The electrical conductivity of eclogite in Tibet and its geophysical ...

SCIENCE CHINA Earth Sciences • RESEARCH PAPER •

September 2014 Vol.57 No.9: 2071–2078 doi: 10.1007/s11430-014-4876-6

The electrical conductivity of eclogite in Tibet and its geophysical implications GUO YingXing1, WANG DuoJun1*, SHI YaoLin1, ZHOU YongSheng2, DONG YongSheng3 & LI Cai3 1

2

Key Laboratory of Computational Geodynamics, University of Chinese Academy of Sciences, Beijing 100049, China; The State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China; 3 College of Earth Sciences, Jilin University, Changchun 130061, China Received April 25, 2013; accepted November 13, 2013; published online July 2, 2014

The electrical conductivity of Tibetan eclogite was investigated at pressures of 1.5–3.5 GPa and temperatures of 500–803 K using impedance spectroscopy within a frequency range of 10−1–106 Hz. The electrical conductivity of eclogite increases with increasing temperature (which can be approximated by the Arrhenius equation), and is weakly affected by pressure. At each tested pressure, the electrical conductivity is weakly temperature dependent below ~650 K and more strongly temperature dependent above ~650 K. The calculated activation energies and volumes are 44±1 kJ/mol and −0.6±0.1 cm3/mol for low temperatures and 97±3 kJ/mol and −1.2±0.2 cm3/mol for high temperatures, respectively. When applied to the depth range of 45–100 km in Tibet, the laboratory data give conductivities on the order of 10−1.5–10−4.5 S/m, within the range of geophysical conductivity profiles. electrical conductivity, high temperature and pressure, eclogite Citation:

Guo Y X, Wang D J, Shi Y L, et al. 2014. The electrical conductivity of eclogite in Tibet and its geophysical implications. Science China: Earth Sciences, 57: 2071–2078, doi: 10.1007/s11430-014-4876-6

Eclogite is a garnet- and clinopyroxene-rich rock that is typically formed during high-pressure metamorphism of mafic igneous rocks (basaltic in composition) subducted to the lower crust or mantle (Anderson, 2007; Peacock et al., 1999). High pressure eclogite is considered to be a marker of collision between oceanic crust and a continental plate (Yang et al., 2009). In subducting oceanic crust, one of the most important reactions involves the transformation from basalt or gabbro to eclogite (Stern et al., 2002; Searle et al., 2011; Hacker et al., 2003). This transformation occurs at depths of ~50 to ~300 km (Kirby et al., 1996), and eclogite can exist at a depth of 100 km, according to Hacker et al. (2003).

*Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2014

Studying the electrical conductivity of minerals and rocks from a variety of regions is important to constraining the composition of the earth’s interior using field observations. Magnetotelluric soundings in the Qiangtang area have revealed widespread high-conductivity layers in the crust (Chen et al., 1996; Wei et al., 2001; Bai et al., 2010), with conductivities of up to 0.01–1 S/m in the crust and upper mantle. This high-conductivity layer is generally considered to be caused by partial melting or aqueous fluid (Wei et al., 2001; Li et al., 2003; Nelson et al., 1996) and is not considered to reflect the conductivity of the rock itself. Figure 1 shows an electrical conductivity profile to a depth of 100 km in the Tibetan Plateau. Recently, eclogite has been found in several high-pressure metamorphic belts in Tibet, where the Indian plate is subducting beneath the SinoPacific plate; the eclogite exists at depths of 45–120 km earth.scichina.com

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Figure 1 Electrical conductivity of the Qiangtang region in the Tibetan Plateau. The electrical conductivity data for the section located in Qiangtang is from lines 500 and 600 collected by Wei et al. (2001, 2006). (a) Line 500; (b) Line 600.

(Giese et al., 1999). Zhai et al. (2011) indicated that peak eclogite-facies metamorphism occurs at 683–733 K and 2.0–2.5 GPa, while Zhang et al. (2006) found that eclogite can form at pressures as low as 2.0–2.5 GPa at temperatures of 755 to 898 K. The stability field of eclogite occurs above ~773 K and ~1 GPa (Peacock et al., 1999). Thus, eclogite may exist in the lower crust and upper mantle in the Qiangtang region. The electrical conductivity of eclogite is important to understanding the distribution of electrical conductivities in this region, and may provide a clue to interpreting the high-conductivity anomaly in the lower crust and upper mantle in the Tibetan Plateau. However, the electrical conductivity of eclogite in this region has not yet been studied. The electrical conductivity of eclogite has been studied much less than that of peridotite. Laštovičková et al. (1976) studied the electrical conductivity of eclogites from the Bohemian Massif at high temperatures (473–1173 K) and pressures (up to 2 GPa) using both direct- and alternatingcurrent methods, and found that the conductivity is strongly affected by the content of symplectite minerals. More recently, Bagdassarov et al. (2011) measured the electrical conductivity of eclogite collected from the Tianshan Mountains at high temperatures and 2.5 GPa using the electrical impedance method, and obtained an activation energy of ~77 kJ/mol. As these studies were conducted at confining pressure (Laštovičková et al., 1976; Bagdassarov et al., 2011), the pressure effect on the electrical conductivity remains unclear. Thus, further measurements at various pressures are important to understanding the electrical conductivity of eclogite in its stability field, and help constrain the relationship between electrical conductivity and depth.

