5.1 A three dimensional global electromagnetic response function Authors: Ikuko Fujii[1][2] and Adam Schultz[2] [1] Earthquake Research Institute, Univ. of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, JAPAN e-mail : fujii@utada-sun.eri.u-tokyo.ac.jp [2] Institute of Theoretical Geophysics, Department of Earth Sciences, Univ. of Cambridge, Downing St., Cambridge, CB2 3EQ, UK e-mail : adam@esc.cam.ac.uk We report on the robust estimation of a three dimensional global electromagnetic response function by using hourly values of the geomagnetic fields at 225 ground-based observatories from 1957 to 1995. We present new estimates of the zeta response, which is an implementation of the conventional MT response on a sphere. Computation of the zeta response is essential to improve the resolution of large scale electrical conductivity structure in the mantle because of its sensitivity to lateral inhomogeneity. Zeta is calculated from a robust section averaging procedure applied to the Fourier coefficients of the geomagnetic field and its spatial gradients at the Earth's surface. To obtain the spatial gradient, we apply a regularised spherical harmonic expansion to the geomagnetic field. We examine the effects of spatial aliasing, the breakdown of orthogonality, and the use of spatial regularization and geographical weight in determining the spherical harmonic coefficients. We discuss the sensitivity of the zeta response to lateral conductivity variations, and the effects of the present and proposed magnetic observatory distribution on our ability to reconstruct models of mantle conductivity. 5.2 Induction anomalies in Magsat data Authors: Grammatika N. (1), Tarits P. (1), Arkani-Hamed J. (2), Dyment J. (1) (1) UBO - IUEM, UMR 'Domaines Oceaniques', F-29280 Plouzane, France Email: naphsica@sdt.univ-brest.fr (2) Earth and Planetary Sciences, Mc Gill University, Montreal, Quebec, Canada H3A 2A7 Email: jafar@planet.eps.mcgill.ca The magnetic field measured by Magsat at an altitude around 400 km results from the superposition of the magnetospheric and ionospheric fields and internal fields including core, crustal and induced fields. In this study, we attempt to determine whether magnetic field effects of induced currents flowing in the Earth are significant in the satellite observations. In order to isolate the contribution of induced anomalies in Magsat data, we subtract field models of the core, the crust, and simplified models of the magnetospheric and ionospheric source fields. The residual field is first studied in areas of supposedly low remanent magnetization then considered globally. Indications about significant induction signals are presented. 5.3 The effect of heterogeneity in "D" layer on the electromagnetic variations of core origin AUTHORS : Takao Koyama & Hisashi Utada Earthquake Research Institute, Univ. of Tokyo Yayoi 1-1-1, Bunkyo-ku, Tokyo, Japan tkoyama@eri.u-tokyo.ac.jp We developed a calculation code for EM induction in the 3D heterogeneous earth based on the Modified Neumann Series by Pankratov et al. [1995], and examined the effect of the heterogeneous conductivity structure in the D"layer on the surface EM variations of core origin. The heterogenity is assumed to be of sectorial (P_2^2 - type) according to seismic tomography result, and EM variations of internal origin, having dipole-type symmetry (S1 or T2), are given on the CMB. We confirmed that the mode conversion of EM fields occurs because of the existence of the heterogeneous D"layer. For example, with T2 variation on the CMB, the magnetic field of S_3^2 mode is dominant on the surface. We tried to explain geomagnetic sixty-year variation with this mode conversion. It has been shown by the calculation that T2 variation of order one-tenth of main field on the CMB can cause the S_3 ^2 field of order 50 nT on the surface of the earth which was detected by Yokoyama & Yukutake [1991]. It was also shown that the mode conversion can also explain the locating of geomagnetic jerk, which is remarkable in the European region. 