- Email: [email protected]
Electrical conductivity of the major upper mantle minerals: a review
Russian Geology and Geophysics 50 (2009) 1139–1145 www.elsevier.com/locate/rgg
Electrical conductivity of the major upper mantle minerals: a review T. Katsura *, T. Yoshino, G. Manthilake, T. Matsuzaki Institute for Study of the Earth’s Interior, Okayama University, Misasa, 682-0193, Japan Received 10 October 2008
Abstract The electrical conductivity of the major upper mantle minerals, namely, olivine, wadsleyite and ringwoodite, is reviewed in this paper. There are mainly three electrical conduction mechanisms for three upper mantle minerals: hopping, ionic and proton conductions. The charge carriers for these conduction mechanisms are an electron hole in Fe ion, a vacancy in Mg site, and a proton, respectively. Hopping conduction is the most essential conduction mechanism for the major upper mantle minerals. Because ionic conduction has high activation energy, it becomes a dominant conduction mechanism only at high temperatures. Proton conduction contributes at relatively low temperatures. If the mantle minerals contain large amount of water (more than 0.1 wt.%), proton conduction can be a dominant conduction mechanism, even at high temperatures. © 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: electrical conductivity; upper mantle; transition layer; olivine; wadsleyite; ringwoodite
Introduction
Electrical conductivity mechanisms
The electrical conductivity profiles in the upper mantle are obtained by means of magnetotellurics (Egbert, 2007) and geomagnetic deep soundings (Lowes, 2007) using the electromagnetic observations. Information about the structure and dynamics of the upper mantle can be obtained by comparing the geophysically obtained electrical conductivity profiles with electrical properties of minerals experimentally determined. Although seismology gives us a variety of reliable information about the structure and dynamics of the Earth’s interior, information obtained from electrical conductivity is independent of that obtained from seismology. Therefore, this kind of information is very valuable to understand the Earth’s interior. For example, as mentioned below, conductivity is sensitive to the presence of water. Therefore, we could obtain information about distribution of water in the mantle from electrical conductivity profiles with knowledge of electrical properites of the mantle minerals. In this paper, we review electrical properties of the major upper mantle minerals, namely, olivine, wadsleyite, and ringwoodite.
In metal, electrical conduction takes place by free electrons. The mantle minerals are, however, essentially ionic crystalline materials, in which almost all electrons are bound to ions. Hence, particles other than free electrons play an important role in electrical conduction of mantle minerals. The major mantle minerals are ferromagnesian silicates, in which the most important charge carrier is an electron hole in an Fe ion. In addition to this, atomic vacancy in a Mg site and proton (H+) also act as a charge carrier. In this paper, the electric conduction mechanisms by these carriers are called as hopping, ionic and proton conductions, respectively. We briefly explain them below. The three conduction mechanisms mentioned above are thermally activated processes, in which conductivity increases with increasing temperature. These conductivity mechanisms are frequently expressed using the Arrhenius formula:
* Corresponding author. E-mail address: [email protected] (T. Katsura)
∆E σ = σ0 exp − , kT where ∆E is activation energy, k is the Boltzmann constant and T is absolute temperature. The σ0 is a constant called pre-exponential term. The σ0 and ∆E could have a special formula according to the conduction mechanism.
1068-7971/$ - see front matter D 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2009.11.012
1140
T. Katsura et al. / Russian Geology and Geophysics 50 (2009) 1139–1145
Fig. 1. A scheme of hopping conduction of Fe•Mg in ferromagnesian silicate. OxO , charge-free oxygen vacancy; e′, electron; h•, hole.
Hopping conduction. In ferromagnesian silicates, an Fe ion usually substitutes for a Mg ion in a Mg site. Hence it is basically a ferrous ion, which is expressed as FexMg in the Kroeger-Vink expression (for this expression, see Chiang et al., 1997). However, according to oxygen fugacity, temperature and pressure, there should be a certain proportion of ferric ion, Fe•Mg, which has an electron hole. Transfer of an electron hole from Fe•Mg to FexMg carries an electric charge (Fig. 1). This is hopping conduction. If only electron holes migrate, the energy barriers for the migration are expected to be relatively low. However, the presence of an electron hole should have a significant effect on the local structure of the ionic crystal, which will impede migration of electric charge. If Fe•Mg exists, there is an extra positive charge, which repulses cations and attracts anions (Fig. 2). This complex of the local strains is called “small polaron”. The migration of electron holes associates small polaron. A relatively large energy is needed for migration of a small polaron in comparison with that of an electron hole. As a result, the hopping conduction of usual ferromagnesian silicates has relatively large activation energy of around 1 eV. As it is explained above, hopping conduction of mantle minerals is due to transport of Fe•Mg. Hence, hopping conduction increases with increasing Fe•Mg. Given the oxygen fugacity in the system is constant, hopping conduction increases with increasing total iron content. In order to verify whether the hopping conduction is dominant, we should measure conductivity with a variety of samples with different iron content. Given the constant total iron content, conductivity should increase with increasing oxygen fugacity. Hence, it is also important to measure conductivity as a function of oxygen fugacity in order to verify whether hopping conduction is dominant. The above discussion is applicable for ferromagnesian minerals that contain no ferric iron in their basic composition. In the case of ferromagnesian minerals that contain both ferrous and ferric irons in their basic composition, electrical conduction occurs between the neighboring ferric and ferrous
Fig. 2. A scheme of small polaron around Mg sites. The lattice is locally distorted by excess charge of Fe•Mg, which attract anions and repulse cations.
ions. For example, in the case of magnetite the following reaction occurs: FexMg + FexAl = Fe•Mg + Fe′Al, in which the sites follow those of MgAl2O4 spinel. In this case, the local excess charge is kept zero. Hence the local strains are very small, and as a result, the activation energy is low. For example, the activation energy of hopping conduction of magnetite is about 0.1 eV (Yamanaka et al., 2001). In this case, electrical conductivity is not expected to largely depend on oxygen fugacity. ′′ ) work as Ionic conduction. Vacancies in a Mg site (VMg a charge carrier in the case of ionic conduction of ferromag′′ , Mg and Fe nesian minerals (Fig. 3). In order to transfer VMg ions themselves should move. Hence, the energy barrier for the charge transfer is very high. As a result, the activation energy of ionic conduction is high, usually higher than 2 eV. For this reason, extremely high temperatures are needed for the ionic conduction to be a dominant conduction mechanism. The relation between electrical conductivity and concentra′′ has to be determined in order to verify whether tion of VMg it contributes to the ionic conduction. However, there is no ′′ in ferromagnesian good way to measure concentration of VMg
′′ . Fig. 3. A scheme of ionic conduction by VMg
T. Katsura et al. / Russian Geology and Geophysics 50 (2009) 1139–1145
minerals. Therefore, it is not easy to identify the ionic conduction in ferromagnesian minerals. Proton conduction. The major mantle minerals can accommodate hydrogen in their crystal structure at high pressures even though they are nominally anhydrous. A proton can move in a crystal very rapidly. Hence, if the amount of protons is sufficiently high, ionic conduction by proton as a carrier can be a dominant conduction mechanism. The activation energy of proton diffusion is relatively small. For example, Mackwell and Kohlstedt (1990) reported that the activation enthalpy of the proton conduction in olivine is 1.3 eV. The activation energies of proton conduction in other mantle minerals are also expected to be small. In order to verify whether the proton conduction is dominant, electrical conductivity should be measured as a function of concentration of proton. In the high pressure and temperature experiments, the samples of the mantle minerals always contain some amount of water, even if dry samples are loaded. The water may come from the pressure medium. As a result, the measured conductivity is always contributed by both hopping and proton conductions. Therefore, in order to determine hopping or ionic conduction separately, it is necessary to determine contribution of proton conduction by the above method and subtract the contribution of hopping conduction from the total conductivity.