In this study, we determine the electrical conductivity of eclogite at high pressures and temperatures (simulating the pressure and temperature conditions in the lower crust and upper mantle) and measure the effect of pressure on electrical conductivity. We also compare the electrical conductivity derived from geophysical observations to constrain the composition of this region, and investigate the formation mechanism of the high-conductivity layer in the Tibetan Plateau.

1 Experimental procedure 1.1

Sample preparation

The material studied was natural eclogite collected from central Pianshi Hill (33°23′46″N, 86°7′7″E), Qiangtang, Tibet. A photomicrograph of the material is shown in Figure 2. The mineral assemblages are 40% garnet (Grt), 30% omphacite (Omp), and 30% accessory minerals, including 25% glaucophane (Gln) and ~5% rutile (Rt) and quartz (Q). The compositions of the bulk rock and minerals were analyzed using X-ray fluorescence and an electron microprobe, and are shown in Tables 1 and 2, respectively. The average garnet composition from microprobe analyses is Alm64.5And0.5Grs26.8Prp7.1Sps1.0Uv0.1, which puts the eclogite into group C (Figure 3) of Coleman et al. (1965). Most garnets are 0.1 to 0.5 mm in diameter, with an average diameter of 0.3 mm. To determine the impact of structural water in eclogite on conductivity, Fourier transform infrared spectroscopy measurements of samples were obtained from 3000 to 4000 cm−1. The thicknesses of samples were 0.170 and 0.150 mm

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Figure 2 Photomicrograph of the starting material. Grt, garnet; Omp, omphacite; Gln, glaucophane; Rt, rutile; Q, quartz.

before and after the experiment, respectively. The main minerals analyzed have two or more peaks within the typical OH infrared absorption region (3000–3800 cm−1). The calibration from Paterson (1982) was used to determine the water content: C

  i 150



K ( ) d , 3780 

(1)

where COH is the molar concentration of hydroxyl (H/106 Si), Bi is the density factor,  is the orientation factor, and K() is the absorption coefficient in cm−1 at wavenumber v in cm−1, with v between 3000 and 3800 cm−1. The average water contents are shown in Table 3: the bulk water content Table 1

Figure 3 Compositions of garnets in eclogite. The three garnet groups are defined by Coleman et al. (1965). The black circle shows the sample used in this study, and the grey square is from Zhai et al. (2011).

is 0.63 wt.%, which assumes 5 % rutile in the sample, and the water content of glaucophane is 2.35 wt.%. The water content changed little during the experiment, which indicates that eclogite dehydration did not occur at the experimental temperature. 1.2

Experimental method

Impedance is a complex quantity that consists of an Ohmic resistance (the real component) and a reactance (the imaginary component), and reflects the total opposition to current flow in response to an alternating current (ac) signal. It was defined as a complex function (Roberts et al., 1991, 1993; Huebner et al., 1995; Zhu et al., 2001; Wang et al., 2002): Z * = Z r  jZ i ,

(2)

Bulk rock analysis of the eclogite sample

Al2O3 Composition SiO2 Content (wt.%) 48.37 12.82 a) LOI denotes the loss on ignition. Table 2

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TiO2 4.11

Fe2O3 16.73

CaO 10.0

MgO 4.44

K2 O 0.1

Na2O 3.17

MnO 0.26

P2O5 0.5

LOIa) −0.42

Total 100.08

Electron-microprobe analysis of representative minerals in the sample

Composition (wt.%) Na2O TiO2 MgO SiO2 Cr2O3 K2O Al2O3 MnO CaO FeO NiO Total

Grt 0.07±0.02 0.18±0.02 1.79±0.18 38.59±0.02 0.02±0.01 0.01±0.01 21.29±0.08 0.46±0.04 9.63±0.26 29.16±0.22 0.02±0.01 101.24±0.09