5.4 Laboratory electrical conductivity studies at high pressures and temperatures: upper mantle minerals Authors: Brent Poe and Yousheng Xu Bayerisches Geoinstitut Universitat Bayreuth, D95440 Bayreuth, Germany email: Brent.Poe@uni-bayreuth.de We have developed a method to determine electrical conductivities of minerals using complex impedance spectroscopy under simultaneous high pressure and high temperature conditions in a 1000 tonne multianvil apparatus. Experiments have now been conducted at pressures to 25 GPa and temperatures to 1650 C. Starting from a natural San Carlos olivine sample with Mg/(Mg+Fe) = 0.9, we have determined the conductivities of olivine to 10 GPa and its high pressure polymorphs wadsleyite at 15 GPa and ringwoodite at 20 GPa. Complex impedances were measured over the frequency range 1 MHz to 0.1 Hz. Data from all samples appeared as single semi-circular arcs in the complex plane (Z' vs -Z'') allowing fits of simple RC parallel equivalent circuits to estimate conductivity. Results for olivine agree well with studies conducted under controlled atmosphere conditions at 1 atm. The measurements, which ranged from 4 to 10 GPa, confirmed only a very slight pressure dependence. Conductivities for wadsleyite and ringwoodite are both about 100 times greater than for olivine. The difference is likely due to the ability of the high pressure polymorphs to accomodate ferric iron in their structures and is consistent with the jump near the transition zone in many geophysical models of mantle conductivity. 5.5 Numerical study of the mechanism of the mantle electrical conductivity Authors: Porokhova L.N.(e-mail: Ludmila.Porokhova.@pobox.spbu.ru), Abramova D. Yu., Porokhov D. A. Porokhova L.N.,Porokhov D. A.:St.Petersburg State University, Research Institute for Physics, Russia; Abramova D. Yu.:Troitsk, Moskow reg., IZMIRAN Ulyanovskaya,1, St.Petersburg, Russia, 198904. The well-known equation that establish a connection between the electroconductivity of solid body and the activation enthalpy in the thermodynamical mantle conditions, is investigated. For this purpose we used the models of temperature (Brown J.M. & Shankland T.J.), pressure (Dsiwonski A.M. & Anderson D.L.) and electroconductivity (Porokhova L.N. et al.) distributions. The negative values of the activation volume were obtained on the depths ranging from 500 to 2400 km. This allows us to suppose that the mechanism of charge transfer in the mantle is realized through electron jumps. The dependencies of the electroconductivity on the activation energy for a free electron, mobility of which has a power-kind temperature dependence, and for a polaron of a little radius with an exponential temperature dependence were studied. An analysis of results allows us to make a conclusion that the hypothesis about the polaron mechanism of the electrical conductivity better agrees with physical processes in the mantle. 5.6 Partial-melt electrical conductivity as a function of temperature and oxygen fugacity: Interconnectivity, melt chemistry, and permeability Authors: Jeffery J. Roberts and James A. Tyburczy Jeffery Roberts: Box 808, L-201, Lawrence Livermore National Laboratory, Livermore, CA, 94551, USA roberts17@llnl.gov James Tyburczy: Box 871404, Dept. of Geology, Arizona State University, Tempe, AZ, 85287-1404, USA jim.tyburczy@asu.edu We have performed measurements of bulk electrical conductivity as a function of temperature on texturally-equilibrated olivine Fo80 basalt partial melts between 684 and 1244=B0C at controlled oxygen fugacity and 1 bar total pressure. Melt-enhanced parallel conduction is observed at temperatures above 1100=B0C. The melt fraction and variation of melt composition with temperature has a major influence on bulk conductivity. Melt fraction as a function of temperature was constrained using the MELTS program (Ghiorso and Sack, 1995), which indicates that melt composition varies continuously with changes in melt fraction and becomes silica and alkali rich at the lowest melt fractions, rather than basaltic. Using the Hashin-Shtrikman upper bound mixing law, we calculate a melt conductivity as a function of temperature that includes the effects of composition. Between 1150 and 1244=B0C, the resulting melt conductivity is independent of temperature indicating that melt compositional and temperature effects counteract each other. The apparent Archie's Law exponent and constant, n =3D 0.98: C =3D 0.73, includes the effects of temperature on melt fraction, crystal conductivity, and melt composition. Application of permeability-conductivity models results in permeability estimates on the order of 100 microD to 20 mD (1 D =3D 1 x 10^-12 m^2) for our experimental grain size of ~45 microns. Partial support for this work was provided by the Geosciences Program of the OBES. Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract W-7405-ENG-48. 