Electrical conductivity of the major upper mantle minerals Laboratory measurement of electrical conductivity. Although olivine is stable at ambient pressure, wadsleyite and ringwoodite are stable at high pressures. In addition, high-pressures are required for measuring proton conductions for all the minerals because water can be incorporated in minerals under pressure. For these reasons, most of measurements cited below, except for hopping and ionic conduction of olivine, were carried out at high pressures (Manthilake et al., 2009; Yoshino and Katsura, 2009; Yoshino et al., 2006, 2008, 2009). The typical assembly for the high-pressure electrical conductivity measurement is shown in Fig. 4. In such assemblies, a cylindrical sample is sandwiched between two metal disks. The metal disks work as electrodes for conductivity measurement and also as a buffer of oxidation state. One pair of thermocouple wires is inserted to one side of the sample, and one wire is to the other side. A sinusoidal signal is applied to the sample. The impedance of the system, Z, is obtained by complex ratio of the applied voltage to the sample current. The equivalent circuit is considered to be a simple parallel combination of a resistor and capacitor. Based on this assumption, the sample resistance is obtained from the impedance by the following equation: 1 1 = + IωC, Z R where R is the sample resistance, I is the imaginary unit, ω is the angular frequency, and C is the capacitance. The sample
1141
conductivity is calculated from the sample resistance, with the sample geometry taken into account. If the angular frequency is too low, electrochemical reactions will occur in the case of hydrous samples. Therefore, the proper angular frequency has to be chosen. For more sophisticated measurement, impedance spectroscopy is applied. Difficulties of conductivity measurement under high P-T conditions are as follows. (1) In high-pressure experiments, samples are surrounded by some materials. The resistance of surrounding materials is not always high enough for measurement. Both the sample and surrounding materials are quite resistive at ambient temperature. Samples resistance largely decreases on heating. However, resistance of the surrounding materials also decreases. Therefore, considerable leakage of signals to the surrounding material could occur if the resistance contrast between the sample and surrounding material is not sufficient. (2) The chemical environment of samples is difficult to control. As mentioned above, the redox states could largely influence the conductivity. Silica activity may also influence it. In order to control the chemical environments, we have to put buffering materials in the assembly. However, it is not always clear whether the buffering material effectively control the chemical environment in a high pressure cell. (3) The sample is first compressed at room temperature. This procedure causes breakage and fracture of the sample. The breakage of the sample would make bad contacts, which increase nominal resistance of the samples. On the other hand, a fractured sample would have lower resistance because electric current flows through defects in the fracture. In order to overcome these problems, the sample should be annealed at very high temperatures (>1700 °C) before measurements. However, highly water-doped samples cannot be annealed at such high temperatures because of decomposition.
Fig. 4. A scheme of a typical assembly for high-pressure electrical conductivity measurement of mantle minerals. The sinusoidal signals are applied to the sample using one side of the thermocouple and the electrode on the opposite side of the sample. The voltage on the sample is measured using the other thermocouple and the electrode on the opposite side of the sample.
1142
T. Katsura et al. / Russian Geology and Geophysics 50 (2009) 1139–1145
The high P-T electrical conductivity measurements carried out at Institute for Study of the Earth’s Interior (ISEI), Okayama University, took these points into account (Manthilake et al., 2009; Yoshino and Katsura, 2009; Yoshino et al., 2006, 2008, 2009). Although another group studied the electrical conductivity of the major upper mantle minerals at high pressures (Huang et al., 2005; Wang et al., 2006), they failed to separate contributions of hopping and proton conductions, and they could have measured conductivity of interstitial fluid phase rather than mantle minerals (Yoshino et al., 2008). Hence, we mainly review the results at ISEI in the following section as for high P-T measurements. Olivine. Figure 5 is a summary of the hopping and ionic conductions of olivine in the three crystallographic orientations taken from Constable et al. (1992). The original measurements for this figure were conducted in temperature ranges below 1773 K. Hence, the possible ionic conduction was able to be observed only in small intervals of reciprocal temperature. Therefore, the magnitude and temperature dependence of ionic conduction are not reliable. In contrast, those of hopping conduction have high reliability. Hopping conduction of olivine in the [001] direction is about twice as high as those in the other two crystallographic orientations. Constable et al. (1992) considered that the magnitudes of anisotropy are constant irrespective of temperature. They reported that the activation energies of hopping and ionic conductions are 1.60 and 4.25 eV, respectively. Contributions of hopping conduction to conductivity of olivine were shown by determining iron content and oxygen fugacity dependences of conductivity. Omura et al. (1989) measured conductivity of olivine as a function of temperature and iron content at pressures from 3 to 7 GPa. They showed that the absolute values of conductivity monotonically increase with increasing iron content. They also showed that the activation energy decreases with increasing iron content. The decrease in activation energy with increasing carriers should be partly because the hopping distance decreases and partly because the local strains by an extra charge is relatively lowered. Wanamaker and Duba (1993) studied electrical conductivity of olivine as a function of temperature and oxygen partial pressure and found that conductivity increases with oxygen partial pressure. Figure 6 shows proton conduction of olivine against reciprocal temperature with various water contents in the three crystallographic orientations with sums of hopping and ionic conductions. Yoshino et al. (2006, 2009) confirmed contribution of proton conduction by measuring conductivity of olivine samples with different water contents. They observed that electrical conductivity of olivine increases with water content at low temperatures. They also found that activation energies of proton conduction slightly decrease with increasing water content. The activation energies of proton conduction of an olivine aggregate are 0.90 and 0.85 eV at water contents of 0.001 and 0.1 wt.%, respectively. In the following discussion, the activation energy of proton conduction is assumed to decrease as a linear function of a cubic root of water content (Debye and Conwell, 1954). Wang et al. (2006) measured
Fig. 5. Hopping and ionic conduction of olivine with different crystallographic orientations. Solid, dotted and broken lines denote conductions in [100], [010] and [001] directions, respectively. Blue, red and green lines denote hopping conductions, ionic conductions, and their sums, respectively.
