SESolid EarthSESolid Earth1869-9529Copernicus GmbHGöttingen, Germany10.5194/se-6-1037-2015Experimental study on the electrical conductivity of quartz andesite at
high temperature and high pressure: evidence of grain boundary transportHuiK. S.ZhangH.LiH. P.DaiL. D.dailidong_2014@hotmail.comHuH. Y.JiangJ. J.SunW. Q.Key Laboratory of High-Temperature and High-Pressure Study of the Earth's Interior, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, Guizhou 550002, ChinaUniversity of Chinese Academy of Sciences, Beijing 100049, ChinaL. D. Dai (dailidong_2014@hotmail.com)2September201563103710435April20156May201523August201525August2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://se.copernicus.org/articles/6/1037/2015/se-6-1037-2015.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/6/1037/2015/se-6-1037-2015.pdf
In this study, the electrical conductivity of quartz andesite
was measured in situ under conditions of 0.5–2.0 GPa and 723–973 K using
a YJ-3000t multi-anvil press and a Solartron-1260 Impedance/Gain-Phase
Analyzer. Experimental results indicate that grain interior transport
controls the higher frequencies (102–106 Hz), whereas the grain
boundary process dominates the lower frequencies (10-1–102 Hz).
For a given pressure and temperature range, the relationship between Log
σ and T-1 follows the Arrhenius relation. As temperature increased,
both the grain boundary and grain interior conductivities of quartz andesite
increased; however, with increasing pressure, both the grain boundary and
grain interior conductivities of the sample decreased. By the virtue of the
dependence of grain boundary conductivity on pressure, the activation
enthalpy and the activation volume were calculated to be 0.87–0.92 eV and
0.56 ± 0.52 cm3 mol-1, respectively. The small polaron conduction
mechanism for grain interior process and the ion conduction mechanism for
grain boundary process are also discussed.
Introduction
Studies of the electrical conductivity of rocks at high temperatures and
high pressures have found that similar to temperature and pressure, grain
boundary greatly affects the electrical properties of rocks. Grain boundary,
a general property of rocks, is therefore receiving increasing attention
from researchers. Peridotite is the most important rock in the upper mantle,
and the influence of grain boundary on its electrical conductivity has been
studied in detail (Tyburczy and Roberts, 1990; Roberts and Tyburczy, 1991,
1993, 1994; Xu et al., 1998, 2000; ten Grotenhuis et al., 2004; Watson et
al., 2010; Wu et al., 2010). However, the relation between the total
conductivity, grain boundary and grain interior conductivity for andesite
remains unclear till now.
Whole rock analysis by X-ray fluorescence (XRF) and chemical
composition of dominant minerals for quartz andesite by the electron
microprobe (EPMA).
OxidesXRF for wholeEPMA forEPMA for EPMA forrock (wt %)anorthoclase (wt %)albite (wt %)amphibole (wt %)SiO266.4759.5665.2552.47Al2O313.5726.2019.753.61MgO0.440.150.0315.72CaO1.122.280.6212.18Na2O4.986.9910.550.20K2O4.163.090.020.18FeO5.020.901.0311.23TiO20.220.030.010.33Cr2O30.020.021.470.03MnO2–0.040.000.12P2O50.81–––L.O.I2.74–––Total99.5599.2698.7396.07
Andesite forms in plate subduction settings, and is thus widely distributed
in the orogenic belts bordering the Pacific Ocean. Extensive studies of its
electrical conductivity have achieved notable results. Waff and Weill (1975)
measured the electrical conductivities of andesite of varying components
(Na2O: 4.96–7.83 wt %; FeO: 4.99–13.7 wt %) using a direct
current (DC) method at room pressure and different oxygen partial pressures
of CO2 and H2. They found that increasing alkali ion content
significantly increased the electrical conductivity of andesite, whereas
oxygen fugacity and iron content had little effect (Waff and Weill, 1975).