5.7 A return to the potential method for a global analysis of daily variations and magnetic storms Author: Ulrich Schmucker Planckstrasse 19, D-37-73 Goettingen The potential method requires a twofold spherical harmonic analysis of global surface fields: to express their magnetic potential and their vertical field by truncated series of spherical terms. The current view is that internal anomalies affect the vertical field so strongly that its series approximation leads to unreproducable results, when different sets of observing sites are used. Hence, other methods for EM response estimates have been devolopped without need for a spherical harmonic analysis of the vertical field. New analyses indicate that this view is too pessimistic. It appears that the required least-squares fit to the vertical field at many sites smoothes most effectively over internal anomalies at coastlines, on islands and elsewhere. Reliable and reproducable linear relations can be established between the expansion coefficients of the two series or, alternatively, between internal and external parts of the potential coefficients. For daily variations the expansions are conducted with time harmonics day-by-day, for storms with hourly means hour-by-hour, with a subsequent time series analysis. Results are global source descriptions and global responses of single spherical terms in reference to a mean layered Earth model response at a given frequency from 6 cpd to 0.025 cpd, yielding also indications for anomalies in the local surface field. 5.8 A new 3D global mantle conductivity reference model Authors: A. Schultz(1), I. Fujii(1,2) and M. Uyeshima(2) Affiliation: 1: Institute of Theoretical Geophysics Department of Earth Sciences University of Cambridge Downing St. Cambridge, CB2 3EQ U.K. email: adam@esc.cam.ac.uk, ikuko@esc.cam.ac.uk 2: Earthquake Research Institute University of Tokyo Tokyo Japan email: uyeshima@utada-sun.eri.u-tokyo.ac.jp Our initial global-scale 3D mantle conductivity inverse model (Schultz & Pritchard, 1995) was obtained by inversion of a limited set of scalar impedance functions (c values) from 20 magnetic observatories. This earlier inversion made use of a 3D perturbation expansion forward solver, and a broad-scale 3D model was developed. More recently, an accurate 3D staggered grid forward solver (Uyeshima & Schultz) based on the integral formulation of Maxwell's equations has made possible accurate and reasonably rapid calculation of the magnetic inductive response of a spherical earth of arbitrary conductivity distribution. The calculation of a new set of global 3D tensor magnetic inductive responses (Fujii & Schultz) makes possible the construction of models of the conductivity of the mantle of higher resolution than previously possible. We review here the progress made in applying the new forward solver toward the inversion of this rich data set. Specific issues include parallelisation of the forward solver to make efficient search of model space possible, and estimation of sensitivity functions in three-dimensions. We present a new, higher resolution inverse model of the global-scale 3D conductivity structure of the mantle, and discuss its geodynamic significance. 5.9 Electrical Conductivity With Depth As Determined By Laboratory Measurements Authors: T. J. Shankland1, A. G. Duba2, Yousheng Xu, Brent Poe, and David Rubie (Bayerisches Geoinstitut, Universitaet Bayreuth D95440 Bayreuth, Germany, thomas.shankland@uni-bayreuth.de), Jeff Roberts (L201 Lawrence Livermore National Laboratory, Livermore, CA 94551) 1on leave from MS D443 Los Alamos National Laboratory, Los Alamos NM 87545 2on leave from L201 Lawrence Livermore National Laboratory, Livermore, CA 94= 551 Chemical buffering (principally of oxygen fugacity) has improved laboratory measurements of electrical conductivity of iron-bearing silicates by (a) retarding iron loss from the sample, (b) controlling the oxygen-sensitive defect populations that are responsible for conduction at equilibrium. For olivine containing about 10% fayalite these improvements have produced general agreement among several groups for conductivity measured to temperatures in excess of 1200 deg.C. Improved oxygen buffering in the multi-anvil device has reduced uncertainties in olivine conductivity measured as a function of pressure to over 10 GPa at temperatures up to 1400 deg. C. Using the same techniques, conductivities of the high pressure phases of olivine--wadsleyite and ringwoodite--have also been measured at elevated temperatures and pressures. Similarly, conductivity of perovskite transformed from natural orthopyroxene has been measured at 25 GPa and up to 1600 deg.C. Combining these laboratory measurements with mantle temperature and pressure profiles constrains mantle electrical conductivity in the 200 - 900 km depth range. With less certainty, conductivity can be extrapolated thruough the lower mantle using activation volume determined from diamond anvil cell measurements. Such a profile resembles that of Banks (1969) based on analysis of world-wide observatory data. 5.10 New constraints on mantle conductivities