conductivity of hydrous olivine, and gave one order of magnitude higher conductivity than Yoshino et al. (2006). Their data are also plotted in Fig. 6. Conductivity anisotropy of proton conduction at relatively low temperature is as follows: [100] > [001] > [010]. The anisotropy is more than one order of magnitude at water content of 0.01 wt.% and temperature of 800 K. In contrast, magnitudes in the activation energies follow the succession: [010] > [001] > [100]. For example, the activation energies are 0.89, 0.75 and 0.93 eV in the [100], [010] and [001] directions, respectively, at 0.01 wt.% water. As a result, although the anisotropy of conductivity is large at low temperatures, it decreases with increasing temperature. At lower pressures, the maximum water solubility in olivine is low. Therefore, proton conduction cannot be a dominant conduction mechanism under pressure-temperature conditions corresponding to the top of the asthenosphere. In the deeper regions, the maximum water solubility becomes high. For
Fig. 6. Proton conduction and sum of hopping and ionic conductions of olivine with different crystallographic orientations. Solid, dotted and broken lines denote conductions in [100], [010] and [001] directions, respectively. Blue and green lines denote hopping conductions at water contents of 0.01 and 0.001 wt.%, respectively. Red lines denote sum of hopping and ionic conductions. Blue color denotes proton conduction at water content of 0.01 wt.% reported by Wang et al. (2006).
T. Katsura et al. / Russian Geology and Geophysics 50 (2009) 1139–1145
1143
example, olivine can contain 0.7 wt.% water at 12–14 GPa (Litasov et al., 2007). In this case, the hopping and proton conductions at 1700 K are 0.01 and 1 S/m, respectively. Thus, proton conduction can be a dominant conduction mechanism in the deep mantle. Wadsleyite is a high-pressure polymorph of olivine, which is considered to be the major constituent mineral in the upper part of the mantle transition zone. The mantle wadsleyite is believed to contain about 10 wt.% fayalite component. Therefore, hopping conduction is expected to be a dominant conduction mechanism, although the iron content and oxygen fugacity dependence of conductivity of wadsleyite have not been studied yet. There is also no study showing contribution of ionic conduction in wadsleyite. Although wadsleyite is a nominally anhydrous mineral, it can contain significant amount of water, which reaches 3.4 wt.% at maximum (Chen et al., 2002; Inoue et al., 1995). Therefore proton conduction should be important for wadsleyite. Manthilake et al. (2009) measured electrical conductivity of polycrystalline wadsleyite with (Mg0.9Fe0.1)2SiO4 composition as a function of temperature and water content, and confirmed contribution of proton conduction. They separated the contributions of hopping and proton conductions, which are shown in Fig. 7. The activation energy of hopping conduction is 1.5 eV. The activation energy of proton conduction of wadsleyite only slightly decreases with increasing water content: from 0.68 to 0.66 eV with increasing water content from 0.01 to 1 wt.%. The magnitude of proton conduction in wadsleyite is smaller than olivine in the same water content at high temperatures. However the maximum water solubility of wadsleyite is higher than that of olivine. If wadsleyite contains the maximum amount of water in it, the hopping conduction will be hidden by proton conduction even at temperatures corresponding to the mantle transition zone. Huang et al. (2005) reported proton conduction of wadsleyite from their conductivity measurement of hydrous wadsleyite. Hae et al. (2006) estimated proton conduction of wadsleyite from proton diffusion data using the Nernst–Ein-
stein relation. Figure 8 compares the proton conductions of wadsleyite given by Huang et al. (2005), Hae et al. (2006) and Manthilake et al. (2009). The proton conduction estimated from the proton diffusion (Hae et al., 2006) gave much larger temperature dependence than those obtained by the actual conductivity measurements. Hae et al. (2006) adopted the Nernst–Einstein relation, and therefore, their conductivity is proportional to the water content. On the other hand, Manthilake et al. (2009) gave larger and Huang et al. (2005) gave smaller water content dependence than that expected from the Nernst-Einstein relation. The reason for the larger water content dependence by Manthilake et al. (2009) is that the activation energy decreases with increasing water content. Ringwoodite is another high-pressure polymorph of olivine. It is considered a major constituent of the lower part of the mantle transition zone. As in the case for olivine and wadsleyite, the hopping conduction is expected to be a dominant conduction mechanism for ringwoodite. Yoshino and Katsura (2009) measured total iron content dependence of conductivity of ringwoodite to confirm contribution of hopping conduction. Like wadsleyite, ringwoodite can contain significant amount of water, which can reach 2.8 wt.% (Yusa et al., 2000). Therefore proton conduction should also be important in ringwoodite as well as wadsleyite. Yoshino et al. (2008) measured electrical conductivity of ringwoodite as a function of temperature and water content, which confirmed contribution of proton conduction. They separated the contributions of hopping and proton conductions, which are shown in Fig. 9. The activation energy of hopping conduction is 1.4 eV. In contrast to wadsleyite, the activation energy of proton conduction largely decreases from 0.98 to 0.45 eV with increasing water content from 0.01 to 1 wt.%. As a result, for example at 1700 K, the contribution of proton conduction is negligible at water content below 0.1 wt.%, but it is much larger than that of wadsleyite at water content above 0.5 wt.%. Huang et al. (2005) reported proton conduction of ringwoodite, which is also shown in Fig. 9. Obviously, the
Fig. 7. Hopping and proton conductions of wadsleyite. Red line denotes hopping conduction. Blue lines denote proton conductions at water contents of 1, 0.1, 0.01, and 0.001 wt.%. Green lines denote sums of hopping and proton conductions.
Fig. 8. Comparison of proton conductions reported by three groups. Red: conductivity measurement by Manthilake et al. (2009); blue: conductivity measurement by Huang et al. (2005), green: estimation from proton diffusion by Hae et al. (2006). Solid and broken lines denote proton conduction at water contents of 1 and 0.01 %.
1144
T. Katsura et al. / Russian Geology and Geophysics 50 (2009) 1139–1145
Fig. 9. Hopping and proton conductions of ringwoodite. Red line denotes hopping conduction. Blue lines denote proton conductions at water contents of 1, 0.1, and 0.01 wt.%. Green lines denote sums of hopping and proton conductions.
