Geoscience Reference
In-Depth Information
degree of melting
and the
melt fraction
. The melt
fraction of a given place could be estimated from
the phase diagram if the system in consideration
behaves like a closed system (''batch melting'').
However, when the melt is mobile (''fractional
melting''), then the melt fraction is
not
deter-
mined by the phase equilibrium, but rather the
melt fraction is determined by the dynamics of
the system (i.e., by the melt production rate and
the rate of melt transfer). For example, the (final)
degree of melting at mid-ocean ridges is
average scheme. The contrasts of conductivity
among different minerals are modest and there-
fore the difference between the upper and the
lower bounds of the Hashin-Shtrikman average is
not large (less than 50%).
6
(a) Continental mid- and lower crust
Structure
and composition of Earth's mid- and lower
crust are expected to be laterally heterogeneous
(Rudnick & Fountain, 1995; Rudnick
et al
., 1998).
The most important variables that may affect
electrical conductivity are (i) the major element
composition (and mineralogy), (ii) temperature,
(iii) the water content, and (iv) the degree of
partial melting.
The rocks in the continental mid- and lower
crust have mafic composition, the dominant min-
erals being orthopyroxene, clinopyroxene, and
plagioclase (
10%,
but the amount of melt beneath a ridge is es-
timated to be
∼
∼
0.1% from the U-Th isotopic
composition (Spiegelman & Elliott, 1993), and
the inferred large difference is attributed to the
dynamic control of the melt fraction (fast melt
migration). In many of recent studies on the influ-
ence of partial melting on electrical conductivity,
this distinction is not appreciated and the influ-
ence of partial melting is often over-estimated
(e.g., Gaillard
et al
., 2008; Yoshino
et al
., 2010).
some hydrous minerals). Electrical
conductivity in the lower crust mineral depends
strongly onminerals (plagioclase has significantly
lower conductivity than clino- and orthopyrox-
ene) (Yang
et al
., 2011, 2012). Also the regional
variation in temperature is large (
+
5.5 Some Applications
800-1300K).
Consequently, it is difficult to interpret the re-
sults of conductivity distribution in the lower
crust uniquely. However, the reported conduc-
tivities (10
−
4
-10
−
1
S/m, (Jones, 1992)) can be ac-
counted for by the combination of composition,
temperature and water content. In particular, a
detailed review by Yang (2011) showed that mod-
estly high conductivity of the continental lower
crust (
∼
5.5.1 Electrical conductivity and the Earth
and planetary interiors
In this section, we will apply experimental re-
sults on electrical conductivity and discuss how
the electrical conductivity distribution inferred
from geophysical studies may be interpreted by
the composition and/or temperature in Earth and
planetary interiors. Among various parameters,
one needs to consider the influence of water,
temperature, major element composition, oxygen
fugacity and of partial melting.
We will calculate the electrical conductivity in
various regions of Earth (and other planets) from
laboratory data and a range of temperature, ma-
jor element chemistry and water distribution. We
assume certain compositional models (e.g., pyro-
lite for the mantle), and calculate the mineralogy
and element partitioning for each mineral. We
will also assume temperature (pressure) and oxy-
gen fugacity, and calculate electrical conductivity
of each mineral, and then calculate the aggre-
gate conductivity using the Hashin-Shtrikman
10
−
3
to 10
−
2
S/m) can be explained by
high Fe (in pyroxenes) and Na (in plagioclase)
content (
∼
high temperature) in addition to the
effects of hydrogen (Figure 5.14).
However, high conductivity on the order of
10
−
2
to 10
−
1
S/m observed in certain regions
(e.g., Tibet; (Chen
et al
., 1996)) is not easy to
be attributed to the conduction by nominally
anhydrous minerals. Recently, Wang
et al
.
(2012a) showed that the dehydration reaction of
+
6
The difference between the upper and the lower bound
is large for the lower crust where the contrast in conduc-
tivity among co-existing minerals is large (Yang
et al
.,
2012).