from simultaneous sea-floor, island and continental electromagnetic measurements Authors: F. Simpson and K. Bahr Geophysics Institute, University of Goettingen, Germany fsimpson@willi.uni-geophys.gwdg.de/Fax: +49-551-397459 J. Watermann SACLANT Undersea Research, La Spezia, Italy It has been suggested that oceanic mantle is more conductive than continental, either due to its different chemical composition or because of its genesis in more recent tectonic activity. Long period magnetotelluric (MT) and geomagnetic data from a sparse array (nominal site spacing 600 km) with coverage of island sites (Montecristo and Mallorca) on both sides of the Mediterranean simultaneously with continental sites in tectonically stable regions of France and Germany, together with ocean bottom magnetometer data at 150-160 m depth in the vicinity of the island of Montecristo are used to examine the hypothesis of mantle heterogeneities to approximately 600 km depth in oceanic/continental regimes. Constraints are evaluated via three independent techniques for calculating the electromagnetic transfer function C (Z:H, MT and vertical gradient) for comparison between sea-bottom, island and continental sites. Corrections due to the attenuation effects of sea-water bathymetry are calculated from 3D thin sheet modelling. The potential for using islands as windows to oceanic mantle despite the shielding and distortion effects of surrounding sea-water layers is discussed in relation to the results. 5.11 3D analysis of very long period geomagnetic data : the conductivity of the lower mantle Authors: Pascal Tarits and Mioara Alexandrescu Pascal Tarits tarits@univ-brest.fr UBO - IUEM UMR CNRS ' Domaines Oceaniques ' Place Nicolas Copernic F-29280 Plouzane France Mioara Alexandrescu mioara@ipgp.jussieu.fr Institut de Physique du Globe 4 Place Jussieu, F-75252 Paris cedex 05 France Monthly mean values of the Geomagnetic field from 78 observatories have been analysed. We selected a time window appropriate for the longest available series of observatory data. After removal of the main field and the secular variation using the model provided by Bloxham and Jackson (1992), we calculated the spectrum of all 3 components X, Y and Z at each observatory. Assuming that the source field is a combination of an axial and two equatorial external dipoles, the response for a given 3D conductivity model of the Earth was computed iteratively using a minimisation procedure that optimized the mantle electrical conductivity to fit the data. The earth is modelled with a homogeneous upper mantle down to 400 km which lies above two heterogeneous layers 260 km thick each. Below these layers the Earth is assumed homogeneous. The heterogeneous conductivity is given on a grid 10 (North-South) by 20 (East-West) corresponding roughly to maximum wavenumbers l=6-8. We present the preliminary results . 5.12p PRELIMINARY RESULTS ON THE DEEP STRUCTURE OF THE OUTER CARPATHIANS Authors: Cornel David, Mihai Micu S.C. Prospectiuni S.A. Caransebes 1,78344, Bucuresti 32 During the last ten years more than 1,200 magnetotelluric soundings (MT) were carried out for a detailed investigation programme of the East Carpathians Flysch Zone in order to evaluate the hydrocarbon perspectives at depths over 5,000 m. A series of oil and gas fields are exploited in this area since several decades from depths rarely exceeding 2,000 - 3,000 m. Only several boreholes reached depths over 4,000 m and from seismic profiles performed here were obtained inconsistent and sometimes contradictory informations about the deep stucture. The feasibility of the MTS method is based upon the resistivity contrasts between the different geological formations. The general structure may be aproximated as a 2-D model whose uniformity axis has the same direction as the stuctures of the Carpathians Flysch. As for the deep structures of the Flysch area we realised MTS profiles which cross entirely the mountain area starting from the eastern border of the Transylvanian Basin up to the foreland zone. They are very useful in deciphering the deep structure of the underthrusted basement in conection with important compresional phenomena since the end of the Upper Cretaceous. 5.13p An Attempt of Laboratory measurements of the electrical conductivity of rocks Author: Kiyoshi Fuji-ta Department of the earth and planetary sciences Kobe University 1-1 Rokkodai, Nada, Kobe, 657-8501, Japan e-mail:fuji-ta@kobe-u.ac.jp FAX:+81-78-302-5337 Electrical conductivity structures were obtained by various Electro-Magnetic sounding methods in many places in the world. However, a lack of laboratory measurement of the electrical conductivity makes evaluations of these soundings difficult. As an attempt of the laboratory experiment, I intend to measure the electrical conductivity of basic rocks. Simulating crustal conditions, samples are heated up to 800 degree( Celsius) and are pressured up to 1 GPa. I also consider the problems of porosity, saline water content, oxygen fugacity and included mineral grains. In this workshop, experimental details of conductivity of samples and a preliminary result will be discussed. 