Omp 8.04±0.08 0.11±0.00 6.81±0.06 56.06±0.21 0.02±0.01 0.01±0.01 10.99±0.15 0.02±0.01 11.55±0.06 6.67±0.25 0.01±0.01 100.28±0.22

Gln 5.19±0.05 0.65±0.05 10.05±0.11 47.39±0.11 0.02±0.01 0.53±0.01 13.42±0.14 0.05±0.00 6.97±0.10 13.26±0.02 0.02±0.00 97.65±0.06

Rt 0.02±0.01 97.17±2.49 0.01±0.01 1.29±1.23 0.03±0.02 0.00±0.00 0.09±0.07 0.01±0.01 1.23±0.98 0.51±0.03 0.00±0.00 100.38±0.24

Q 0.02±0.02 0.00±0.00 0.01±0.01 98.54±0.12 0.00±0.00 0.00±0.00 0.00±0.00 0.02±0.01 0.03±0.01 0.23±0.04 0.03±0.02 98.88±0.10

Table 3 Water contents of the minerals in the sample using the calibration from Paterson (1982) Mineral Water content (wt.%)

Grt 0.084

Omp 0.032

Gln ~2.35

Rt 0.039

Q –

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where Z* is the complex impedance (the asterisk denotes a complex quantity), Zr and Zi are the real and imaginary elements of impedance, respectively and j  1 . The real and imaginary parts of the impedance are obtained at a given frequency by (3) Z r  | Z | cos  , Z i  | Z | sin  ,

(4)

where | Z | is the modulus of the impedance and  denotes phase angle. High-pressure experiments in our study were carried out in a YJ-3000 cubic-anvil high-pressure apparatus at the Institute of Geochemistry, Chinese Academy of Sciences; this apparatus was also described elsewhere (Wang et al., 2008, 2010). The high-pressure sample cell for electrical conductivity measurements is shown in Figure 4. A cubic pyrophyllite sintered at 1173 K was used as the pressure-transmission medium. The sample was cylindrical, with a diameter of 8 mm and a length of 5 mm. It was sandwiched by Ni disk electrodes. The oxygen fugacity in the sample cell was controlled by the Ni-NiO buffer. The sample was insulated by an Al2O3 tube, and the furnace contained three layers of stainless steel foil. The sample temperature was monitored by a NiCr-NiAl thermocouple. A Solartron 1260 impedance/gain-phase analyzer was used to measure the complex impedance. It can simultaneously measure the modulus | Z | and phase angle θ at various frequencies. A 1 V sinusoidal signal was applied in the frequency range 0.1–106 Hz, and the complex impedance measurements were performed from 500 to 803 K at 1.5 GPa, from 500 to 774 K at 2.5 GPa, and from 500 to 781 K at 3.5 GPa for several heating/cooling cycles. The complex impedance was analyzed using ZPlot software to fit it to an equivalent circuit comprising a resistor in parallel with a capacitor.

2 Results and discussion The resulting data sets for eclogite at selected temperatures


and pressures are shown in Figure 5, in the form of complex plane plots showing the real (Z′) versus imaginary (Z″) components. The data points form ideal semicircular patterns and incomplete arcs. The frequencies decrease from the left to the right, near the intersection of the data with real axis (Z′). There are two arcs at different pressures, implying two charge-transport processes (Roberts et al., 1991; Huebner et al., 1995). The ideal semicircular arc at a high frequency range represents the conductivity of the grain interior, and the incomplete arc at low frequencies gives the conductivity associated with the grain boundaries. Huebner et al. (1995) have demonstrated that the electrical conductivity of a grain boundary weakly affects the total electrical conductivity, which is dominated by the conduction of the grain interior. The diameter of the impedance arc decreases with increasing temperature, which indicates that the electrical conductivity of eclogite increases with increasing temperature, behaving similarly to a semiconductor. These complex impedance arcs are equivalent to a parallel circuit of resistors and capacitors in Figure 5. The sample’s resistance was calculated by fitting the first arc to an equivalent circuit containing a resistor (R1) and a capacitor (C); the fitting error less than 1%. The electrical conductivity  was calculated by (5)   (d /s)/R , where d is the thickness of the sample, s is the crosssectional area of the electrode and R is the resistance. Figure 6 shows the electrical conductivity of eclogite at 1.5 GPa during the three heating/cooling cycles. The sample was heated to 803 K and then cooled to 500 K. The electrical conductivity shows excellent reproducibility during the heating/cooling cycles, with the exception of some data points. The resulting electrical conductivities at 1.5, 2.5, and 3.5 GPa are shown in Figure 7. These results are similar throughout the heating/cooling cycles. The pressure weakly affects the electrical conductivity in the experimental temperature range. Xu et al. (2000) carefully studied the electrical conductivity of olivine at 4, 7, and 10 GPa, and found

Figure 4 Sample assembly (right) and impedance measuring system (left) for electrical conductivity measurements at high pressures. 1, pyrophyllite; 2, thermocouple; 3, heating element ; 4, pyrophyllite space; 5, Al2O3 space; 6, Al2O3 sleeve; 7, sample; 8, electrode wires; 9, Ni electrode.