Tyburczy and Waff (1983) employed the alternating current (AC) at pressures
of 0–2.55 GPa and temperatures of 1473–1673 K to observe the electrical
conductivity of andesite melt from Crater Lake. By combining the electrical
conductivity data from andesite melt and tholeiite to model the high
conductivity zone, they concluded that the electrical conductivity of
andesite melts increases with the rise of pressure, and that a minimum melt
fraction of 5–10 % can account for the anomalously high electrical
conductivity of the upper mantle in typical andesite regions (Tyburczy and Waff, 1983).
More recently, Laumonier et al. (2015) measured
the electrical conductivity of dacitic melts with H2O contents up to 12 wt %
at pressures of 0.15–3.0 GPa and temperatures of 673–1573 K, and
demonstrated that the electrical conductivity is strongly dependent on the
water content. Likewise, the influence of pressure on the activation
enthalpy is strongly correlated with the sample's water content. By means of
T–P–H2O model (temperature–pressure–H2O model), crustal and mantle wedge conductive bodies have been
interpreted by the presence of silica-rich, hydrous, partially crystallized
magma (Laumonier et al., 2015). However, previous studies mainly focused on
the grain interior conductivity of andesite rather than the effect of grain
boundary conductivity.
In this study, the grain boundary electrical conductivity of quartz andesite
was measured at pressures of 0.5–2.0 GPa and temperatures of 723–973 K
within the frequency range of 10-1 to 106 Hz. The characteristic
parameters of the electrical conductivity of quartz andesite acquired here
include the activation enthalpy and the activation volume. These parameters
allow for discussion of the relationship between the contributions from the grain
interior and grain boundary conductivity, and the total conductivity. The
conduction mechanism was also discussed.
Experimental procedureSample preparation
Quartz andesite was collected from Shizhu Town, Yongkang City, Zhejiang
Province, China. The samples were fresh and unaltered, and appeared
pale-yellow in color. According to observation under the optical microscope,
the quartz andesite mainly consists of fine-grained plagioclase, amphibole,
quartz, and feldspathic matrix, without any accessory mineral.
Before experiment, the samples were cut into cylinders of 6 mm diameter and
6 mm height, and cleaned ultrasonically using deionized water, acetone, and
ethanol in turn. Finally, they were placed in an oven at 323 K for 24 h. The
chemical composition and mineralogical proportion of the sample (Table 1)
were analyzed by an X-ray fluorescence spectrometer (XRF) and an electron
microprobe analysis (EPMA) at the State Key Laboratory of Ore Deposit
Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences,
Guiyang, China.
High-pressure conductivity cell and impedance measurements
The electrical conductivity in situ measurements at high pressures and high
temperatures were performed in a YJ-3000t multi-anvil apparatus and 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.
The equipment and experimental process are described in detail by Dai et al. (2012)
and Hu et al. (2014). A diagram of the cross-section of the high-pressure
cell assembly is shown in Fig. 1. In order to avoid the effect of
dehydration on the impedance spectroscopy measurement, a pyrophyllite (32.5 × 32.5 × 32.5 mm)
pressure-transmitting medium was heated
at 1173 K for 12 h in a muffle furnace. The heater was composed of
three-layer stainless steel sheets (total thickness: 0.5 mm) in the shape of
a tube. Similar to previous studies (Dai et al., 2012; Hu et al., 2014), an
alumina and magnesia sleeve were used to ensure that the sample was in a
relatively insulated environment. A grounded 0.025 mm thick nickel foil
located in the middle of the alumina and magnesia sleeve shielded against
external electromagnetic and spurious signal interference. The electrodes
were composed of two nickel disks (0.5 mm in thickness and 6 mm in
diameter). Temperature was monitored using a NiCr–NiAl thermocouple in
contact with the middle of the sample.
Experimental setup of electrical conductivity measurements.
