Electrical properties of montmorillonite studied together with the processes occurring under thermal activation

The results of laboratory experiments on studying the electrical conductivity σ of a clay mineral montmorillonite from different sedimentary mineral deposits of Dagestan in the temperature interval from 100 to 1000°C are presented. The general regularities in the dependence of the electrical conductivity σ of the studied samples on the absolute temperature T are accounted for by the existence of the associated complexes of elementary defects of the crystal lattice. These complexes play important role in a variety of kinetic processes under the conditions of the Earth’s interior, and their existence is demonstrated by the experiments. The activation energy of the electrical conductivity and the preexponential factors are determined for all the temperature zones. The relationship between the pattern of temperature variations in electrical conductivity and the processes of releasing interlayer water and hydroxyls from different energy sites is established. It is concluded that the anomalous change in electrical conductivity in some samples reflects the postsedimentation changes of montmorillonite manifesting themselves by the emergence of a hydromuscovite component.     

Evidence from pseudomorphous β-quartz phenocryst for decompression of rock-forming and ore-forming processes in Shapinggou porphyry Mo deposit

The Shapinggou porphyry molybdenum(Mo) deposit, located in Jinzhai County, Anhui Province, China, is the largest in the Qinling-Dabie Mo Metallogenic Belt. The intrusive rocks in the Shapinggou Mo ore district formed in the Yanshanian can be divided into two stages based on zircon U-Pb dating and geochemical features. This study focuses on the late stage intrusions(quartz syenite and granite porphyry), which are closely genetically related to molybdenum mineralization. Petrographic observations identified two quartz polymorphs in the quartz syenite and granite porphyry, which were derived from the same magmatic sources and similar evolutionary processes. The quartzes were identified as a xenomorphic β-quartz within quartz syenite, while the quartz phenocrysts within the granite porphyry were pseudomorphous b-quartz, characterized by a hexagonal bipyramid crystallography. The pseudomorphous b-quartz phenocrysts within the granite porphyry were altered from b-quartz through phase transformation. These crystals retained b-quartz pseudomorph. Combined with titanium-inzircon thermometry, quartz phase diagrams, and granitic Q-Ab-Or-H_2O phase diagrams, it is suggested that the quartz syenite and granite porphyry were formed under similar magmatic origins, including similar depths and magmatic crystallization temperatures. However, the β-quartz within quartz syenite indicated that the crystallization pressure was greater than 0.7 GPa, while the original b-quartz within the granite porphyry was formed under pressures between 0.4 and 0.7 GPa. The groundmass of the granite porphyry which formed after the phenocryst indicated a crystallizing pressure below 0.05 GPa. This indicates that the granite porphyry was formed under repetitive and rapid decompression. The decompression was significant as it caused the exsolution of the ore-forming fluids, and boiling and material precipitation during the magmatic-fluid process. The volumetric difference during the phase transformation from b-quartz to β-quartz caused extensive fracturing on the granite porphyry body and the wall rocks. As the main ore-transmitting and ore-depositing structures, these fractures benefit the hydrothermal alteration and stockwork-disseminated mineralization of the porphyry deposit. It is considered that the pseudomorphous β-quartz phenocrysts of the porphyritic body are metallogenic indicators within the porphyry deposits. The pseudomorphous β-quartzes therefore provide evidence for the formation of the porphyry deposit within a decompression tectonic setting.     

