SESolid EarthSESolid Earth1869-9529Copernicus PublicationsGöttingen, Germany10.5194/se-9-233-2018Effect of chemical composition on the electrical conductivity of gneiss at
high temperatures and pressuresDaiLidongdailidong@vip.gyig.ac.cnSunWenqingLiHepingHuHaiyingWuLeiJiangJianjunKey Laboratory of High-Temperature and High-Pressure Study of the
Earth's Interior, Institute of Geochemistry, Chinese Academy of Sciences,
Guiyang, 550081, ChinaUniversity of Chinese Academy of Sciences, Beijing, 100049, ChinaLidong Dai (dailidong@vip.gyig.ac.cn)6March20189223324531August201727September20175February201811February2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://se.copernicus.org/articles/9/233/2018/se-9-233-2018.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/9/233/2018/se-9-233-2018.pdf
The electrical
conductivity of gneiss samples with different chemical
compositions (WA= Na2O + K2O + CaO = 7.12, 7.27 and
7.64 % weight percent) was measured using a complex impedance
spectroscopic technique at 623–1073 K and 1.5 GPa and a frequency range of
10-1 to 106 Hz. Simultaneously, a pressure effect on the electrical
conductivity was also determined for the WA= 7.12 % gneiss. The results
indicated that the gneiss conductivities markedly increase with total alkali
and calcium ion content. The sample conductivity and temperature conform to
an Arrhenius relationship within a certain temperature range. The influence
of pressure on gneiss conductivity is weaker than temperature, although
conductivity still increases with pressure. According to various ranges of
activation enthalpy (0.35–0.52 and 0.76–0.87 eV) at 1.5 GPa, two main
conduction mechanisms are suggested that dominate the electrical conductivity
of gneiss: impurity conduction in the lower-temperature region and ionic
conduction (charge carriers are K+, Na+ and Ca2+) in the
higher-temperature region. The electrical conductivity of gneiss with various
chemical compositions cannot be used to interpret the high conductivity
anomalies in the Dabie–Sulu ultrahigh-pressure metamorphic belt. However, the
conductivity–depth profiles for gneiss may provide an important constraint on
the interpretation of field magnetotelluric conductivity results in the
regional metamorphic belt.
Introduction
According to magnetotelluric (MT) and geomagnetic depth-sounding results,
the electrical conductivity of geological samples at high temperature and
pressure can be used to extrapolate the mineralogical composition and
thermodynamic state in the Earth's interior (Maumus et al., 2005; Dai et
al., 2008; Hui et al., 2015; Manthilake et al., 2015; Li et al., 2016; Hu et
al., 2017). High conductivity anomalies are widely distributed in the middle
to lower crust and upper mantle, and there are various causes of these
anomalies in different regions (Xiao et al., 2007, 2011; Pape et al., 2015;
Novella et al., 2017). Hence, it is crucial to comprehensively measure the
electrical conductivities of minerals and rocks that are distributed in the
deep Earth. A series of electrical conductivity results of the main minerals
and rocks have been reported in previous studies under high temperature and
pressure conditions (Fuji-ta et al., 2007; Hu et al., 2011, 2013; Dai et
al., 2012; Yang et al., 2012; Sun et al., 2017a). However, the electrical
conductivity of most metamorphic rocks has not been explored at high
temperature and pressure, and thus the interpretation of high conductivity
anomalies distributed in representative regional metamorphic belts is still
not comprehensive.
A regional metamorphic ultrahigh-pressure belt for the Dabie–Sulu orogen is
a complexly giant geotectonic unit in central-eastern China. Geophysical
exploration results confirmed that a large number of high conductivity
anomalies have been observed in metamorphic belts (Xiao et al., 2007;
Wannamaker et al., 2009; Zeng et al., 2015). Metamorphic rocks (e.g., slate,
schist, gneiss, granulite and eclogite) with different degrees of
metamorphism play an important role because of their widespread distribution
in regional metamorphic belts. Dai et al. (2016) measured the electrical
conductivity of dry eclogite at 873–1173 K, 1.0–3.0 GPa and different oxygen
partial pressures (using Cu + CuO, Ni + NiO and Mo + MoO2 solid oxygen
buffers) and found that the hopping of small polaron is the dominant
conduction mechanism for dry eclogite at high temperature and pressure. The
electrical conductivity of natural eclogite is much lower than the high
conductivity anomaly in the Dabie–Sulu ultrahigh-pressure metamorphic (UHPM)
belt of eastern China. Granulite is another important metamorphic rock
distributed in a majority of regional metamorphic belts. The electrical
conductivity of granulite is lowered by repetitive heating cycles with a
conductivity range of about 10-7–10-2 S m-1 at 1.0 GPa and up to about
900 K (Fuji-ta et al., 2004). Due to the complicated mineralogical
assemblage of granulite and rock structure, the features of the electrical
conductivity values over heating cycles have not been explained and the
conduction mechanism for granulite not definitively stated. Gneiss is formed
at middle- to lower-crustal pressure and temperature conditions and widely
distributed in regional metamorphic belts. The main rock-forming minerals of
gneiss are feldspar, quartz and biotite. The electrical conductivity of
gneiss increases with temperature, and the conductivity values range from
about 10-4–10-2 S m-1 at up to 1000 K and 1.0 GPa (Fuji-ta et al.,
2007). On the basis of the dominant rock-bearing mineralogical assembly of
the metamorphic rock, gneiss can generally be divided into types, such as
plagioclase gneiss, quartz gneiss and biotite gneiss. Therefore, it is
crucial to investigate the electrical conductivity of gneisses with various
chemical compositions and mineralogical constituents. The electrical
conductivity of granite dramatically increases with alkaline and calcium ion
content at 623–1173 K and 0.5–1.5 GPa (Dai et al., 2014). Impurity
conduction has been proposed to be the dominant conduction mechanism for
granite in the lower-temperature region, and alkaline ions, including
K+, Na+ and Ca2+, are probable charge carriers at higher
temperatures.
In the present study, we measured the electrical conductivity of gneiss
samples in situ under 0.5–2.0 GPa, 623–1073 K and three different chemical
compositions. The influences of temperature, pressure and chemical
composition on the gneiss electrical conductivity were determined, and the
dominant conduction mechanism for gneiss is discussed in detail. On the
basis of the conductivity results, the geophysical implications for the high
conductivity anomalies of the Dabie–Sulu UHPM belt were explored in depth.
Photomicrographs and electron backscattered images of three
natural gneiss samples under the polarizing microscope. Pl: plagioclase,
Qtz: quartz and Bt: biotite.