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Figure 6 Logarithm (electrical conductivity) versus reciprocal temperature for eclogite at 1.5 GPa. The complex impedance measurements were performed from 506 to 803 K for three heating/cooling cycles.

temperature. The activation enthalpy is given by H = U + P  V ,

(7)

where ∆U is the activation energy and P is pressure. The pre-exponential factor, activation energy, and volume were calculated to fit the points from the three curves corresponding to the three pressures; the results are summarized in Table 4. The calculated conductivities using the parameters listed in Table 4 at 1.5, 2.5, and 3.5 GPa are plotted in Figure 7. This figure also shows a comparison with previous studies (Laštovičková et al., 1976; Bagdassarov et al., 2011). The relations    0 exp(H /kT ) and H =U+ P  V were used to calculate the values of the parameters. The listed errors are one standard deviation, and include contributions from errors in the individual measurements, such as temperature and electrical conductivity.

Figure 5 Complex impedance arcs and equivalent circuit of eclogite from 554 to 803 K (281–530°C) at different pressures. For the low-temperature results, only parts of the spectra are shown. In the equivalent circuit, R0 and R1 are resistors, and C is a capacitor.

the pressure has a weak effect on the electrical conductivity of olivine. Our results are consistent with their results. A change in slope occurs near 651–665 K at 1.5 GPa; 650– 665 K at 2.5 GPa and 639–667 K at 3.5 GPa. The electrical conductivities were fitted separately to the following equation for lower and higher temperatures:    0 exp(H /kT ) ,

(6)

where σ0 is the pre-exponential factor, ∆H is the activation enthalpy, k is the Boltzmann constant, and T is the absolute

Figure 7 Logarithm (electrical conductivity) versus reciprocal temperature for eclogite at different pressures. .

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2012):

Electrical conductivity parameters of eclogite

P (GPa) (T (K))

3

log10σ0

∆U (kJ/mol)

∆V (cm /mol)

0.3±0.1

44±1

−0.6±0.1

1.5 (500–651) 2.5 3.5 1.5 2.5 3.5

(500–650) (500–639) (665–803) (665–774) (667–781)

4.4±0.2

97±3


−1.2±0.2

Laštovičková et al. (1976) studied the electrical conductivity of eight species of eclogites from the Bohemian Massif under high temperatures (473–1173 K) and pressures (up to 2 GPa), and found electrical conductivities between 10−5 and 10−2.5 S/m. Bagdassarov et al. (2011) measured the electrical conductivity of eclogite with an iron content of 8.2 wt.% and calculated an activation energy of 80 kJ/mol. The electrical conductivities obtained in this study are higher than those from Bagdassarov et al. (2011) and Laštovičková et al. (1976); this is probably because of differences in chemical compositions, however, the chemical compositions, particularly the iron contents, of the eclogites used in the previous studies were not always specified. The electrical conductivity of eclogite is controlled by minerals such as garnet, pyroxene, and amphibole. From Table 4, activation energy values of 97 and 44 kJ/mol may correspond to different mechanisms of conduction. An electrical conductivity study on garnet with a Fe# (Fe#=100*Fe/ (Fe+Mg)) between 0 and 100 showed a wide activation enthalpy range from 54 to 245 kJ/mol (Romano et al., 2006), which is related to the small polaron, i.e., the electron or hole transfer from Fe2+ and Fe3+. Schmidbauer et al. (2000) found that Fe-bearing silicates were dominated by the electron-hopping processes of Fe2+→Fe3+. The activation enthalpy (97 kJ/mol) obtained at high temperatures in our study falls into this range. However, the water content of their previous sample is unclear. Tolland (1973) measured activation enthalpies of 52 and 55 kJ/mol along and across, respectively, strips of amphibole and determined the conduction mechanism of amphibole. Usually, the activation enthalpy of amphibole is less than 67 kJ/mol before dehydration, which suggests that the activation energy at low temperatures in our study is probably dominated by amphibole (glaucophane). Most previous studies (Schock et al., 1989; Yoshinoa et al., 2008, 2010) suggest ionic conduction occurs at the high-temperature range. Therefore we suggest that the conduction of eclogite at low temperatures is mainly attributable to small polarons. Previous studies have indicated that hydrogen-related defects significantly affect electrical conductivity (Wang et al., 2006; Yang et al., 2011). We argue that the electrical conductivity here is dominated by water and iron. To identify this phenomenon, the self-diffusion coefficients of iron and hydrogen were calculated from the electrical conductivity according to the Nernst-Einstein equation (Karato et al.,