斜长石、辉石混合模型的电导率有限元数值计算研究

以岩石实验中矿物的几何形态及空间分布为建模依据,以实验条件及单矿物电导率的测量结果为约束条件,用有限元数值方法模拟了不同微观结构的斜长石、辉石混合物在施加电压后电势及电流的分布情况,并计算了混合模型在不同温度条件下的电导率.研究结果显示,数值模型网格数及矿物颗粒数的选取对电导率计算结果的精度有较大影响,在体导电情况下,模型电导率因矿物比例含量和排列结构而异.当斜长石及辉石随机分布时,随着辉石含量的增加,混合模型电导率在不同温度下均有所增加,且温度越高,增加幅度越大,电导率的有限元模拟计算结果接近于有效介质渗透理论模型,且位于并、串联模型之间以及HS模型的上、下边界范围内;在斜长石及辉石含量一定的情况下,各矿物的排列分布对电导率计算结果也有一定的影响,当矿物颗粒大小接近且分布均匀时,模型中电势沿电流传导方向变化较为均匀,模拟计算得出的电导率相对较高,当矿物颗粒大小差别较大及分布不均匀时,电势分布受到一定的扰动,电导率计算结果也较低.将混合模型电导率有限元计算结果与辉长岩、辉绿岩及玄武岩实验测量结果进行比较,显示这3种岩石样品电导率与温度变化关系的斜率均与混合模型计算结果的斜率相接近,表明这些岩石在所选温度段导电机制与斜长石、辉石混合模型相似,用斜长石、辉石混合模型的电导率研究玄武岩、辉长岩及辉绿岩的导电性具有适用性.将混合模型有限元计算结果与玄武岩、辉长岩、辉绿岩覆盖区地壳大地电磁实测结果对比,发现大地电磁电导率结果位于混合模型计算结果范围内,用斜长石、辉石混合模型模拟玄武岩、辉长岩等岩石地壳具有一定的可行性.     

Where fast weathering creates thin regolith and slow weathering creates thick regolith

Weathering disaggregates rock into regolith – the fractured or granular earth material that sustains life on the continental land surface. Here, we investigate what controls the depth of regolith formed on ridges of two rock compositions with similar initial porosities in Virginia (USA). A priori, we predicted that the regolith on diabase would be thicker than on granite because the dominant mineral (feldspar) in the diabase weathers faster than its granitic counterpart. However, weathering advanced 20× deeper into the granite than the diabase. The 20 × ‐thicker regolith is attributed mainly to connected micron‐sized pores, microfractures formed around oxidizing biotite at 20 m depth, and the lower iron (Fe) content in the felsic rock. Such porosity allows pervasive advection and deep oxidation in the granite. These observations may explain why regolith worldwide is thicker on felsic compared to mafic rock under similar conditions. To understand regolith formation will require better understanding of such deep oxidation reactions and how they impact fluid flow during weathering. Copyright © 2012 John Wiley & Sons, Ltd.     

Critical phenomena in thermal conductivity: Implications for lower mantle dynamics

Microscopic mechanisms for heat transport in dense minerals (phonon scattering and photon attenuation) exhibit aspects of threshold behavior, discussed qualitatively here. For all minerals examined so-far using laser-flash analysis, the lattice component of the thermal conductivity of the mantle asymptotes to a constant above a critical temperature of ∼1500 K. Radiative transfer calculated from absorption spectra has thresholds in both grain-size and Fe content, and a rather complex dependence on temperature. These critical phenomena impact convection of the lower mantle, because the lattice contribution tends to destabilize the cold boundary layers, whereas radiative transfer mostly promotes stability in the lower mantle, unless the grains are large and Fe-rich, which makes convection chaotic and time-dependent. The specific behavior suggests that flow in the lower mantle is sluggish, whereas flow in the upper mantle-transition zone is time-dependent. The decrease in k_rad as Fe/(Fe + Mg) increases beyond ∼0.1 may be connected with formation of lower mantle, thermo-chemical plumes through positive feedback.     

Electrical conductivity of magmatic liquids: effects of temperature,oxygen fugacity and composition

The effects of temperature, f_O2 and composition on the electrical conductivity of silicate liquids have been experimentally determined from 1200 to 1550°C under a range of f_O2 conditions sufficient to change the oxidation state of Fe from predominantly Fe²⁺ to Fe³⁺. Oxidation of ferrous to ferric iron in the melt has no measurable effect on the conductivity of melts with relatively low ratios of divalent to univalent cations. Under strongly oxidizing conditions a minor decrease of conductivity is detected inth highΣM²/ΣM⁺ ratios. It is concluded that for purposes of estimating the conductivity of magmatic liquids, f_O2 may be ignored to a first approximation. Both univalent and divalent cation transport is involved in electrical conduction. Melts relying heavily on divalent cations for conduction, i.e. melts with relatively large ΣM²⁺/ΣM⁺ ratios, show strong departures from Arrheenius temperature dependence with the apparent activation energies decreasing steadily as the temperature increases. Conductivities dominated by the univalent cations, in melts with relatively small ΣM²⁺/ΣM⁺ ratios, show classical Arrhenius temperature dependence. These observations are discussed in terms of the general characteristics of the melt structure.Compositional variations within the magmatic range account for much less than an order of magnitude variation in electrical conductivity at a fixed temperature. This observation, combined with previous measurements of the conductivity of olivine (A. Duba, H.C. Heard and R. Schock, 1974) make it possible to state with reasonable confidence that melts occurring within the mantle will be more conductive by 3–4 orders of magnitude than their refractory residues. Potential applications to geothermometry are discussed.     