Experimental proceduresSample preparation
Three relatively homogeneous natural gneiss samples with parallel to
foliation direction were collected from Xinjiang, China. The sample surfaces
were fresh, non-fractured and non-oxidized, without evidence of alteration
before and after the experiments. To determine the gneiss mineralogical
assemblage, we used optical microscopy and scanning electron microscopy
(SEM) at the State Key Laboratory of Ore Deposit Geochemistry, Institute of
Geochemistry, Chinese Academy of Sciences, Guiyang, China. The major
elemental content of the gneiss samples was analyzed by X-ray fluorescence
spectrometry (XRF) at Australian Laboratory Services, Shanghai, China. The
main rock-forming minerals of three gneiss samples were feldspar, quartz and
biotite (Fig. 1). The volume percentage varied for each corresponding
rock-forming mineral in different gneiss samples (Table 1). Three gneiss
samples had the same mineralogical assemblage, and all of them belong to the
biotite-bearing felsic gneiss. Table 2 shows the results of whole rock
analysis by XRF for the three gneiss samples. We found that the total
alkali, such as K+ and Na+, and the divalent cationic calcium
metal ion content varied for each sample (Table 2).
Mineralogical assemblage of three natural gneiss samples.
Pl: plagioclase, Qz: quartz and Bi: biotite.
Run no.Mineralogical associationsDS12Pl (50 %) + Qz (40 %) + Bi (10 %)DS13Pl (25 %) + Qz (40 %) + Bi (35 %)DS14Pl (60 %) + Qz (25 %) + Bi (15 %)Impedance measurements
High temperatures and pressures for the experiments were generated in a
YJ-3000t multi-anvil apparatus, and the impedance spectra were collected
using a Solartron-1260 impedance–gain-phase analyzer at the Key Laboratory
of High-Temperature and High-Pressure Study of the Earth's Interior,
Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China. All
components of the experimental assemblage (ceramic tubes, pyrophyllite,
Al2O3 and MgO sleeves) were previously baked at 1073 K for 12 h in
a muffle furnace to avoid the influence of absorbed water on the electrical
conductivity measurements. The sample was then loaded into an MgO insulation
tube (Fig. 2). Two nickel disks (6.0 mm in diameter and 0.5 mm in thickness)
were applied to the top and bottom of the sample to act as electrodes. To shield
against external electromagnetic and spurious signal interference, a layer
of nickel foil with a thickness of 0.025 mm was installed between the
alumina and magnesia sleeves. These sleeves have good insulating properties
for current and transmitting pressure. A pyrophyllite cube (edge length:
32.5 mm) was used as the pressure medium, and the heater was composed of
three-layer stainless steel sheets with a total thickness of 0.5 mm. The
sample assembly was placed in an oven at 330 K to keep it dry before the
experiment.
Chemical composition of whole rock analysis by X-ray
fluorescence (XRF) for three gneiss samples.
Experimental setup for electrical conductivity measurements
at high temperatures and pressures.
In the experiments, the pressure was slowly increased to the desired value
at a rate of 1.0 GPa h-1, and then the temperature was increased at a rate of
300 K h-1 to the designated values. The Solartron-1260 impedance–gain-phase
analyzer with an applied voltage of 3 V and frequency range of
10-1–106 Hz was used to collect impedance spectra when the
pressure and temperature were stable. At the desired pressure, the spectra
were measured at a certain temperature, which was changed in 50 K intervals.
The impedance spectra of gneiss samples with WA
(Na2O + K2O + CaO) = 7.12 % were collected under conditions of
0.5–2.0 GPa and 623–1073 K. The spectra of the other two gneiss samples
(WA= 7.27 and 7.64 %) were measured at 623–1073 K and 1.5 GPa.
To confirm the data reproducibility, we measured the electrical conductivity
of gneiss over two heating and cooling cycles at a constant pressure. The
errors of temperature and pressure were ±5 K and ±0.1 GPa,
respectively.
Results
The typical complex impedance spectra for the run DS12 gneiss samples at 1.5 GPa and 623–1073 K are shown in Fig. 3. All of these obtained spectra are
composed of an almost ideal semicircle in the high-frequency domain and an
additional tail in the lower-frequency domain. Other complex impedance
spectra of the gneiss samples at other pressures displayed the same
characteristics as those shown in Fig. 3. Figure 4 displays the real and
imaginary parts of complex impedance for the runs DS13 and DS14 gneiss
samples as a function of the measured frequency at 1.5 GPa and 623–1073 K.
The real part values almost remain unchanged over a frequency range of
106–104 Hz and sharply increased at 104–102 Hz; these
values then slowly increased within the 102 to 10-1 Hz lower-frequency region. The values of imaginary parts almost remain unchanged
within a frequency range of 106–105 Hz, and the values gradually
increased at 105–103 Hz and decreased at 103–101 Hz;
these values then slowly increased in the 101 to 10-1 Hz lower-frequency region. Roberts and Tyburczy (1991) and Saltas et al. (2013) have
suggested that the ideal semicircle represents the bulk electrical
properties of a sample, and the additional tail is characteristic of
diffusion processes at the sample–electrode interface. Hence, the bulk
sample resistance can be obtained by fitting the ideal semicircle in the
high-frequency domain. A series connection of resistance and constant phase
elements (RS–CPES) and the interaction of charge carriers with
the electrode (RE–CPEE) was applied to be the equivalent circuit.
All fitting errors of the electrical resistance were less than 5 %. Based
on the sample size and electrical resistance, the electrical conductivity of
the sample was calculated with
σ=L/SR,
where L is the height of the sample (m), S is the cross-sectional area of the
electrodes (m2), R is the fitting resistance (Ω) and σ
is the electrical conductivity of the sample (S m-1).
Representative complex impedance spectra for run DS12 gneiss
under conditions of 1.5 GPa and 623–1073 K.
The logarithmic electrical conductivities of the gneiss samples were plotted
against the reciprocal temperatures under conditions of 623–1073 K and
0.5–2.0 GPa. The electrical conductivities of gneiss with
XA= 7.12 % were measured in two sequential heating and cooling
cycles at 1.5 GPa (Fig. 5). After the first heating cycle, the electrical
conductivities of the gneiss at the same temperature were close to each
other in other cycles. We confirmed that our experimental data were
reproducible, and the gneiss sample was kept at a steady state after the
first heating cycle. Two different linear relationships of logarithmic
electrical conductivity and reciprocal temperature were separated by an
inflection point. The electrical conductivity of gneiss with
WA= 7.12 % significantly increased with temperatures above 723 K at
0.5–1.0 GPa, and this phenomenon occurred after 773 K at 1.5–2.5 GPa (Fig. 6). The electrical conductivity of the samples increased with pressure, but
the effect of pressure on conductivity was weaker than temperature. For
other gneiss samples (WA= 7.27 and 7.64 %), the inflection
points appeared at 773 K under all designated pressures (Fig. 7). In a
specific temperature range, the relationship between electrical conductivity
and temperature fits the Arrhenius formula:
σ=σ0exp(-ΔH/kT),ΔH=ΔU+PΔV,
where σ0 is the pre-exponential factor (S m-1), ΔH is the
activation enthalpy (eV), k is the Boltzmann constant (eV K-1), T is the
absolute temperature (K), ΔU is the activation energy (eV), P is the
pressure (GPa) and ΔV is the activation volume (cm3 mole-1). All
fitting parameters for the electrical conductivities of three gneiss samples
are listed in Table 3. The activation enthalpy values (ΔH) for the
gneiss samples are 0.35–0.58 eV in the lower-temperature region and
0.71–1.05 eV in the higher-temperature region, respectively. In addition,
the logarithms of pre-exponential factor values (Log σ0) were
transformed from negative to positive from the correspondent lower to the
higher temperature ranges.