Fig. 10. Comparison of hopping conductions of olivine (red), wadsleyite (green), and ringwoodite (blue) at Mg/Mg + Fe ratio of 0.9.
dependence of conductivity on their water content is much weaker than the dependence reported by Yoshino et al. (2008). Comparison of the three phases. Figure 10 shows comparison of hopping conductions of olivine, wadsleyite and ringwoodite. The hopping conduction increases in the succession: olivine, wadsleyite, and ringwoodite. The activation energies of hopping conduction of these phases are similar, but slightly decrease in the succession: 1.6, 1.5, and 1.4 eV, respectively. At the temperatures of the mantle, conductivity increases by 0.5 and 0.6 log units associated with the olivine-wadsleyite and wadsleyite-ringwoodite transitions, respectively. Figure 11 compares proton conductions of olivine, wadsleyite and ringwoodite. The magnitude of proton conduction of olivine at the mantle temperatures is the largest among these three minerals at the same water content. That of ringwoodite is the smallest at low water content. At water content higher than 0.1 wt.%, the proton conduction of ringwoodite becomes larger than that of wadsleyite. The maximum water solubility in olivine is smallest in these minerals, suggesting that hydrogen in olivine is the least bound in the crystal structure
among these minerals. Hence hydrogen in olivine is relatively mobile, which should cause the highest proton conduction. Wadsleyite has only one distinct peak for OH bonding in FT-IR spectra (3330 cm–1). On the other hand, the peak of OH bonding of ringwoodite is very broad at high water contents (Yoshino et al., 2008, supplementary information). The peak broadening may be related to possible high mobility of proton. In the case of water content of 0.1 wt.%, the proton conduction decreases by 0.4 log unit associated with the olivine-wadsleyite transition. It does not change significantly in the wadsleyite-ringwoodite transition. At water content of 1 wt.%, the conductivity increases by 1.2 log units associated with the wadsleyite-ringwoodite transition.
Fig. 11. Comparison of proton conduction of olivine (red), wadsleyite (green), and ringwoodite (blue). The lines labeled by “2”, “3”, “4”, and “5” denote proton conduction of each phase at water contents of 1, 0.1, 0.01, and 0.001 wt.%, respectively.
Conclusions Electrical conductivity of the mantle minerals are very sensitive to temperature. With increasing temperature, conductivity increases by orders of magnitude. Therefore, conductivity is a useful tool to estimate temperature in the mantle. It is also sensitive to the water content in the mantle. Hydration largely changes the mechanical properties of minerals. Electrical conductivity can be used for estimation of degree of hydration of the mantle material, and therefore important for discussing the mantle dynamics. Acknowledgements. Part of this paper was presented at the international symposium “Lithosphere Petrology and Origin of Diamond” held in Novosibirsk in June, 2008. We thank Yu. Paly’anov and other conveners for giving us an opportunity to psrticipate in the symposium and inviting us to write this paper for this special volume. This work was supported by Grant-in-Aids for Scientific Research, No. 13440164 and 18740280 to TK and TY, respectively, from the Japan Society for Promotion of Science. It was also supported by the COE-21 program of the Institute for Study of the Earth’s Interior, Okayama University.
T. Katsura et al. / Russian Geology and Geophysics 50 (2009) 1139–1145
References Chen, J., Inoue, T., Yurimoto, T., Weidner, D.J., 2002. Effect of water on olivine-wadsleyite phase boundary in the (Mg,Fe)2SiO4 system. Geophys. Res. Lett. 29, doi: 10.1029/2001GL014429. Chiang, Y-M., Birnie, III, D., Kingery, W., 1997. Physical Ceramics. John Wiley & Sons, New York. Constable, S., Shankland, T.J., Duba, A., 1992. The electrical conductivity of an isotropic olivine mantle. J. Geophys. Res. B 97, 3397–3404. Debye, P.P., Conwell, E.M., 1954. Electrical properties of N-type germanium. Phys. Rev. 93, 693–706, 1954. Egbert, G.D., 2007. EM modeling, inverse, in: Gubbins, D., Herrero-Bervera (Eds.), Encyclopedia of Geomagnetism and Paleomagnetism. Springer, Berlin, Heidelberg, New York, pp. 219–223. Hae, R., Ohtani, E., Kubo, T., Koyama, T., Utada, H., 2006. Hydrogen diffusivity in wadsleyite and water distribution in the mantle transition zone. Earth Planet. Sci. Lett. 243, 141–148. Huang, X.G., Xu, Y.S., Karato, S.I., 2005. Water content in the transition zone from electrical conductivity of wadsleyite and ringwoodite. Nature, 434 (7034), 746–749. Inoue, T., Yurimoto, H., Kudoh, Y., 1995. Hydrous modified spinel, Mg1.75SiH0.5O4: a new water reservoir in the mantle transition region. Geophys. Res. Lett. 22, 117–120. Litasov, KD, Ohtani, E., Kagi, H., Jacobsen, S.D., Ghosh, S., 2007. Temperature dependence and mechanism of hydrogen incorporation in olivine at 12.5–14.0 GPa. Geophys. Res. Lett. 34, L16314. Lowes, F., 2007. Geomagnetic deep soundings, in: Gubbins, D., Herrero-Bervera (Eds.), Encyclopedia of Geomagnetism and Paleomagnetism. Springer, Berlin, Heidelberg, New York, pp. 307–311.
1145
Mackwell, S.J., Kohlstedt, D.L., 1990. Diffusion of hydrogen in olivine: implications for water in the mantle. J. Geophys. Res. 95, 5079–5088. Manthilake, M.A.G.M., Tatsuzaki, M., Yoshino, T., Yamashita, S., Ito, E., Katsura, T., 2009. Electrical conductivity of wadsleyite as a function of temperature and water content. Phys. Earth Planet. Inter. 174, 10–18. Omura, K., Kurita, K., Kumazawa, M., 1989. Experimental study of pressure dependence of electrical conductivity of olivine at high temperatures. Phys. Earth Planet. Inter. 57, 291–303. Wanamaker, B.J., Duba, A.G., 1993. Electrical conductivity of San Carlos olivine along [100] under oxygen- and pyroxene-buffered conditions and implications for defect equilibria, J. Geophys. Res. 98, 489–500. Wang, D.J., Mookherjee, M., Xu, Y.S., Karato, S., 2006. The effect of water on the electrical conductivity of olivine. Nature, 443 (7114), 977–980. Yamanaka, T., Shimazu, H., Ota, K., 2001. Electric conductivity of Fe2SiO4Fe3O4 spinel solid solutions. Phys. Chem. Miner. 28, 110–118, 2001. Yoshino, T., Katsura, T., 2009. Effect of iron content on electrical conductivity of ringwoodite, with implications for electrical structure in the transition zone. Phys. Earth Planet. Inter. 174, 3–9. Yoshino, T., Matsuzaki, T., Yamashita, S., Katsura, T., 2006. Hydrous olivine unable to account for conductivity anomaly at the top of the asthenosphere. Nature 443, 973–976. Yoshino, T., Manthilake, G., Matsuzaki, T., Katsura, T., 2008. Dry mantle transition zone inferred from the electrical conductivity of wadsleyite and ringwoodite. Nature 451, 326–329. Yoshino, T., Matsuzaki, T., Shatskiy, A., Katsura, T., 2009. Effect of water on electrical conductivity of olivine aggregates with implications for electrical structure in the upper mantle. Earth Planet. Sci. Lett. (in press). Yusa, H., Inoue, T., Ohishi, Y., 2000. Isothermal compressibility of hydrous ringwoodite and its relation to the mantle discontinuities. Geophys. Res. Lett. 27, 413–416, 2000.