During the experiment, a Solartron-1260 Impedance/Gain-Phase Analyzer was
adopted to collect the impedance spectroscopy with a signal voltage of 1 V
and frequency range of 10-1–106 Hz. To explore the influence
of pressure on electrical conductivity, electrical conductivity was
conducted in the pressure range of 0.5–2.0 GPa. With pressure increased at
1.0 GPa h-1 to each designated pressure, sample was then heated at 100 K h-1,
and the complex impedance of quartz andesite was measured at temperature
intervals of 50 K. To obtain credible data, the temperature was stabilized
for several minutes at each step before measurement. Experimental errors in
the temperature and pressure gradients during each measurement were no more
than ±10 K and ±0.1 GPa, respectively. The obtained impedance
spectra were fitted by an equivalent circuit made of a series of
R1-C1 and R2-C2-W (R1 and C1 correspond respectively
to the resistance and capacitance of grain interior conduction process, and
R2, C2, and W correspond respectively to the resistance, capacitance,
and Warburg element of grain boundary conduction process).
Nyquist plot of the complex impedance of quartz andesite under
conditions of 1.0 GPa and 723–973 K.
Experimental results
In this study, the Nyquist and Bode plots, respectively, for the complex
impedance of typical quartz andesite were obtained under conditions of 1.0
GPa, 723–973 K and 10-1–106 Hz (Figs. 2 and 3). Similar
results were also obtained under different pressures. The presence of
different relaxation time constant led to the appearance in the Nyquist plot
of both a semicircular arc and a 45∘ slope in the complex
impedance plane at the given frequency range. The first semicircle impedance
arc (102–106 Hz) represents the grain interior conduction
mechanism; it crosses the origin, and its center lies on the real axis. The
45∘ slope in the complex impedance plane at the end of the first
semicircle (10-1–102 Hz) represents grain boundary diffusion.
With the rise of temperature, the diameter of impedance arc and value of
impedance decreased rapidly; hence, the electrical conductivity increased.
The Bode plot (Fig. 3) reflects the dependence of modulus (|Z|)
and phase angle (θ) on frequency. From high to low
frequency, the impedance modulus increased rapidly, and the absolute value
of the phase angle tended toward zero. Impedance spectroscopy theory (Nover
et al., 1992; Huebner and Dillenburg, 1995; Huang et al., 2005) relates the
real part (Z′), imaginary part (Z′′), modulus (|Z|) and
phase angle (θ) as follows: Z′=|Z|cosθ and
Z′′=|Z|sinθ. According to previous studies (Dai and
Karato, 2014a, b), the resistance of the grain interior can be determined by
modeling the electrical response with equivalent circuit of resistance and
capacitance (R1C1). However, the impedance at low frequency is not wholly semicircular. By combing the 45∘ of slope in the complex
impedance at low frequency, a Warburg element was adopted to fit the grain
boundary resistance. The equivalent circuit is shown in Fig. 2. Another
equivalent circuit composed of a resistor and capacitor in parallel was used
simultaneously to fit the total resistance. Furthermore, the grain interior,
grain boundary and total electrical conductivity are in accordance with the
following expression:
σ=L/SR,
where L is the sample length (m), S is the cross-sectional area of the
electrode (m2), and R is the resistance for the given conduction process
(Ω).
Bode plot of dependence of modulus and phase angle on frequency
quartz andesite under conditions of 1.0 GPa and 723–973 K.
At pressures of 0.5–2.0 GPa and temperatures of 723–973 K, the
relationship between the electrical conductivity (σ) of the quartz
andesite and reciprocal temperature (T-1) was fitted using the Arrhenius
relation:
σ=σ0exp(-ΔH/kT),
where σ0 is the pre-exponential factor (S m-1), ΔH is the
activation enthalpy (eV), k is the Boltzmann constant, and T is the absolute
temperature (K). The relationship between activation energy ΔU (eV),
pressure P (GPa) and activation volume ΔV (cm3 mol-1) is expressed
as follows:
ΔH=ΔU+P×ΔV.
The grain interior, grain boundary, and total conductivity at different
pressures and temperatures are plotted against reciprocal temperature in
Figs. 4–6. Figures 4 and 5 show the plots for grain interior and grain
boundary conductivity, respectively. The relationship between grain
interior, grain boundary and total conductivity at 1.0 GPa is shown in
detail in Fig. 6, and the value of electrical conductivity under 1.0 GPa
is summarized in Table 2. Similar results were obtained under 0.5–2.0 GPa.