Fitted parameters of the Arrhenius relation for the
electrical conductivity of three gneiss samples. Two equations, σ=σ0exp(-ΔH/kT) and ΔH=ΔU+PΔV, are
adopted in which
σ0 is the pre-exponential factor (S m-1), ΔH is the
activation enthalpy (eV), k is the Boltzmann constant (eV K-1), T is the
absolute temperature (K), ΔU is the activation energy (eV), P is the
pressure (GPa) and ΔV is the activation volume (cm3 mole-1). The
symbol R2 is denoted as the fitted correlation coefficient.
The total alkali and calcium ion content of K2O, Na2O and CaO is a
remarkable influence on the electrical conductivities of the gneiss samples.
As shown in Fig. 7, the electrical conductivity of the gneiss samples
increased with the total weight percent of K2O, Na2O and CaO. This
reflects the fact the electrical conductivity of the gneiss samples is
controlled mainly by minerals that contain abundant K2O, Na2O and
CaO. The cations of feldspar are K+, Na+ and Ca2+, and
K+ is also the main cation of biotite. Furthermore, impurity ions
(K+, Na+ and Al3+) have been suggested to be the charge
carriers for quartz samples (Wang et al., 2010). In addition, the electrical
conductivity of the gneiss samples does not regularly change with variations
in biotite-bearing content (Fig. 7 and Table 1). Based on all of the
experimental results, the biotite content is not the main influential factor
on the electrical conductivity of gneiss. Therefore, we cannot distinguish
the specific mineral that controls the electrical conductivity of the gneiss
samples. However, it was reasonable to consider the gneiss sample as a
complex whole and analyze the electrical conductivity of gneiss with
various chemical compositions at high temperature and pressure.
Real and imaginary parts of complex impedance as functions
of the measured frequencies for the runs DS13 and DS14 gneiss samples under
conditions of 1.5 GPa and 623–1073 K. (a) real and (b) imaginary parts for
the run DS13 gneiss; (c) Real and (d) imaginary parts for the run DS14
gneiss.
Logarithm of the electrical conductivities versus the
reciprocal temperatures for run DS12 gneiss during two heating and cooling
cycles at 1.5 GPa.
DiscussionA comparison with previous studies
As three constituent minerals of gneiss, feldspar, biotite and quartz
dominated the electrical conductivity of the whole rock at high temperature
and pressure. Due to their sophisticated mineralogical assemblage and rock
structure, the gneiss samples were unstable in the first heating cycle. In
this process, the impurity ions may have been distributed, the grain size
slightly changed and the microfractures gradually closed. After the first
cycle, the electrical conductivity of the gneiss samples had good
repeatability. This suggested that the gneiss samples were in a stable
state. The electrical conductivity range of the gneiss samples with various
chemical compositions was about 10-5–10-1 S m-1 at 623–973 K and
0.5–2.0 GPa. The electrical conductivity was slightly related to pressure
and conforms to previous conclusions that the influence of pressure on
mineral and rock conductivity is much weaker than temperature (Xu et al.,
2000; Hu et al., 2011; Dai and Karato, 2014a, b). The possible reason is
that the effect of pressure on the activity of the charge carriers is weaker
than temperature. The total alkaline ion content of K2O, Na2O and
CaO has a crucial influence on the electrical conductivity of gneiss. Previous
studies have investigated the electrical conductivity of minerals and rocks
with various chemical compositions, and the conclusions were similar to ours
(Dai et al., 2014). Fiji-ta et al. (2007) performed the electrical
conductivity of gneiss perpendicular and parallel to foliation at up to 1000 K and a constant pressure of 1.0 GPa. The conductivity of gneiss measured
perpendicular to foliation was 1 order of magnitude lower than the value
measured parallel to foliation. However, the influence of pressure and
chemical composition on the electrical conductivity of gneiss has not been
studied. In the present work, we investigated the electrical conductivity of
gneiss parallel to foliation. As shown in Fig. 8, the electrical
conductivity of gneiss from Fuji-ta et al. (2007) was higher than our
results in the lower temperature range, and values were lower than the
conductivity of gneiss with WA= 7.27 and 7.64 % in this study.
This discrepancy is probably caused by varying chemical compositions of the
gneiss samples. Dai et al. (2014) measured the electrical conductivity of
granite at 0.5–1.5 GPa and 623–1173 K, and the main rock-forming minerals
were also quartz, feldspar, and biotite. They found that the content of
calcium and alkali ions significantly affected the electrical conductivity
of granite under conditions of high temperature and high pressure.
Electrical conductivities of granite and gneiss increased with calcium and
alkali ion content. However, the electrical conductivity of granite was much
lower than gneiss (Fig. 8). This difference may be caused by the various
chemical compositions and rock structures between granite and gneiss.
Feldspars are the main constituent rock-forming minerals in gneiss, and thus it
is important to compare the electrical conductivity of feldspar. The
electrical conductivity of K-feldspar is 1 order of magnitude lower than
albite, and K+ and Na+ ions are the charge carriers of K-feldspar
and albite, respectively (Hu et al., 2013). As shown in Fig. 8, the
electrical conductivity of alkali feldspar is much higher than the gneiss
samples. This may be because the concentration of alkali ions in feldspar is
higher than gneiss. In addition, granulite is another significant
metamorphic rock and usually coexists with gneiss. The electrical
conductivity of granulite is moderately higher than gneiss. The electrical
conductivity of quartz at 1.0 GPa is slightly lower than gneiss with
XA= 7.27 % at 1.5 GPa, and the slope of the linear relationship
between the logarithm of electrical conductivity and the reciprocal of
temperature for quartz is close to gneiss in a lower temperature range (Wang
et al., 2010). The conductivity of phlogopite is higher than gneiss with
XA= 7.64 % at higher temperatures (> 773 K) and lower
than gneiss samples at lower temperatures (< 773 K). Furthermore,
the slope of the linear relationship between the logarithm of electrical
conductivity for the phlogopite and the reciprocal temperature is much
higher than the slope of the gneiss samples (Li et al., 2016). Compared with
Ferri et al. (2013), the electrical conductivity of the
garnet–biotite–sillimanite residual enclave (JOY2-X4) was very close to
our conductivity results for the run DS13 and DS14 gneiss samples in the
lower-temperature and higher-temperature regions, respectively. The
electrical conductivity of sample JOY2-X4 was slightly lower than the run
DS12 gneiss sample. In addition, the electrical conductivity of natural
metapelite (PP216) from Hashim et al. (2013) was close to the values of the
run DS12 gneiss sample in the lower-temperature region, and the slope
between logarithmic conductivities and reciprocal temperature for the PP216
metapelite was higher than the gneiss samples in the higher-temperature
region.