Electrical conductivity of the major upper mantle minerals: a review T. Katsura *, T. Yoshino, G. Manthilake, T. Matsuzaki Institute for Study of the Earth’s Interior, Okayama University, Misasa, 682-0193, Japan Received 10 October 2008
Abstract The electrical conductivity of the major upper mantle minerals, namely, olivine, wadsleyite and ringwoodite, is reviewed in this paper. There are mainly three electrical conduction mechanisms for three upper mantle minerals: hopping, ionic and proton conductions. The charge carriers for these conduction mechanisms are an electron hole in Fe ion, a vacancy in Mg site, and a proton, respectively. Hopping conduction is the most essential conduction mechanism for the major upper mantle minerals. Because ionic conduction has high activation energy, it becomes a dominant conduction mechanism only at high temperatures. Proton conduction contributes at relatively low temperatures. If the mantle minerals contain large amount of water (more than 0.1 wt.%), proton conduction can be a dominant conduction mechanism, even at high temperatures. © 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: electrical conductivity; upper mantle; transition layer; olivine; wadsleyite; ringwoodite
Introduction
Electrical conductivity mechanisms
The electrical conductivity profiles in the upper mantle are obtained by means of magnetotellurics (Egbert, 2007) and geomagnetic deep soundings (Lowes, 2007) using the electromagnetic observations. Information about the structure and dynamics of the upper mantle can be obtained by comparing the geophysically obtained electrical conductivity profiles with electrical properties of minerals experimentally determined. Although seismology gives us a variety of reliable information about the structure and dynamics of the Earth’s interior, information obtained from electrical conductivity is independent of that obtained from seismology. Therefore, this kind of information is very valuable to understand the Earth’s interior. For example, as mentioned below, conductivity is sensitive to the presence of water. Therefore, we could obtain information about distribution of water in the mantle from electrical conductivity profiles with knowledge of electrical properites of the mantle minerals. In this paper, we review electrical properties of the major upper mantle minerals, namely, olivine, wadsleyite, and ringwoodite.
In metal, electrical conduction takes place by free electrons. The mantle minerals are, however, essentially ionic crystalline materials, in which almost all electrons are bound to ions. Hence, particles other than free electrons play an important role in electrical conduction of mantle minerals. The major mantle minerals are ferromagnesian silicates, in which the most important charge carrier is an electron hole in an Fe ion. In addition to this, atomic vacancy in a Mg site and proton (H+) also act as a charge carrier. In this paper, the electric conduction mechanisms by these carriers are called as hopping, ionic and proton conductions, respectively. We briefly explain them below. The three conduction mechanisms mentioned above are thermally activated processes, in which conductivity increases with increasing temperature. These conductivity mechanisms are frequently expressed using the Arrhenius formula:
* Corresponding author. E-mail address: [email protected] (T. Katsura)
∆E σ = σ0 exp − , kT where ∆E is activation energy, k is the Boltzmann constant and T is absolute temperature. The σ0 is a constant called pre-exponential term. The σ0 and ∆E could have a special formula according to the conduction mechanism.
1068-7971/$ - see front matter D 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2009.11.012
1140
T. Katsura et al. / Russian Geology and Geophysics 50 (2009) 1139–1145
Fig. 1. A scheme of hopping conduction of Fe•Mg in ferromagnesian silicate. OxO , charge-free oxygen vacancy; e′, electron; h•, hole.
Hopping conduction. In ferromagnesian silicates, an Fe ion usually substitutes for a Mg ion in a Mg site. Hence it is basically a ferrous ion, which is expressed as FexMg in the Kroeger-Vink expression (for this expression, see Chiang et al., 1997). However, according to oxygen fugacity, temperature and pressure, there should be a certain proportion of ferric ion, Fe•Mg, which has an electron hole. Transfer of an electron hole from Fe•Mg to FexMg carries an electric charge (Fig. 1). This is hopping conduction. If only electron holes migrate, the energy barriers for the migration are expected to be relatively low. However, the presence of an electron hole should have a significant effect on the local structure of the ionic crystal, which will impede migration of electric charge. If Fe•Mg exists, there is an extra positive charge, which repulses cations and attracts anions (Fig. 2). This complex of the local strains is called “small polaron”. The migration of electron holes associates small polaron. A relatively large energy is needed for migration of a small polaron in comparison with that of an electron hole. As a result, the hopping conduction of usual ferromagnesian silicates has relatively large activation energy of around 1 eV. As it is explained above, hopping conduction of mantle minerals is due to transport of Fe•Mg. Hence, hopping conduction increases with increasing Fe•Mg. Given the oxygen fugacity in the system is constant, hopping conduction increases with increasing total iron content. In order to verify whether the hopping conduction is dominant, we should measure conductivity with a variety of samples with different iron content. Given the constant total iron content, conductivity should increase with increasing oxygen fugacity. Hence, it is also important to measure conductivity as a function of oxygen fugacity in order to verify whether hopping conduction is dominant. The above discussion is applicable for ferromagnesian minerals that contain no ferric iron in their basic composition. In the case of ferromagnesian minerals that contain both ferrous and ferric irons in their basic composition, electrical conduction occurs between the neighboring ferric and ferrous
Fig. 2. A scheme of small polaron around Mg sites. The lattice is locally distorted by excess charge of Fe•Mg, which attract anions and repulse cations.
ions. For example, in the case of magnetite the following reaction occurs: FexMg + FexAl = Fe•Mg + Fe′Al, in which the sites follow those of MgAl2O4 spinel. In this case, the local excess charge is kept zero. Hence the local strains are very small, and as a result, the activation energy is low. For example, the activation energy of hopping conduction of magnetite is about 0.1 eV (Yamanaka et al., 2001). In this case, electrical conductivity is not expected to largely depend on oxygen fugacity. ′′ ) work as Ionic conduction. Vacancies in a Mg site (VMg a charge carrier in the case of ionic conduction of ferromag′′ , Mg and Fe nesian minerals (Fig. 3). In order to transfer VMg ions themselves should move. Hence, the energy barrier for the charge transfer is very high. As a result, the activation energy of ionic conduction is high, usually higher than 2 eV. For this reason, extremely high temperatures are needed for the ionic conduction to be a dominant conduction mechanism. The relation between electrical conductivity and concentra′′ has to be determined in order to verify whether tion of VMg it contributes to the ionic conduction. However, there is no ′′ in ferromagnesian good way to measure concentration of VMg
′′ . Fig. 3. A scheme of ionic conduction by VMg
T. Katsura et al. / Russian Geology and Geophysics 50 (2009) 1139–1145
minerals. Therefore, it is not easy to identify the ionic conduction in ferromagnesian minerals. Proton conduction. The major mantle minerals can accommodate hydrogen in their crystal structure at high pressures even though they are nominally anhydrous. A proton can move in a crystal very rapidly. Hence, if the amount of protons is sufficiently high, ionic conduction by proton as a carrier can be a dominant conduction mechanism. The activation energy of proton diffusion is relatively small. For example, Mackwell and Kohlstedt (1990) reported that the activation enthalpy of the proton conduction in olivine is 1.3 eV. The activation energies of proton conduction in other mantle minerals are also expected to be small. In order to verify whether the proton conduction is dominant, electrical conductivity should be measured as a function of concentration of proton. In the high pressure and temperature experiments, the samples of the mantle minerals always contain some amount of water, even if dry samples are loaded. The water may come from the pressure medium. As a result, the measured conductivity is always contributed by both hopping and proton conductions. Therefore, in order to determine hopping or ionic conduction separately, it is necessary to determine contribution of proton conduction by the above method and subtract the contribution of hopping conduction from the total conductivity.