The ratio of grain boundary (σgb) to grain interior (σgi) conductivity represents their respective contributions to total
conductivity; it varies with temperature and pressure, and is plots in the
range 0.5–2.0 GPa in Fig. 7. Fitting parameters of the grain interior
and grain boundary conductivity are listed in Table 3.
The relationship of the logarithmic grain boundary conductivity vs.
reciprocal temperature under conditions of 0.5–2.0 GPa and 723–973 K.
The relationship of the logarithmic grain interior conductivity vs.
reciprocal temperature under conditions of 0.5–2.0 GPa and 723–973 K.
The relationship of grain interior, grain boundary and total
conductivity under conditions of 1.0 GPa and 723–973 K.
Grain boundary/grain interior conductivity (σgb/σgi) versus reciprocal temperature (T-1) under
conditions of 1.0 GPa and 723–973 K. The ratio represents the leading role
of grain boundary or grain interior conductivity in the conduction process.
The value of grain interior, grain boundary and total electrical
conductivity under 1.0 GPa and 723–973 K. The estimate error for grain
interior conductivity is lower than 5 %, for grain boundary conductivity
is lower than 7 %, and for total conductivity is lower than 5 %.
In the present work, the grain interior (σgi), grain boundary
(σgb) and total electrical conductivity (σt) of
quartz andesite were measured in situ at the pressures of 0.5–2.0 GPa and
temperatures of 723–973 K. With the rise of pressure, the grain boundary
conductivity decreases, while the activation enthalpy and pre-exponential
factor increase (Fig. 4 and Table 3). From Fig. 6, it is clear that the
grain boundary conductivity was higher than either the grain interior or
total conductivity, and the total conductivity was lower than the grain
interior conductivity.
The activation energy and activation volume for grain boundary conduction
process under the experimental conditions were 0.90 ± 0.10 eV and
0.56 ± 0.52 cm3 mol-1, respectively. The ratio of grain boundary to
grain interior conductivity (σgb/σgi) at 0.5–2.0 GPa
(Fig. 7) gradually decreased with increasing temperature and pressure;
and thus the contribution of grain boundary conductivity to the total
conductivity of quartz andesite continually decreased with increasing
temperature and pressure. Dai et al. (2008) presented a functional model of
the variation of grain boundary conductivity with pressure in which the
grain boundary conductivity of peridotite varies with the width of grain
boundary, as follows:
σgb-micro=σgb-bulk(d/D),
where σgb-micro is the microscopic grain boundary conductivity
(S m-1), σgb-bulk is the bulk grain boundary conductivity (S m-1),
d is the grain boundary width (µm), and D is the grain size (µm).
According to Eq. (4), the diffusivity of cements between feldspar and
amphibole in the quartz andesite increased with the rise of pressure,
reducing the grain boundary width along the direction of current
transmission and decreasing the grain conductivity accordingly. These
results are consistent with those of ten Grotenhuis et al. (2004) and Dai et
al. (2008) on the effect of pressure on the grain boundary electrical
conductivity of peridotite.
Figure 5 shows that the grain interior conductivity of the quartz andesite
decreased with increasing pressure; the activation enthalpy and
pre-exponential factor increased accordingly. The variation of grain
interior conductivity with pressure observed here is similar to previous
studies, which were concentrated on the effect of partially molten andesite
(Waff and Weill, 1975; Tyburczy and Waff, 1983; Laumonier et al., 2015)
(Fig. 8). The activation enthalpy (0.81–1.05 eV) and activation volume
(4.96 ± 0.52 cm3 mol-1) of quartz andesite are within the same range
as results for andesite (0.78–1.17 eV and 3.25–17.9 cm3 mol-1,
respectively) from Crater Lake (Tyburczy and Waff, 1983), and are also
similar to those of dacitic melts (0.69–1.0 eV and 3.9–24.7 cm3 mol-1,
respectively) measured by Laumonier et al. (2015). However,
discrepancies in pressure, temperature, melting conditions, and chemical
composition of the samples are the important factors that might have led to
1–2 orders of magnitudes lower found here, compared with previous studies.