Logarithm of the electrical conductivities versus the
reciprocal temperatures for run DS12 gneiss at 0.5–2.5 GPa and 623–1073 K.
Logarithm of the electrical conductivities versus the
reciprocal temperatures of the gneiss samples with various chemical
compositions at 1.5 GPa and 623–1073 K.
Conduction mechanism
The logarithm of electrical conductivities and reciprocal temperatures
showed linear relationships at the lower and higher temperature ranges,
respectively. This implies that the dominant conduction mechanism for our
gneiss samples in the lower temperature range is different from the higher
temperature range. The mineral assemblage and chemical composition of gneiss
samples are very complicated, and thus the conduction mechanisms for gneiss
samples are difficult to determine. Feldspar, quartz and biotite are the
dominant minerals in the gneiss samples. Previous studies have suggested
that the conduction mechanism for feldspar minerals is ionic conduction and
the charge carriers are K+, Na+ and Ca2+ (Hu et al., 2013).
The conduction mechanism for biotite has not been studied, whereas the
charge carriers of phlogopite were proposed to be F- and K+ (Li et
al., 2016). For quartz, the conduction mechanism was impurity ionic
conduction, and the dominant charge carriers migrate by moving the alkali
ions in channels (Wang et al., 2010). Therefore, we deduced that the
conduction mechanism for gneiss samples may be related to ions. The
activation enthalpy is one crucial piece of evidence for the conduction mechanism of
minerals and rocks (Dai et al., 2016). The activation enthalpies for gneiss
samples are 0.35–0.58 eV in the lower-temperature region and 0.77–0.87 eV
in the higher-temperature region (Table 3). Dai et al. (2014) studied the
electrical conductivity of granite that had the same mineralogical
assemblage as the gneiss samples. They proposed that the conduction
mechanism at the lower temperature range was the impurity conduction owing
to the low activation enthalpy (0.5 eV), whereas the mechanism was ionic
conduction with a high activation enthalpy (1.0 eV) at the higher
temperature range. The activation enthalpy for gneiss was close to the
values for granite at the lower and higher temperature ranges. The
activation enthalpies for albite and K-feldspar were 0.84 and 0.99 eV,
respectively (Hu et al., 2013). With increasing pressure, the electrical
conductivity of gneiss increased accordingly. The activation volumes for one
gneiss sample (DS12) were -7.10 and -2.69 cm3 mole-1 in the
low- and high-temperature regions, respectively. We can compare gneiss with
the electrical conductivity of eclogite, another representative metamorphic
rock. Recently, Dai et al. (2016) measured the electrical conductivity of
dry eclogite and the negative activation volume for eclogite was -2.51 cm3 mole-1 under 1.0–3.0 GPa and 873–1173 K. It was proposed that the
main conduction mechanism for dry eclogite is intrinsic conduction (Dai et
al., 2016). In addition, Fig. 7 shows that the increasing content of
alkali and calcium ions significantly enhances the electrical conductivity
of gneiss samples. Therefore, the impurity conduction (possible charge
carriers: K+, Na+, Ca2+ and H+) and ionic conduction
(possible charge carriers: K+, Na+ and Ca2+) are suggested to
be the conduction mechanisms at lower and higher temperature ranges,
respectively.
Comparisons of the electrical conductivities of the gneiss
samples measured at 1.5 GPa in this study and in previous studies. The
dashed blue and green lines represent the electrical conductivities of
granulite and gneiss at 1.0 GPa from Fuji-ta et al. (2004, 2007), respectively. The dashed orange line represents the electrical
conductivity of quartz at 1.0 GPa from Wang et al. (2010), and the dashed dark
green line represents the electrical conductivity of alkali feldspars at 1.0 GPa from Hu et al. (2013). The dashed sky blue line represents the
electrical conductivity of natural PP216 metapelite at 0.3 GPa from Hashim
et al. (2013), and the dashed violet line represents the electrical conductivity
of the residual JOY2-X4 enclave at 0.3 GPa from Ferri et al. (2013). The
dashed red lines represent the electrical conductivity of granite at 0.5 GPa
from Dai et al. (2014), and the dashed pink line represents the electrical
conductivity of phlogopite at 1.0 GPa from Li et al. (2016).
Geological sketch map of the Dabie–Sulu orogenic belt (a) and its correspondent lithological distribution diagram in the southern
counterpart of the Dabie–Sulu region (b, modified after Xu et al., 2013; Liu et
al., 2014).
Effect of chemical composition on electrical conductivity
The influence of chemical composition (Na2O + K2O + CaO) on the
electrical conductivity of the gneiss samples was very significant, as shown
in a previous study that the electrical conductivity of granite samples is
closely related to the alkali and calcium ion content (Dai et al., 2014).
The electrical conductivity of granite samples at high temperature and
pressure can be fitted as a function of
(Na2O + K2O + CaO)/SiO2 (Dai et al., 2014). However, the
electrical conductivity of gneiss samples does not regularly change with
variations in (Na2O + K2O + CaO)/SiO2. This may be due to the
more complicated mineralogical assemblage and chemical composition of
gneiss: for mineralogical assemblage, the biotite content of the gneiss
sample is higher than granite. As for the chemical compositions, the
contents of SiO2 for gneiss are lower than those of granite samples, and
the contents of the calc-alkali ions are approximate between gneiss and
granite samples. Hu et al. (2013) demonstrated that the electrical
conductivity of alkali feldspar significantly depends on the value of
Na/(Na + K). This suggests that the electrical conductivity of gneiss is
affected by the total content of alkali and calcium ions, as well as the
ratios between various ions.
Geophysical implications
As a typical metamorphic rock in the present research region, gneiss is
widespread in the UHPM zone (Zheng et al., 2003; Liu et al., 2005; Hashim et
al., 2013). The geological map of the Dabie–Sulu orogenic belt and its
corresponding lithological distribution in the southern Dabie–Sulu region
are displayed in Fig. 9. As one of the largest UHPM belts in the world for
Dabie–Sulu orogen, gneiss is the outcropping rock directly in contact with
eclogite and occupies up to 90 % of the exposed metamorphic rock area.