f  D  n  q2 , RgasT

(8)

where f (~1) is a non-dimensional constant representing the geometrical factor, D is the diffusivity, n is the concentration of the charged species, q is the electrical charge of the species, Rgas is the gas constant, and T is the temperature in Kelvin. The diffusion coefficients of hydrogen and iron in the garnet, pyroxene, and amphibole are shown in Figure 8. The diffusion coefficients of iron in garnet, pyroxene, and amphibole are significant smaller than those calculated in our study, while the diffusion coefficients of hydrogen in garnet and pyroxene are relatively close to those in this study, indicating that the electrical conductivity of eclogite at high temperatures is most likely controlled by hydrogen.

3 Geophysical implications Magnetotelluric measurements reveal the distribution of electrical conductivity with depth. To properly interpret field observations, the electrical conductivity of the rock at the study location must be known. Figure 9 displays a comparison of geophysical profiles with those calculated from laboratory measurements at lower-crustal and upper-mantle conditions. It shows that electrical conductivities at 45–100 km depths in Qiangtang, Tibet vary from 10−3 to 10−0.5 S/m. The electrical conductivity derived from experiments is 10−1.5 –10−4.5 S/m, according to eq. (5). The temperatures are from 500 to 803 K, which is within the stability field of eclogite. Thus, electrical conductivities from 10−3 to 10−1.5 S/m are attributed to eclogite prior to its dehydration. However,

Figure 8 The diffusivities inferred using the Nernst-Einstein relation at 1.5, 2.5, and 3.5 GPa. ‘H’ and ‘Fe’ in brackets represent hydrogen and iron diffusion coefficients, respectively. The hydrogen diffusion coefficients for different minerals were derived from Farver (2010). The iron diffusion coefficients for diopside are from Cherniak et al. (2010), and those for garnet are from Ganguly (2010).

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Nos. 41074063, 41374095, 41272240), the CAS/CAFEA International Partnership Program for Creative Research Teams (Grant No. KZZDEW-TZ-19), the National Science and Technology Project (Grant No. SinoProbe-07). The authors are grateful to Heping Li, Lidong Dai and Chunming Wu for their helpful suggestions. We also thank Zaiyang Liu, Yingjie Yu and Danyang Li for their technical assistance.

Figure 9 A comparison of laboratory-based conductivities with geophysically inferred electrical conductivity for the lower crust and upper mantle. The light gray and hatched regions correspond to the electrical conductivities for lines 500 and 600. The dark gray region indicates the electrical conductivity of eclogite from this study. The solid and dashed lines indicate the electrical conductivity of amphibole-bearing rocks derived from Wang et al. (2012).

eclogite cannot be used to interpret abnormally high electrical conductivity from 10−1.5 to 10−0.5 S/m. The high-conductivity anomalies could be interpreted as partial melting and aqueous fluids from the motion of a fault (Zhao et al., 2008, 2012). Zhao et al. (2008) argued that partial melting is a cause of the low-resistivity body at a depth of 15–20 km beneath the Himalayas of southern Tibet. They suggested that the low-resistivity layer beneath the eastern Tibetan Plateau is blocked by stable lithosphere beneath the Sichuan Basin, which caused the uplift of eastern Tibet (Zhao et al., 2012). In contrast, hydrous minerals such as amphibole may explain the high conductivities measured in the region. Figure 9 also shows the electrical conductivity of amphibolebearing rock (Wang et al. 2012), which ranges from 10−6 to 1 S/m, consistent with geophysical observations. Thus, the dehydration of amphibole-bearing rock may provide a clue to interpreting the high electrical conductivities (10−1.5– 10−0.5 S/m), which may be because of retrograde metamorphism from eclogite to amphibole. Tibet has undergone many stages of subduction, collision, and uplift, and retrograde metamorphism often occurs when eclogite is exhumed to shallower conditions. Therefore, the eclogite may partially or completely retrogress to amphibolite during exhumation (Sajeev et al., 2009). Our experimental observations suggest that eclogite is likely at this depth, and some conductive rock-forming minerals related to eclogite may also be present in the Qiangtang region because of subduction and uplift. This work was supported by the Important Field Program of Knowledge Innovation Program of Chinese Academy of Sciences (Grant No. KZCX2YW-QN608), the National Natural Science Foundation of China (Grant

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