A comparison of grain interior conductivity of quartz andesite with
previous studies.
On the other hand, the logarithmic conductivity (Log σ) and
reciprocal temperature (T-1) show a strong linear relationship
(> 99 %). On the base of the result, including FeO = 5.02 wt %
in the quartz andesite (Table 1), ΔH=0.81–1.05 eV and
ΔV=4.96± 0.52 cm3 mol-1 (Table 3); this implies that there is
only one single dominant conduction mechanism for quartz andesite. Numerous
studies have reported similar results, indicating that the conduction
mechanism is the small polaron (Xu et al., 1998; Scarlato et al., 2004; Dai
et al., 2008; Yang and Heidelbach, 2011). We consider that the conduction
mechanism of grain interior conduction process is the small polaron
conduction. The hopping process can be described as follows:
FeMgx+h⚫⇌FeMg⚫.
In ferromagnesian silicate, the presence of FeMg⚫ generates an
extra positive charge that repulses cations, causing the lattice deformation
is a small polaron (Dai et al., 2013). The small polaron is an important
conduction mechanism at low temperature; it is characterized by the transfer
of an electron hole (h⚫) from FeMg⚫ to FeMgx
(Schmidbauer et al., 2000; Huang et al., 2005; Poe et al., 2008; Dai et
al., 2014, 2015). In light of the above-mentioned results, the low energy
barrier for the transmission process resulted in the low activation enthalpy
of quartz andesite. Two further factors, oxygen fugacity and iron content,
also affect the small polaron conduction of quartz andesite. The proportion
of ferric iron in the total iron (Fe3+/ΣFe) increases with
increasing oxygen fugacity; with the rise of iron content, the charge
carrier concentration also increases. However, understanding the effects of oxygen
fugacity and iron content on the grain interior conductivity of quartz
andesite requires further research.
As mentioned above, a Warburg element was adopted to fit the grain boundary
resistance; and it indicates that the grain boundary conduction process
occurred via ion diffusion. A large quantity of alkali ions are contained in
the quartz andesite (Na2O: 4.98 wt %; K2O: 4.16 wt %), and
requires only low activation energy (Hu et al., 2014). Combing the
activation energy of the grain boundary conduction process (0.87–0.92 eV), we
conclude that the grain boundary conduction mechanism for quartz andesite is
the ion conduction. However, understanding the effects of alkali ion content on the grain
boundary conductivity of quartz andesite requires further
research.
Conclusions
At pressures of 0.5–2.0 GPa and temperatures of 723–973 K, and within
the frequency 10-1–106 Hz, the grain boundary conductivity of
quartz andesite ranged from 10-4.2 to 10-2.2 S m-1; the activation
enthalpy and activation volume were 0.87–0.92 eV and 0.56 ± 0.52 cm3 mol-1,
respectively. The grain boundary conductivity varied greatly
with pressure, temperature. Its effect on the total conductivity increased
with the rise of temperature. The grain boundary conductivity tended to
decrease with increasing pressure. At 0.5–2.0 GPa, the total conductivity
of quartz andesite is slightly lower than grain interior conductivity due to
the presence of grain boundary. These obtained physical parameters, combined
with data on the chemical and mineralogical composition of the andesite,
suggest that the conduction mechanism for grain interior of quartz andesite
is the small polaron conduction, and for grain boundary is the ion
conduction.
Acknowledgements
We thank the editor Juan Carlos Afonso and two reviewers (Nover Georg and Clark Simon)
for their very constructive comments and
suggestions during the reviewing process, which helped us greatly in improving
the manuscript. This research was financially supported by the “135”
Program of Institute of Geochemistry of CAS, Hundred Talents Program of CAS,
Youth Innovation Promotion Association of CAS and NSF of China (41474078,
41304068 and 41174079).
Edited by: J. C. Afonso
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