Therefore, the in situ laboratory-based electrical conductivity of gneiss at
high temperature and pressure is very significant to interpret the
conductivity structure in the Dabie–Sulu belt deep in the Earth's interior.
The Dabie terrane is a major segment bounded by the Tan-Lu fault to the east
and separated into a series of continuous zones by several large-scale E–W-trending faults; the Sulu terrane is segmented into a number of blocks by
several NE–SW-trending faults subparallel to the Tan-Lu fault (Zheng, 2008;
Xu et al., 2013). The discovery of coesite and/or diamond inclusions in
various types of rock (e.g., gneiss, eclogite, amphibolite, marble and
jadeite quartzite) through the Dabie–Sulu orogen indicates that continental
crust has been subducted at a depth of 80–200 km and subsequently exhumed
to the Earth's surface. During subduction, dehydration reactions of some
hydrous minerals (e.g., lawsonite, phengite and chlorite) and partial
melting of other regional metamorphic rocks (e.g., gneiss and eclogite)
occur at high temperature and pressure (Xu et al., 2013; Liu et al., 2014).
Previous field MT results have found that high conductivity anomalies with
magnitudes of 10-1 S m-1 are widely distributed at 10–20 km in the
Dabie–Sulu UHPM belt (Xiao et al., 2007). In addition, the slab-like high-velocity anomaly results have also confirmed a depth of ≥ 110 km for the
uppermost mantle beneath the Dabie–Sulu orogen, which represents a remnant
of the subducted Yangtze block after Triassic continent–continent collision
(Xu et al., 2001). However, the origin and causal mechanisms of these high
conductivity anomalies for the Dabie–Sulu UHPM belt are still unknown.
Together with the two main constituent rocks (natural eclogite and
granulite) in the UHPM belt, it is crucial to explore whether the gneiss
electrical conductivity can be used to interpret the high conductivity
anomalies distributed in the Dabie–Sulu tectonic belt. The relationship
between temperature and depth in the Earth's stationary crust can be
obtained by a numerical solution of the heat conduction equation (Selway et
al., 2014):
Laboratory-based conductivity–depth profiles constructed
from data of the gneiss samples and the thermodynamic parameters;
comparison with geophysically inferred field results from the Dabie–Sulu UHPM
belt, China. The red solid lines represent the conductivity–depth profiles
based on the conductivities of the samples described in Fig. 3 and based on
a surface heat flow of 75 mW m-2 in the Dabie–Sulu UHPM belt. The dashed
blue lines represent the conductivity–depth profiles based on the
conductivities of eclogite, and the dashed brown line represents the
conductivity–depth profiles based on the conductivities of granulite
(Fuji-ta et al., 2004; Dai et al., 2016). The green region represents the MT
data derived from a high conductivity anomaly in the Dabie–Sulu UHPM belt (Xiao
et al., 2007; He et al., 2009).
T=T0+QkZ-A02kZ2,
where T0 is the surface temperature (K), Q is the surface heat flow
(mW m-2), Z is the lithosphere layer depth (km), k is thermal conductivity
(W mK-1) and A0 is the lithospheric radiogenic heat productivity (µW m-3). Based on previous studies, the corresponding thermal
calculation parameters for the Dabie–Sulu orogen are Q= 75 mW m-2 (He et
al., 2009), A0= 0.31 µW m-3 and k= 2.6 W mK-1 (Zhou et al.,
2011).
Based on the heat conduction equation (Eq. 4) and thermal calculation
parameters, the conductivity–temperature results of gneiss with various
chemical compositions (WA= Na2O + K2O + CaO = 7.12,
7.27 and 7.64 %) can be converted to a conductivity–depth profile for
the Dabie–Sulu orogen (Fig. 10). A similar transformation was also conducted
for granulite by Fuji-ta et al. (2004) and eclogite with different oxygen
fugacity (Cu + CuO, Ni + NiO and Mo + MoO2) by Dai et al. (2016).
Figure 10 makes clear that the high conductivity anomaly of
10-1.5–10-0.5 S m-1 from the field MT results in the Dabie–Sulu UHPM
belt occurs at 12–21 km compared with three dominant constituent rock
conductivities of gneiss, granulite and eclogite in the region. Although our
obtained electrical conductivity of gneiss with different chemical
compositions is moderately higher than granulite and eclogite, it is not
high enough to explain the high conductivity anomaly observed in field MT
results in the Dabie–Sulu orogen. In other words, three dominant outcrops of
metamorphic rocks, including gneiss, eclogite and granulite, are not
substances that produce the high conductivity anomalies of the Dabie–Sulu
orogen. However, the conductivity–depth profile for gneiss with various
chemical compositions may provide an important constraint on the
interpretation of field magnetotelluric conductivity results in the regional
UHPM belt.
Aside from the chemical composition, other available alternative causes for
high conductivity anomalies can be considered, such as water in nominally
anhydrous minerals (Wang et al., 2006; Yang, 2011; Dai and Karato, 2009,
2014a), interconnected saline (or aqueous) fluids (Hashim et al., 2013;
Shimojuku et al., 2014; Sinmyo and Keppler, 2017; Guo et al., 2015;
Li et al., 2018), partial melting (Wei et al., 2001; Maumus et al., 2005;
Gaillard et al., 2008; Ferri et al., 2013; Laumonier et al., 2015, 2017;
Ghosh and Karki, 2017), interconnected secondary high conductivity phases
(e.g., FeS, Fe3O4; Jones et al., 2005; Bagdassarov et al., 2009;
Padilha et al., 2015), dehydration of hydrous minerals (Wang et al., 2012,
2017; Manthilake et al., 2015, 2016; Hu et al., 2017; Sun et al., 2017a, b;
Chen et al., 2018) and graphite films on mineral grain boundaries (Freund,
2003; Pous et al., 2004; Chen et al., 2017). In consideration of the similar
formation conduction and geotectonic environments, the Himalaya–Tibetan
orogenic system was compared with the Dabie–Sulu UHPM belt and explained
high electrical conductivity anomalies. Previous evidence from
magnetotelluric and elastic seismic velocity data in the southern Tibet and
northwestern Himalaya zones has confirmed that the high conductivity and
low-seismic-velocity anomalies are widespread at 10–25 km in the
Himalaya–Tibetan orogenic system (Wei et al., 2001; Unsworth et al., 2005;
Arora et al., 2007; Caldwell et al., 2009). Some studies have hypothesized
that partial melting is the cause of the high conductivity anomalies in the
Himalaya–Tibetan orogenic system (Wei et al., 2001; Gaillard et al., 2004;
Hashim et al., 2013). Nevertheless other researchers think they are closely
related with interconnected aqueous fluid (Makovsky and Klempere, 1999). As
argued by Li et al. (2003), five possible hypotheses could explain the cause
of the high conductivity anomalies in the INDEPTH magnetotelluric data of
the southern Tibet mid-crust. The authors found that the high conductivity
anomalies may be a result of interconnected melt and fluids. Recently, Naif
et al. (2018) suggested that the high conductivity anomaly at 50–150 km can
be explained by either a small amount of water stored in nominally anhydrous
minerals or interconnected partial melts. In the present study, the
electrical conductivity of gneiss with various chemical compositions at high
temperature and pressure cannot be used to interpret the high conductivity
anomalies of the Dabie–Sulu UHPM belt. Therefore, we propose that it is
possibly caused by interconnected fluids or melts that result in high
conductivity anomalies for the Dabie–Sulu UHPM belt.