5.14p Constraints on the electrical conductivity beneath the Japan Sea by MT response of the Japan Sea Cable (JASC) Authors: Tadanori Goto (Aichi University of Education, Kariya, Japan), Hisayoshi Shimizu, Hisashi Utada (University of Tokyo, Tokyo, Japan), Yoshikazu Tanaka (Kyoto University, Beppu, Japan), Kiyofumi Yumoto (Kyushu University, Fukuoka, Japan), Valerian Nikiforov (Pacific Oceanographic Institute, Vladivistok, Russia) Nick Palshin, Renat MEDZHITOV, and Leonid VANYAN (Shirshov Institute of Oceanology, Moscow, Russia ) Electrical potential difference between Nakhodka (Russia) and Naoetsu (Japan) has been observed by using the Japan Sea Cable (JASC) since 1996. MT resp- onses were obtained from the observed data to constrain the conductivity structure beneath the Japan sea and the West Pacific region for frequency range from 300 sec to 30,000 sec. Magnetic data from several observatories were used for testing the inhomogeneity of the source field. Then, three-dimensio- nal forward modeling, using a code by Mackie et al. (1993), was carried out to examine how the structure is constrained by the observed JASC MT response. Following implications were obtained: (1) The high conductance of the upper mantle (the depth range of 100 - 400km) beneath the Japan sea is well con- strained by the JASC MT response. It is estimated as about 15,000 - 25,000 S from the response curve at the periods longer than 6,000 sec. (2) The litho- sphere conductivity is also sensitive to the MT response by the JASC, but it cannot be constrained by using JASC only. On-land MT responses are needed in the modeling process to estimate the lithosphere conductivity beneath the Japan Sea. 5.15p Investigation of Natural Electromagnetic fields in a frequency range of the Shumann Resonances and Geomagnetic Pulsations on the earth and in the Sea. Authors: A.S.Lisin (Troitsk Institute for Innovation and Fusion Research (TRINITI), 142092, Troitsk,Moscow Reg., Russia). Natural EM-waves in the frequency range ~0.001 - 40 c/s are cousideved both as an informative field, that can be used to obtain quantitative data about electromagnetic parameters of cosmo-geological media and as a sourse of the electromagnetic noise for receiving artificial signals. The results of experimental measurements and processing of EM-noise realizations both in time and frequency domains are presented. The following parameters are considered: - time variations of the noise level; - space-correlation coefficient; - compesation coefficient; - probability low of errors; - autocorrelation functions; - power spectrum distributions. The measurements of EM-noise was carried out in various parts of Russia, including the Black Sea, the Barents Sea and the Polar Zone of the Arctic Ocean. 5.16p On Electromagnetic Soundings of the Earth's Mantle. Authors: Semenov V.Yu., Jozwiak W. Institute of Geophysics Polish Academy of Sciences ul. Ksiecia Janusza 64, Warsaw 01-452. E-mail: sem@igf.edu.pl, Fax: +048-22-691-59-15. The conductivity structure of the Earth's mantle was estimated for the Europe-Asian region down to depths of the core-mantle boundary. For this purpose, the response functions in the period range from 1 day until 11 years were considered. The hourly and monthly data of the geomagnetic observatories with codes IRT, KIV, MOS, NVS, HLP, WIT, NGK were used to estimate responses with periods from 3.5 until 700 days and these data were combined with published responses for other oscillations. Parker's (D+), OCCAM and spherical inversions as well as the forward spherical modelling were applied to interpret all those data. Zharkov's prediction about the existence of two additional conductive zones in the mantle, at depths of 700-900 km and in the lower mantle deeper than 2200 km, is very probable. 