Electrical conductivity of the major upper mantle minerals Laboratory measurement of electrical conductivity. Although olivine is stable at ambient pressure, wadsleyite and ringwoodite are stable at high pressures. In addition, high-pressures are required for measuring proton conductions for all the minerals because water can be incorporated in minerals under pressure. For these reasons, most of measurements cited below, except for hopping and ionic conduction of olivine, were carried out at high pressures (Manthilake et al., 2009; Yoshino and Katsura, 2009; Yoshino et al., 2006, 2008, 2009). The typical assembly for the high-pressure electrical conductivity measurement is shown in Fig. 4. In such assemblies, a cylindrical sample is sandwiched between two metal disks. The metal disks work as electrodes for conductivity measurement and also as a buffer of oxidation state. One pair of thermocouple wires is inserted to one side of the sample, and one wire is to the other side. A sinusoidal signal is applied to the sample. The impedance of the system, Z, is obtained by complex ratio of the applied voltage to the sample current. The equivalent circuit is considered to be a simple parallel combination of a resistor and capacitor. Based on this assumption, the sample resistance is obtained from the impedance by the following equation: 1 1 = + IωC, Z R where R is the sample resistance, I is the imaginary unit, ω is the angular frequency, and C is the capacitance. The sample
1141
conductivity is calculated from the sample resistance, with the sample geometry taken into account. If the angular frequency is too low, electrochemical reactions will occur in the case of hydrous samples. Therefore, the proper angular frequency has to be chosen. For more sophisticated measurement, impedance spectroscopy is applied. Difficulties of conductivity measurement under high P-T conditions are as follows. (1) In high-pressure experiments, samples are surrounded by some materials. The resistance of surrounding materials is not always high enough for measurement. Both the sample and surrounding materials are quite resistive at ambient temperature. Samples resistance largely decreases on heating. However, resistance of the surrounding materials also decreases. Therefore, considerable leakage of signals to the surrounding material could occur if the resistance contrast between the sample and surrounding material is not sufficient. (2) The chemical environment of samples is difficult to control. As mentioned above, the redox states could largely influence the conductivity. Silica activity may also influence it. In order to control the chemical environments, we have to put buffering materials in the assembly. However, it is not always clear whether the buffering material effectively control the chemical environment in a high pressure cell. (3) The sample is first compressed at room temperature. This procedure causes breakage and fracture of the sample. The breakage of the sample would make bad contacts, which increase nominal resistance of the samples. On the other hand, a fractured sample would have lower resistance because electric current flows through defects in the fracture. In order to overcome these problems, the sample should be annealed at very high temperatures (>1700 °C) before measurements. However, highly water-doped samples cannot be annealed at such high temperatures because of decomposition.
Fig. 4. A scheme of a typical assembly for high-pressure electrical conductivity measurement of mantle minerals. The sinusoidal signals are applied to the sample using one side of the thermocouple and the electrode on the opposite side of the sample. The voltage on the sample is measured using the other thermocouple and the electrode on the opposite side of the sample.
1142
T. Katsura et al. / Russian Geology and Geophysics 50 (2009) 1139–1145
The high P-T electrical conductivity measurements carried out at Institute for Study of the Earth’s Interior (ISEI), Okayama University, took these points into account (Manthilake et al., 2009; Yoshino and Katsura, 2009; Yoshino et al., 2006, 2008, 2009). Although another group studied the electrical conductivity of the major upper mantle minerals at high pressures (Huang et al., 2005; Wang et al., 2006), they failed to separate contributions of hopping and proton conductions, and they could have measured conductivity of interstitial fluid phase rather than mantle minerals (Yoshino et al., 2008). Hence, we mainly review the results at ISEI in the following section as for high P-T measurements. Olivine. Figure 5 is a summary of the hopping and ionic conductions of olivine in the three crystallographic orientations taken from Constable et al. (1992). The original measurements for this figure were conducted in temperature ranges below 1773 K. Hence, the possible ionic conduction was able to be observed only in small intervals of reciprocal temperature. Therefore, the magnitude and temperature dependence of ionic conduction are not reliable. In contrast, those of hopping conduction have high reliability. Hopping conduction of olivine in the [001] direction is about twice as high as those in the other two crystallographic orientations. Constable et al. (1992) considered that the magnitudes of anisotropy are constant irrespective of temperature. They reported that the activation energies of hopping and ionic conductions are 1.60 and 4.25 eV, respectively. Contributions of hopping conduction to conductivity of olivine were shown by determining iron content and oxygen fugacity dependences of conductivity. Omura et al. (1989) measured conductivity of olivine as a function of temperature and iron content at pressures from 3 to 7 GPa. They showed that the absolute values of conductivity monotonically increase with increasing iron content. They also showed that the activation energy decreases with increasing iron content. The decrease in activation energy with increasing carriers should be partly because the hopping distance decreases and partly because the local strains by an extra charge is relatively lowered. Wanamaker and Duba (1993) studied electrical conductivity of olivine as a function of temperature and oxygen partial pressure and found that conductivity increases with oxygen partial pressure. Figure 6 shows proton conduction of olivine against reciprocal temperature with various water contents in the three crystallographic orientations with sums of hopping and ionic conductions. Yoshino et al. (2006, 2009) confirmed contribution of proton conduction by measuring conductivity of olivine samples with different water contents. They observed that electrical conductivity of olivine increases with water content at low temperatures. They also found that activation energies of proton conduction slightly decrease with increasing water content. The activation energies of proton conduction of an olivine aggregate are 0.90 and 0.85 eV at water contents of 0.001 and 0.1 wt.%, respectively. In the following discussion, the activation energy of proton conduction is assumed to decrease as a linear function of a cubic root of water content (Debye and Conwell, 1954). Wang et al. (2006) measured
Fig. 5. Hopping and ionic conduction of olivine with different crystallographic orientations. Solid, dotted and broken lines denote conductions in [100], [010] and [001] directions, respectively. Blue, red and green lines denote hopping conductions, ionic conductions, and their sums, respectively.