Conclusions
The electrical conductivity range of gneiss samples with various chemical
compositions was about 10-5–10-1 S m-1 at 623–1073 K and 0.5–2.0 GPa.
Electrical conductivity of the gneiss samples significantly increased
with temperature and weakly increased with pressure. The total alkaline ion
content of K2O, Na2O and CaO is a remarkable influence on the
electrical conductivity of the gneiss samples. Based on various activation
enthalpy ranges (0.35–0.52 and 0.76–0.87 eV) corresponding to higher- and
lower-temperature regions at 1.5 GPa, two main conduction mechanisms are
suggested to dominate the conductivity of gneiss: impurity conduction in the
lower-temperature region and ionic conduction (charge carriers are K+,
Na+ and Ca2+) in the higher-temperature region. Because of the
much lower conductivity of gneiss samples at high temperature and pressure,
we confirmed that gneiss with various chemical compositions cannot cause the
high conductivity anomalies in the Dabie–Sulu UHPM belt.
The underlying data can be found in the Supplement.
The Supplement related to this article is available online at https://doi.org/10.5194/se-9-233-2018-supplement.
The authors declare that they have no conflict of
interest.
Acknowledgements
We thank the editor, Ulrike Werban, and three anonymous
reviewers for their very constructive comments and suggestions in the
reviewing process, which helped us greatly in improving the paper. We
appreciate Kara Bogus from the Edanz Group (www.edanzediting.com/ac)
scientific editing company for support with English improvement on a
previous version of the paper. This research was financially supported by the Strategic
Priority Research Program (B) of the Chinese Academy of Sciences (XDB
18010401), the Key Research Program of Frontier Sciences of CAS
(QYZDB-SSW-DQC009), the “135” Program of the Institute of Geochemistry of CAS,
the Hundred Talents Program of CAS and the NSF of China (41474078, 41774099 and
41772042).
Edited by: Ulrike Werban
Reviewed by: Fabrice Gaillard and two anonymous referees
ReferencesArora, B. R., Unsworth, M. J., and Rawat, G.: Deep resistivity structure of
the northwest Indian Himalaya and its tectonic implications, Geophys. Res.
Lett., 34, L04307, 10.1029/2006GL029165, 2007.
Bagdassarov, N., Golabek, G. J., Solferion, G., and Schmidt, M. W.:
Constraints on the Fe-S melt connectivity in mantle silicates from
electrical impedance measurements, Phys. Earth Planet. In., 177,
139–146, 2009.
Caldwell, W. B., Klemperer, S. L., Rai, S. S., and Lawrence, J. F.: Partial melt
in the upper-middle crust of northwest Himalaya are veiled by Rayleigh wave
dispersion, Tectonophysics, 477, 58–65, 2009.
Chen, J. Y., Yang, X. S., and Chen, J. Y.: Experimental studies on the
relationship between carbonaceous structure and electrical conductivity of
the Longmenshan fault zone, Chinese J. Geophys., 60, 3475–3492, 2017.
Chen, S. B., Guo, X. Z., Yoshino, T., Jin, Z. M., and Li, P.: Dehydration of
phengite inferred by electrical conductivity measurements: Implication for
the high conductivity anomalies relevant to the subduction zones, Geology,
46, 11–14, 2018.
Dai, L. D. and Karato, S. I.: High and highly anisotropic electrical
conductivity of the asthenosphere due to hydrogen diffusion in olivine,
Earth Planet. Sc. Lett., 408, 79–86, 2014a.
Dai, L. D. and Karato, S.: Influence of FeO and H on the electrical
conductivity of olivine, Phys. Earth Planet. In., 237, 73–79, 2014b.
Dai, L. D. and Karato, S. I.: Electrical conductivity of wadsleyite at high
temperatures and high pressures, Earth Planet. Sc. Lett., 287, 277–283,
2009.Dai, L. D., Li, H. P., Hu, H. Y., and Shan S. M.: Experimental study of grain
boundary electrical conductivities of dry synthetic peridotite under
high-temperature, high-pressure, and different oxygen fugacity conditions,
J. Geophys. Res., 113, B12211, 10.1029/2008JB005820, 2008.
Dai, L. D., Li, H. P., Hu, H. Y., Shan, S. M., Jiang, J. J., and Hui, K. S.: The
effect of chemical composition and oxygen fugacity on the electrical
conductivity of dry and hydrous garnet at high temperatures and pressures,
Contrib. Mineral. Petr., 163, 689–700, 2012.
Dai, L. D., Hu, H. Y., Li, H. P., Jiang, J. J., and Hui, K. S.: Influence of
temperature, pressure, and chemical composition on the electrical
conductivity of granite, Am. Mineral., 99, 1420–1428, 2014.
Dai, L. D., Hu, H. Y., Li, H. P., Wu, L., Hui, K. S., Jiang, J. J., and Sun,
W. Q.: Influence of temperature, pressure, and oxygen fugacity on the
electrical conductivity of dry eclogite, and geophysical implications,
Geochem. Geophy. Geosy., 17, 2394–2407, 2016.
Ferri, F., Gibert, B., Violay, M., and Cesare, B.: Electrical conductivity in
a partially molten crust from measurements on metasedimentary enclaves,
Tectonophysics, 586, 84–94, 2013.
Freund, F.: On the electrical conductivity structure of the stable
continental crust, J. Geodyn., 35, 353–388, 2003.
Fuji-ta, K., Katsura, T., and Tainosho, Y.: Electrical conductivity
measurement of granulite under mid- to lower crustal pressure-temperature
conditions, Geophys. J. Int., 157, 79–86, 2004.Fuji-ta, K., Katsura, T., Matsuzaki, T., Ichiki, M., and Kobayashi, T.:
Electrical conductivity measurement of gneiss under mid- to lower crustal
P-T conditions, Tectonophysics, 434, 93–101, 2007.
Gaillard, F., Scaillet, B., and Pichavant, M.: Evidence for present-day
leucogranite pluton growth in Tibet, Geology, 32, 801–804, 2004.
