.1 wt %) of water.
These results should not be interpreted as results
for ''dry'' minerals. Yoshino
et al
. (2008a)'s con-
clusion of ''dry mantle transition zone'' is based
on the use of the data from water-rich samples as
''dry'' samples and therefore unwarranted.
Also there is an issue on the water content
measurements. FTIR (Fourier-Transform In-
frared) spectroscopy and SIMS (Secondary Ion
Mass Spectroscopy) are most frequently used
techniques. For olivine, a significant difference
is found between these methods if Paterson
(1982) calibration is used (see Bell
et al
., 2003).
However such a discrepancy does not exist for
other minerals investigated (Aubaud
et al
., 2007).
In the following we use the calibration based on
SIMS (appropriate corrections have been made on
the results from FTIR measurements).
In summary, the critical conditions that need to
be met for an experimental study of the influence
of hydrogen on electrical conductivity are:
(1)
Impedance spectroscopy is used.
(2)
Water content is measured both before and
after the conductivity measurements and only a
small change in water content during the mea-
surements is documented.
(3)
Water content of a ''dry'' sample is small
(smaller than a few wt ppm for a typical mantle
mineral).
In the following, when we analyze the experi-
mental results, we choose the results where all of
these conditions are met. In some cases, experi-
mental results that meet all of these conditions
are not available. In these cases, we use the avail-
able data with some notes on potential problems.
∼
1/ T
Fig. 5.4
A schematic drawing showing a curvature in
the log
σ
1
/T
plot when two independent
conduction mechanisms are present. In these cases,
extrapolation from the low-temperature regime
assuming a linear relation will underestimate the true
conductivity at mantle temperatures.
⇔
electrical conductivity of hydrous samples were
made at relatively low temperatures (to minimize
hydrogen loss). The influence of enhanced Mg
(Fe) diffusion will be evaluated theoretically
based on the experimental results on diffusion in
the later section.
Although only hydrogen loss is usually consid-
ered, attention should also be drawn to hydrogen
addition. A notable example is the case of a study
by Xu
et al
. (1998b). In that pioneering study,
they found that wadsleyite and ringwoodite have
much higher electrical conductivity than olivine,
and interpreted that wadsleyite and ringwoodite
always
have higher electrical conductivity.
Huang
et al
. (2005) measured the water contents
in the samples studied by Xu
et al
. (1998b)
and found that these samples of wadsleyite and
ringwoodite have a large water content and the
higher electrical conductivities of wadsleyite
and ringwoodite than olivine is largely due to
the higher water contents of wadsleyite and
ringwoodite than olivine. In fact, because these
minerals have high water solubility, it is difficult
to synthesize truly dry wadsleyite and ringwood-
ite (Nishihara
et al
., 2006). Truly dry wadsleyite
has similar electrical conductivity to dry olivine
(Dai & Karato, 2009c; Karato, 2011). Similarly
''dry'' majorite used by (Yoshino
et al
., 2008b)
5.4 Experimental Results
Table 5.1 summarizes most of the available ex-
perimental results on electrical conductivity of
minerals where the influence of water was de-
termined using the impedance spectroscopy with
water content measurements both before and af-
ter each experiments. A generic relationship,
f
O
2
f
O
2
q
exp
H
∗
RT
C
r
w
·
σ
=
A
·
·
−
(5.15)
