Geoscience Reference
In-Depth Information
10
1
T (K)
P=5 GPa
10
1800
1700
10
0
1600
1500
1
10
-1
1400
10
-2
10
-1
SIMS
FT-IR
10
-3
“average”
10
-2
10
-4
10
-3
10
-2
10
-1
100
200
300
400
500
600
C
w
(wt %)
depth, km
(a)
Fig. 5.16
Electrical conductivity-depth profiles for the
Earth's upper mantle and transition zone determined
from the analysis of electromagnetic induction (after
Karato, 2011). The thick lines show ''average''
conductivity and the gray regions show the range of
conductivity in various regions. Reproduced with
permission of Elsevier.
10
0
T (K)
P=5 GPa
1800
1700
1600
10
-1
1500
1400
10
−
1
S/m, but in south-central
Europe, the conductivity is low
On average, it is
∼
10
−
2
-10
−
1
S/m,
whereas in eastern Asia, conductivity is higher
(
∼
10
-2
1S/m).
The electrical conductivity of the astheno-
sphere can be explained by a modest amount of
water (
∼
10
−
2
wt %) which is consistent with the
geochemical inference (e.g., Dixon
et al
., 2002;
Hirschmann, 2006) (Figure 5.17). The influence of
hydrogen will be somewhat higher (by
∼
10
-3
10
-3
10
-2
10
-1
C
w
(wt %)
10-20%)
when the effects of enhanced Mg (Fe) diffusion
(Hier-Majumder
et al
., 2005) are included. There
is no need for partial melting to explain this
commonly observed conductivity of the astheno-
sphere. Most of lateral variation in conductivity
is likely due to the lateral variation in the water
content. Unusually low electrical conductivity
in the western Pacific asthenosphere (Baba
et al
.,
2010) can also be interpreted by a model by Karato
(2008b) who proposed that the central-western
Pacific asthenosphere is depleted with water due
∼
(b)
Fig. 5.17
(a) The partition coefficient of water
(hydrogen) between olivine and orthopyroxene (Dai &
Karato, 2009a). (b) The influence of water content and
temperature on the electrical conductivity of the
asthenosphere (from Dai & Karato, 2009a). For the
plausible temperature (
∼
1600 K), the water content of
10
−
2
wt % is consistent with most of the geophysical
observations on the asthenosphere (with some regional
variations). The enhanced Mg (Fe) diffusion
(Hier-Majumder
et al
., 2005) will increase the influence
of water but its effect is small (
∼
∼
10%) (see text).