Gaillard, F., Malki, M., Iacono-Marziano, G., Pichavant, M., and Scaillet,
B.: Carbonatite melts and electrical conductivity in the asthenosphere,
Science, 322, 1363–1365, 2008.Ghosh, D. B. and Karki, B. B.: Transport properties of carbonated silicate
melt at high pressure, Sci. Adv., 3, e1701840, 10.1126/sciadv.1701840, 2017.
Guo, X. Z., Yoshino, T., and Shimojuku, A.: Electrical conductivity of
albite–(quartz)–water and albite–water–NaCl systems and its implication
to the high conductivity anomalies in the continental crust, Earth Planet. Sc. Lett., 412, 1–9, 2015.
Hashim, L., Gaillard, F., Champallier, R., Breton, N. L., Arbaret, L., and
Scaillet, B.: Experimental assessment of the relationships between
electrical resistivity, crustal melting and strain localization beneath the
Himalayan-Tibetan Belt, Earth Planet. Sc. Lett., 373, 20–30, 2013.
He, L., Hu, S., Yang, W., and Wang, J.: Radiogenic heat production in the
lithosphere of Sulu ultrahigh-pressure metamorphic belt, Earth Planet. Sc. Lett., 277, 525–538, 2009.
Hu, H. Y., Li, H. P., Dai, L. D., Shan, S. M., and Zhu, C. M.: Electrical
conductivity of albite at high temperatures and high pressures, Am.
Mineral., 96, 1821–1827, 2011.
Hu, H. Y., Li, H. P., Dai, L. D., Shan, S. M., and Zhu, C. M.: Electrical
conductivity of alkali feldspar solid solutions at high temperatures and
high pressures, Phys. Chem. Miner., 40, 51–62, 2013.
Hu, H. Y., Dai, L. D., Li, H. P., Hui, K. S., and Sun, W. Q.: Influence of
dehydration on the electrical conductivity of epidote and implications for
high conductivity anomalies in subduction zones, J. Geophys. Res., 122,
2751–2762, 2017.Hui, K. S., Zhang, H., Li, H. P., Dai, L. D., Hu, H. Y., Jiang, J. J., and Sun, W. Q.: Experimental study on the electrical conductivity of
quartz andesite at high temperature and high pressure: evidence of grain boundary transport, Solid Earth, 6, 1037–1043, 10.5194/se-6-1037-2015, 2015.
Jones, A. G., Ledo, J., and Ferguson, I. J.: Electromagnetic images of the
Trans-Hudson Orogen: The North American Central Plains anomaly revealed,
Can. J. Earth Sci., 42, 457–478, 2005.
Laumonier, M., Gaillard, F., and Sifre, D.: The effect of pressure and water
concentration on the electrical conductivity of dacitic melts: Implication
for magnetotelluric imaging in subduction areas, Chem. Geol., 418, 66–76,
2015.
Laumonier, M., Gaillard, F., Muir, D., Blundy, J., and Unsworth, M.: Giant
magmatic water reservoirs at mid-crustal depth inferred from electrical
conductivity and the growth of the continental crust, Earth Planet. Sc. Lett., 457, 173–180, 2017.Li, P., Guo, X. Z., Chen, S. B., Wang, C., Yang, J. L., and Zhou, X. F.:
Electrical conductivity of the plagioclase–NaCl–water system and its
implication for the high conductivity anomalies in the mid-lower crust of
Tibet Plateau, Contrib. Mineral. Petr., 173, 16,
10.1007/s00410-018-1442-9, 2018.
Li, S., Unsworth, M., Booker, J., Wei, W., Tan, H., and Jones, A. G.: Partial
melt or aqueous fluid in the mid-crust of Southern Tibet? Constraints from
INDEPTH magnetotelluric data, Geophys. J. Int., 153, 289–304, 2003.Li, Y., Yang, X. Z., Yu, J. H., and Cai, Y. F.: Unusually high electrical
conductivity of phlogopite: The possible role of fluorine and geophysical
implications, Contrib. Mineral. Petr., 171, 37,
10.1007/s00410-016-1252-x, 2016.
Liu, F. L., Xu, Z. Q., Yang, J. S., Zhang, Z. M., Xue, H. M., Meng, F. C., Li,
T. F., and Cheng, S. Z.: Geochemical characteristics and genetic mechanism of
orthgneiss and paragneiss in the depth intervals of 2000–3000 m from main
drill hole of Chinese Continental Scientific Drilling Project, Acta Petrol.
Sin., 21, 305–324, 2005.
Liu, P., Wu, Y., Liu, Q., Zhang, J. F., Zhang, L., and Jin, Z. M.: Partial
melting of UHP calc-gneiss from the Dabie Mountains, Lithos, 192–195,
86–101, 2014.
Makovsky, Y. and Klemperer, S. L.: Measuring the seismic properties of
Tibetan bright spots: Evidence for free aqueous fluids in the Tibetan middle
crust, J. Geophys. Res., 104, 10795–10825, 1999.
Manthilake, G., Mookherjee, M., Bolfan-Casanova, N., and Andrault, D.:
Electrical conductivity of lawsonite and dehydrating fluids at high
pressures and temperatures, Geophys. Res. Lett., 42, 7398–7405, 2015.Manthilake, G., Bolfan-Casanova, N., Novella, D., Mookherjee, M., and
Andrault, D.: Dehydration of chlorite explains anomalously high electrical
conductivity in the mantle wedges, Sci. Adv., 2, e1501631, 10.1126/sciadv.1501631, 2016.
Maumus, J., Bagdassarov, N., and Schmeling, H.: Electrical conductivity and
partial melting of mafic rocks under pressure, Geochim. Cosmochim. Ac., 69,
4703–4718, 2005.
Naif, S.: An upper bound on the electrical conductivity of hydrated oceanic
mantle at the onset of dehydration melting, Earth Planet. Sc. Lett., 482,
357–366, 2018.Novella, D., Jacobsen, B., Weber, P. K., Tyburczy, J. A., Ryerson, F. J., and
Du Frane, W. L.: Hydrogen self-diffusion in single crystal olivine and
electrical conductivity of the earth's mantle, Sci. Rep., 7, 5344,
10.1038/s41598-017-05113-6, 2017.
Padilha, A. L., Vitorello, I., Antunes, C. E., and Padua, M. B.: Imaging
three-dimensional crustal conductivity structures reflecting continental
flood basalt effects hidden beneath thick intracratonic sedimentary basin,
J. Geophys. Res., 120, 4702–4719, 2015.
Pape, L. P., Jones, A. G., Unsworth, M. J., Vozar, J., Wei, W. B., Jin, S., Ye,
G. F., Jing, J. N., Dong, H., Zhang, L. T., and Xie, C. L.: Constraints on the
evolution of crustal flow beneath Northern Tibet, Geochem. Geophy. Geosy., 12, 4237–4260, 2015.