conductivity of hydrous olivine, and gave one order of magnitude higher conductivity than Yoshino et al. (2006). Their data are also plotted in Fig. 6. Conductivity anisotropy of proton conduction at relatively low temperature is as follows: [100] > [001] > [010]. The anisotropy is more than one order of magnitude at water content of 0.01 wt.% and temperature of 800 K. In contrast, magnitudes in the activation energies follow the succession: [010] > [001] > [100]. For example, the activation energies are 0.89, 0.75 and 0.93 eV in the [100], [010] and [001] directions, respectively, at 0.01 wt.% water. As a result, although the anisotropy of conductivity is large at low temperatures, it decreases with increasing temperature. At lower pressures, the maximum water solubility in olivine is low. Therefore, proton conduction cannot be a dominant conduction mechanism under pressure-temperature conditions corresponding to the top of the asthenosphere. In the deeper regions, the maximum water solubility becomes high. For
Fig. 6. Proton conduction and sum of hopping and ionic conductions of olivine with different crystallographic orientations. Solid, dotted and broken lines denote conductions in [100], [010] and [001] directions, respectively. Blue and green lines denote hopping conductions at water contents of 0.01 and 0.001 wt.%, respectively. Red lines denote sum of hopping and ionic conductions. Blue color denotes proton conduction at water content of 0.01 wt.% reported by Wang et al. (2006).
T. Katsura et al. / Russian Geology and Geophysics 50 (2009) 1139–1145
1143
example, olivine can contain 0.7 wt.% water at 12–14 GPa (Litasov et al., 2007). In this case, the hopping and proton conductions at 1700 K are 0.01 and 1 S/m, respectively. Thus, proton conduction can be a dominant conduction mechanism in the deep mantle. Wadsleyite is a high-pressure polymorph of olivine, which is considered to be the major constituent mineral in the upper part of the mantle transition zone. The mantle wadsleyite is believed to contain about 10 wt.% fayalite component. Therefore, hopping conduction is expected to be a dominant conduction mechanism, although the iron content and oxygen fugacity dependence of conductivity of wadsleyite have not been studied yet. There is also no study showing contribution of ionic conduction in wadsleyite. Although wadsleyite is a nominally anhydrous mineral, it can contain significant amount of water, which reaches 3.4 wt.% at maximum (Chen et al., 2002; Inoue et al., 1995). Therefore proton conduction should be important for wadsleyite. Manthilake et al. (2009) measured electrical conductivity of polycrystalline wadsleyite with (Mg0.9Fe0.1)2SiO4 composition as a function of temperature and water content, and confirmed contribution of proton conduction. They separated the contributions of hopping and proton conductions, which are shown in Fig. 7. The activation energy of hopping conduction is 1.5 eV. The activation energy of proton conduction of wadsleyite only slightly decreases with increasing water content: from 0.68 to 0.66 eV with increasing water content from 0.01 to 1 wt.%. The magnitude of proton conduction in wadsleyite is smaller than olivine in the same water content at high temperatures. However the maximum water solubility of wadsleyite is higher than that of olivine. If wadsleyite contains the maximum amount of water in it, the hopping conduction will be hidden by proton conduction even at temperatures corresponding to the mantle transition zone. Huang et al. (2005) reported proton conduction of wadsleyite from their conductivity measurement of hydrous wadsleyite. Hae et al. (2006) estimated proton conduction of wadsleyite from proton diffusion data using the Nernst–Ein-
stein relation. Figure 8 compares the proton conductions of wadsleyite given by Huang et al. (2005), Hae et al. (2006) and Manthilake et al. (2009). The proton conduction estimated from the proton diffusion (Hae et al., 2006) gave much larger temperature dependence than those obtained by the actual conductivity measurements. Hae et al. (2006) adopted the Nernst–Einstein relation, and therefore, their conductivity is proportional to the water content. On the other hand, Manthilake et al. (2009) gave larger and Huang et al. (2005) gave smaller water content dependence than that expected from the Nernst-Einstein relation. The reason for the larger water content dependence by Manthilake et al. (2009) is that the activation energy decreases with increasing water content. Ringwoodite is another high-pressure polymorph of olivine. It is considered a major constituent of the lower part of the mantle transition zone. As in the case for olivine and wadsleyite, the hopping conduction is expected to be a dominant conduction mechanism for ringwoodite. Yoshino and Katsura (2009) measured total iron content dependence of conductivity of ringwoodite to confirm contribution of hopping conduction. Like wadsleyite, ringwoodite can contain significant amount of water, which can reach 2.8 wt.% (Yusa et al., 2000). Therefore proton conduction should also be important in ringwoodite as well as wadsleyite. Yoshino et al. (2008) measured electrical conductivity of ringwoodite as a function of temperature and water content, which confirmed contribution of proton conduction. They separated the contributions of hopping and proton conductions, which are shown in Fig. 9. The activation energy of hopping conduction is 1.4 eV. In contrast to wadsleyite, the activation energy of proton conduction largely decreases from 0.98 to 0.45 eV with increasing water content from 0.01 to 1 wt.%. As a result, for example at 1700 K, the contribution of proton conduction is negligible at water content below 0.1 wt.%, but it is much larger than that of wadsleyite at water content above 0.5 wt.%. Huang et al. (2005) reported proton conduction of ringwoodite, which is also shown in Fig. 9. Obviously, the
Fig. 7. Hopping and proton conductions of wadsleyite. Red line denotes hopping conduction. Blue lines denote proton conductions at water contents of 1, 0.1, 0.01, and 0.001 wt.%. Green lines denote sums of hopping and proton conductions.
Fig. 8. Comparison of proton conductions reported by three groups. Red: conductivity measurement by Manthilake et al. (2009); blue: conductivity measurement by Huang et al. (2005), green: estimation from proton diffusion by Hae et al. (2006). Solid and broken lines denote proton conduction at water contents of 1 and 0.01 %.
1144
T. Katsura et al. / Russian Geology and Geophysics 50 (2009) 1139–1145
Fig. 9. Hopping and proton conductions of ringwoodite. Red line denotes hopping conduction. Blue lines denote proton conductions at water contents of 1, 0.1, and 0.01 wt.%. Green lines denote sums of hopping and proton conductions.