5.17p Interpretation of long-period electromagnetic array-data.Z:H method, Sq field. Authors: Wolfgang Soyer, Joerg Leibecker, Karsten Bahr; Institute of Geophysics, Georg-August-University of Goettingen, Herzberger Landstrasse 180, 37085 Goettingen, Germany. woso@willi.uni-geophys.gwdg.de Due to geochemical studies the elevation of the Rhenish Massif and its recent volcanic activity in the Eifel is supposed to be associated to the same geophysical feature as in the Massif Central in France, where the ascent of a thermally and chemically anomalous mantle plume could be detected by teleseismic tomography. Such a zone of partial melt material being associated with a high conductivity anomaly, electromagnetic array measurements have been done simultaneously on 23 field stations for a period of six weeks in summer 1997. To resolve expected 3D inhomogeneities in the period range of daily variations, where the skin depth is larger than the extent of the whole array, the magnetic data have been analysed areawide with the 1D Z:H method - with the astonishing result, that the skin depth varies enormously at comparably very short lateral distances, so that two 'plumeheads' seem to get visualised. On the poster, a discussion on the data quality, some analytic approach to this unexpected result and hopefully 3D inhomogenic source modelling in continuation to homogenic modelling of shorter periods is presented. 5.18p A New Seafloor Electromagnetic Station with an Overhauser Magnetometer, a Magnetotelluric Variograph and an Acoustic Telemetry Authors: Toh.Hiroaki* and Hamano Yozo** *Ocean Research Institute, University of Tokyo at School of Earth SciencesFlinders University of South Australia c/o Dr Graham S. Heinson, G.P.O. Box 2100 Adelaide SA 5001, Australia Phone: +61-8-8201-2898,Fax:+61-8-8201-2676 e-mail: toh@ori.u-tokyo.ac.jp **Institute of Earth and Planetary Phisics, Graduate School of University of Tokyo A new type of SeaFloor ElectroMagnetic Station (SFEMS) has been developed in conjunction with the Ocean Hemisphere Project (OHP),which is a Japanese sesmic/ EM project to reveal the Earth`s structure at great depths using the ocean as a search window. New SFEMS is able to conduct long-term electromagnetic (EM) observations at the seafloor, which enables us to probe into the deep Earth (both the mantle and the core) by improving the spatial coverage of the existing magnetic observatory network. The SFEMS has been tested in three sea experiments to yield 3 components of the geomagnetic field, 2 horizontal components of the geoelectric field and 2 components of tilts in addition to the absolute geomagnetic total force in 30 sec sampling. SFEMS is designed for measuring these EM signals at the seafloor continuously for as long as 2 yrs. SFEMS mainly consists of the following three parts: (1) An Overhauser proton precession magnetometer for the absolute measurements of the geomagnetic total force with a possible bias of less that 10 nT. (2) A MT variograph that measures the rest of the EM components and the tilts. (3) An Acoustic Telemetry Modem (ATM) that allows us to control/monitor the seafloor instrument as well as data transmission at the maximum rate of 1200 baud. Construction of seafloor EM observatories in regions where significant EM data have never been collected is now quite feasible by development of SFEMS. 5.19pTitle: The Effect of Alumina on the Electrical Conductivity of Silicate Perovskite Authors: Yousheng Xu, Brent T. Poe Bayerisches Geoinstitut, Universitaet Bayreuth, D-95440 Bayreuth, Germany Xu@uni-bayreuth.de We have measured the effect of alumina on the electrical conductivity of (Mg,Fe)SiO3 perovskite transformed from two pyroxene samples having similar iron content: San Carlos orthopyroxene containing 3.3wt% Al2O3, and a synthetic orthopyroxene with similar iron content, (Mg0.915,Fe0.085)SiO3. Samples were first transformed to perovskite at 25 GPa and 1600 degC in a multi-anvil apparatus, and then prepared as disks for in-situ complex impedance spectroscopy in a second run at 25 GPa and 1400-1600 degC. Both the syntheses and conductivity measurements were performed in the presence of a Mo-MoO2 buffer which maintains an oxygen fugacity close to the iron-wustite buffer at these conditions. Our results show that the activation energy of 0.70 eV for conduction in perovskite containing Al2O3 is close to that of 0.62 eV for conduction in Al-free perovskite, but much higher than that of about 0.35 eV determined by diamond anvil cell studies at much lower temperatures. The electrical conductivity of the Al-bearing perovskite is about three times as that of the Al-free perovskite. Because Mossbauer studies of perovskites with similar compositions have shown that the Al-bearing perovskite has about 3 times the amount of Fe3+ as the Al-free sample (McCammon, 1997), we conclude that the conduction mechanism in perovskite between 1400-1600 degC is most likely by polarons. For geophysical purposes, the effect of Al2O3 on the electrical conductivity of silicate perovskite should be considered when laboratory data are used to interpret geophysical models.