Pous, J., Munoz, G., Heise, W., Melgarejo, J. C., and Quesada, C.:
Electromagnetic imaging of Variscan crustal structures in SW Iberia: The
role of interconnected graphite, Earth Planet. Sc. Lett., 217, 435–450,
2004.
Roberts, J. J. and Tyburczy, J. A.: Frequency dependent electrical properties
of polycrystalline olivine compacts, J. Geophys. Res., 96, 16205–16222,
1991.
Saltas, V., Chatzistamou, V., Pentari, D., Paris, E., Triantis, D., Fitilis,
I., and Vallianatos, F.: Complex electrical conductivity measurements of a
KTB amphibolite sample at elevated temperatures, Mater. Chem. Phys., 139,
169–175, 2013.
Selway, K., Yi, J., and Karato, S.: Water content of the Tanzanian
lithosphere from magnetotelluric data: Implications for cratonic growth and
stability, Earth Planet. Sc. Lett., 388, 175–186, 2014.
Shimojuku, A., Yoshino, T., and Yamazaki, D.: Electrical conductivity of
brine-bearing quartzite at 1 GPa: Implications for fluid content and
salinity of the crust, Earth Planets Space, 66, 1–9, 2014.Sinmyo, R. and Keppler, H.: Electrical conductivity of NaCl-bearing aqueous
fluids to 600 ∘C and 1 GPa, Contrib. Mineral. Petr., 172, 4,
10.1007/s00410-016-1323-z, 2017.
Sun, W. Q., Dai, L. D., Li, H. P., Hu, H. Y., Wu, L., and Jiang, J. J.:
Electrical conductivity of mudstone before and after dehydration at high
temperatures and pressures, Am. Mineral., 102, 2450–2456, 2017a.
Sun, W. Q., Dai, L. D., Li, H. P., Hu, H. Y., Jiang, J. J., and Hui, K. S.: Effect
of dehydration on the electrical conductivity of phyllite at high
temperatures and pressures, Mineral. Petrol., 111, 853–863, 2017b.
Unsworth, M. J., Jones, A. G., Wei, W., Marquis, G., Gokarn, S. G., and Spratt,
J. E.: Project ININDEPTH Team: Crustal rheology of the Himalaya and southern
Tibet inferred from magnetotelluric data, Nature, 438, 78–81, 2005.Wang, D. J., Li, H. P., Matsuzaki, T., and Yoshino, T.: Anisotropy of
synthetic quartz electrical conductivity at high pressure and temperature,
J. Geophys. Res., 115, B09211, 10.1029/2009JB006695, 2010.
Wang, D. J., Mookherjee, M., Xu, Y. S., and Karato, S.: The effect of water on
the electrical conductivity of olivine, Nature, 443, 977–980, 2006.
Wang, D. J., Guo, X. Y., Yu, Y. J., and Karato, S.: Electrical conductivity of
amphibole-bearing rocks: Influence of dehydration, Contrib. Mineral. Petr., 164, 17–25, 2012.Wang, D. J, Liu, X. W., Liu, T., Shen, K. W., Welch, D. O., and Li, B. S.:
Constraints from the dehydration of antigorite on high-conductivity
anomalies in subduction zones, Sci. Rep., 7, 16893, 10.1038/s41598-017-16883-4, 2017.
Wannamaker, P. E., Caldwell, T. G., Jiracek, G. R., Maris, V., Hill, G. J.,
Ogawa, Y., Bibby, H. M., Bennie, S. L., and Heise, W.: Fluid and deformation
regime of an advancing subduction system at Marlborough, New Zealand,
Nature, 460, 733–736, 2009.Wei, W., Unsworth, M., Jones, A. G., Booker, J., Tan, H., Nelson, K. D., Chen,
L., Li, S., Solon, K., and Bedrosian, P.: Detection of wide spread fluids in
the Tibetan crust by magnetotelluric studies, Science, 292, 716–718, 2001.
Xiao, Q. B., Zhao, G. Z., Zhan, Y., Chen, X. B., Tang, J., Wang, J. J., and
Deng, Q. H.: A preliminary study on electrical structure and dynamics of the
ultra-high pressure metamorphic belt beneath the Dabie Mountains, Chinese J.
Geophys., 50, 710–721, 2007.
Xiao, Q. B., Cai, X. P., Liang, G. H., Xu, X. W., and Zhang, B. L.: Application
of 2D magnetotelluric methods in a geological complex area, Xinjiang, China,
J. Appl. Geophys., 75, 19–30, 2011.
Xu, H. J., Ye, K., Song, Y. R., Chen, Y., Zhang, J. F., Liu, Q., and Guo, S.:
Prograde metamorphism, decompressional partial melting and subsequent melt
fractional crystallization in the Weihai migmatitic gneisses, Sulu UHP
terrane, eastern China, Chem. Geol., 341, 16–37, 2013.
Xu, P. F., Liu, F. T., Wang, Q. C., Cong, B. L., and Chen, H.: Slab-like high
velocity anomaly in the uppermost mantle beneath the Dabie-Sulu orogen,
Geophys. Res. Lett., 28, 1847–1850, 2001.
Xu, Y. S., Shankland, T. J., and Duba, A. G.: Pressure effect on electrical
conductivity of mantle olivine, Phys. Earth Planet. In., 118, 149–161,
2000.
Yang, X. Z.: Origin of high electrical conductivity in the lower continental
crust: A review, Surv. Geophys., 32, 875–903, 2011.
Yang, X. Z., Keppler, H., McCammon, C., and Ni, H. W.: Electrical conductivity
of orthopyroxene and plagioclase in the lower crust, Contrib. Mineral. Petr., 163, 33–48, 2012.
Zeng, S. H., Hu, X. Y., Li, J. H., Xu, S., Fang, H., and Cai, J. C.: Detection
of the deep crustal structure of the Qiangtang terrane using magnetotelluric
imaging, Tectonophysics, 661, 180–189, 2015.
Zheng, Y. F.: A perspective view on ultrahigh-pressure metamorphism and
continental collision in the Dabie-Sulu orogenic belt, Chinese Sci. Bull.,
53, 3081–3104, 2008.
Zheng, Y. F., Fu, B., Gong, B., and Li, L.: Stable isotope geochemistry of
ultrahigh pressure metamorphic rocks from the Dabie-Sulu orogen in China:
Implications for geodynamics and fluid regime, Earth-Sci. Rev., 62,
105–161, 2003.Zhou, W. G., Fan, D. W., Liu, Y. G., and Xie, H. S.: Measurements of wave
velocity and electrical conductivity of an amphibolite from southwestern
margin of the Tarim Basin at pressures to 1.0 GPa and temperatures to 700 ∘C: Comparison with field observations, Geophys. J. Int., 187,
1393–1404, 2011.