Fig. 10. Comparison of hopping conductions of olivine (red), wadsleyite (green), and ringwoodite (blue) at Mg/Mg + Fe ratio of 0.9.
dependence of conductivity on their water content is much weaker than the dependence reported by Yoshino et al. (2008). Comparison of the three phases. Figure 10 shows comparison of hopping conductions of olivine, wadsleyite and ringwoodite. The hopping conduction increases in the succession: olivine, wadsleyite, and ringwoodite. The activation energies of hopping conduction of these phases are similar, but slightly decrease in the succession: 1.6, 1.5, and 1.4 eV, respectively. At the temperatures of the mantle, conductivity increases by 0.5 and 0.6 log units associated with the olivine-wadsleyite and wadsleyite-ringwoodite transitions, respectively. Figure 11 compares proton conductions of olivine, wadsleyite and ringwoodite. The magnitude of proton conduction of olivine at the mantle temperatures is the largest among these three minerals at the same water content. That of ringwoodite is the smallest at low water content. At water content higher than 0.1 wt.%, the proton conduction of ringwoodite becomes larger than that of wadsleyite. The maximum water solubility in olivine is smallest in these minerals, suggesting that hydrogen in olivine is the least bound in the crystal structure
among these minerals. Hence hydrogen in olivine is relatively mobile, which should cause the highest proton conduction. Wadsleyite has only one distinct peak for OH bonding in FT-IR spectra (3330 cm–1). On the other hand, the peak of OH bonding of ringwoodite is very broad at high water contents (Yoshino et al., 2008, supplementary information). The peak broadening may be related to possible high mobility of proton. In the case of water content of 0.1 wt.%, the proton conduction decreases by 0.4 log unit associated with the olivine-wadsleyite transition. It does not change significantly in the wadsleyite-ringwoodite transition. At water content of 1 wt.%, the conductivity increases by 1.2 log units associated with the wadsleyite-ringwoodite transition.
Fig. 11. Comparison of proton conduction of olivine (red), wadsleyite (green), and ringwoodite (blue). The lines labeled by “2”, “3”, “4”, and “5” denote proton conduction of each phase at water contents of 1, 0.1, 0.01, and 0.001 wt.%, respectively.
Conclusions Electrical conductivity of the mantle minerals are very sensitive to temperature. With increasing temperature, conductivity increases by orders of magnitude. Therefore, conductivity is a useful tool to estimate temperature in the mantle. It is also sensitive to the water content in the mantle. Hydration largely changes the mechanical properties of minerals. Electrical conductivity can be used for estimation of degree of hydration of the mantle material, and therefore important for discussing the mantle dynamics. Acknowledgements. Part of this paper was presented at the international symposium “Lithosphere Petrology and Origin of Diamond” held in Novosibirsk in June, 2008. We thank Yu. Paly’anov and other conveners for giving us an opportunity to psrticipate in the symposium and inviting us to write this paper for this special volume. This work was supported by Grant-in-Aids for Scientific Research, No. 13440164 and 18740280 to TK and TY, respectively, from the Japan Society for Promotion of Science. It was also supported by the COE-21 program of the Institute for Study of the Earth’s Interior, Okayama University.
T. Katsura et al. / Russian Geology and Geophysics 50 (2009) 1139–1145
References Chen, J., Inoue, T., Yurimoto, T., Weidner, D.J., 2002. Effect of water on olivine-wadsleyite phase boundary in the (Mg,Fe)2SiO4 system. Geophys. Res. Lett. 29, doi: 10.1029/2001GL014429. Chiang, Y-M., Birnie, III, D., Kingery, W., 1997. Physical Ceramics. John Wiley & Sons, New York. Constable, S., Shankland, T.J., Duba, A., 1992. The electrical conductivity of an isotropic olivine mantle. J. Geophys. Res. B 97, 3397–3404. Debye, P.P., Conwell, E.M., 1954. Electrical properties of N-type germanium. Phys. Rev. 93, 693–706, 1954. Egbert, G.D., 2007. EM modeling, inverse, in: Gubbins, D., Herrero-Bervera (Eds.), Encyclopedia of Geomagnetism and Paleomagnetism. Springer, Berlin, Heidelberg, New York, pp. 219–223. Hae, R., Ohtani, E., Kubo, T., Koyama, T., Utada, H., 2006. Hydrogen diffusivity in wadsleyite and water distribution in the mantle transition zone. Earth Planet. Sci. Lett. 243, 141–148. Huang, X.G., Xu, Y.S., Karato, S.I., 2005. Water content in the transition zone from electrical conductivity of wadsleyite and ringwoodite. Nature, 434 (7034), 746–749. Inoue, T., Yurimoto, H., Kudoh, Y., 1995. Hydrous modified spinel, Mg1.75SiH0.5O4: a new water reservoir in the mantle transition region. Geophys. Res. Lett. 22, 117–120. Litasov, KD, Ohtani, E., Kagi, H., Jacobsen, S.D., Ghosh, S., 2007. Temperature dependence and mechanism of hydrogen incorporation in olivine at 12.5–14.0 GPa. Geophys. Res. Lett. 34, L16314. Lowes, F., 2007. Geomagnetic deep soundings, in: Gubbins, D., Herrero-Bervera (Eds.), Encyclopedia of Geomagnetism and Paleomagnetism. Springer, Berlin, Heidelberg, New York, pp. 307–311.
1145
Mackwell, S.J., Kohlstedt, D.L., 1990. Diffusion of hydrogen in olivine: implications for water in the mantle. J. Geophys. Res. 95, 5079–5088. Manthilake, M.A.G.M., Tatsuzaki, M., Yoshino, T., Yamashita, S., Ito, E., Katsura, T., 2009. Electrical conductivity of wadsleyite as a function of temperature and water content. Phys. Earth Planet. Inter. 174, 10–18. Omura, K., Kurita, K., Kumazawa, M., 1989. Experimental study of pressure dependence of electrical conductivity of olivine at high temperatures. Phys. Earth Planet. Inter. 57, 291–303. Wanamaker, B.J., Duba, A.G., 1993. Electrical conductivity of San Carlos olivine along [100] under oxygen- and pyroxene-buffered conditions and implications for defect equilibria, J. Geophys. Res. 98, 489–500. Wang, D.J., Mookherjee, M., Xu, Y.S., Karato, S., 2006. The effect of water on the electrical conductivity of olivine. Nature, 443 (7114), 977–980. Yamanaka, T., Shimazu, H., Ota, K., 2001. Electric conductivity of Fe2SiO4Fe3O4 spinel solid solutions. Phys. Chem. Miner. 28, 110–118, 2001. Yoshino, T., Katsura, T., 2009. Effect of iron content on electrical conductivity of ringwoodite, with implications for electrical structure in the transition zone. Phys. Earth Planet. Inter. 174, 3–9. Yoshino, T., Matsuzaki, T., Yamashita, S., Katsura, T., 2006. Hydrous olivine unable to account for conductivity anomaly at the top of the asthenosphere. Nature 443, 973–976. Yoshino, T., Manthilake, G., Matsuzaki, T., Katsura, T., 2008. Dry mantle transition zone inferred from the electrical conductivity of wadsleyite and ringwoodite. Nature 451, 326–329. Yoshino, T., Matsuzaki, T., Shatskiy, A., Katsura, T., 2009. Effect of water on electrical conductivity of olivine aggregates with implications for electrical structure in the upper mantle. Earth Planet. Sci. Lett. (in press). Yusa, H., Inoue, T., Ohishi, Y., 2000. Isothermal compressibility of hydrous ringwoodite and its relation to the mantle discontinuities. Geophys. Res. Lett. 27, 413–416, 2000.