Temperature profiles in the earth of importance to deep electrical conductivity models

Deep in the Earth, the electrical conductivity of geological material is extremely dependent on temperature. The knowledge of temperature is thus essential for any interpretation of magnetotelluric data in projecting lithospheric structural models. The measured values of the terrestrial heat flow, radiogenic heat production and thermal conductivity of rocks allow the extrapolation of surface observations to a greater depth and the calculation of the temperature field within the lithosphere. Various methods of deep temperature calculations are presented and discussed. Characteristic geotherms are proposed for major tectonic provinces of Europe and it is shown that the existing temperatures on the crust-upper mantle boundary may vary in a broad interval of 350–1,000°C. The present work is completed with a survey of the temperature dependence of electrical conductivity for selected crustal and upper mantle rocks within the interval 200–1,000°C. It is shown how the knowledge of the temperature field can be used in the evaluation of the deep electrical conductivity pattern by converting the conductivity-versustemperature data into the conductivity-versus-depth data.     

The influence of oxidation state on the electrical conductivity of olivine

The electrical conductivity of a single crystal of San Carlos olivine (Fo₉₂, 0.16 wt.% Fe₂O₃) has been measured as a function of temperature and oxygen fugacity (fO₂). After heating to 1338°C at fO₂ = 10^?12 atm., the conductivity at 950°C was 10^?5 (ohm-m)^?1, almost 3 orders of magnitude less than that measured in air. This decrease is due to the reduction of Fe³⁺ to Fe²⁺. Further heating to 1500°C at fO₂ = 10^?14 atm., decreased the electrical conductivity at 950°C to 10^?6 (ohm-m)^?1. When recovered at room temperature, the speciment had Fo₉₆ composition and contained small, opaque blebs distributed throughout the crystal. Derivations of temperature profiles for the earth's mantle from conductivity-depth models must take account of the important role played by iron oxidation state in the electrical conductivity of olivine.     

粤北下庄矿田小水铀矿床地球化学特征分析

粤北小水矿床是下庄矿田典型的交点型铀矿床。通过对矿床内发育的花岗岩、辉绿岩及铀矿石采样分析．发现小水矿床矿石具有轻稀土富集的稀土元素特征和富集大离子亲石元素Rb、Th的微量元素特征。与辉绿岩的相应特征十分吻合,与花岗岩的相应特征差异很大,推测其成矿物质来源于深部地幔流体（富含U、F、CO2）,且交点型矿石的形成很有可能伴有幔源成矿流体对与花岗岩有关的早期红化矿石的叠加改造作用。     

Critical phenomena in thermal conductivity: Implications for lower mantle dynamics

Microscopic mechanisms for heat transport in dense minerals (phonon scattering and photon attenuation) exhibit aspects of threshold behavior, discussed qualitatively here. For all minerals examined so-far using laser-flash analysis, the lattice component of the thermal conductivity of the mantle asymptotes to a constant above a critical temperature of 1500 K. Radiative transfer calculated from absorption spectra has thresholds in both grain-size and Fe content, and a rather complex dependence on temperature. These critical phenomena impact convection of the lower mantle, because the lattice contribution tends to destabilize the cold boundary layers, whereas radiative transfer mostly promotes stability in the lower mantle, unless the grains are large and Fe-rich, which makes convection chaotic and time-dependent. The specific behavior suggests that flow in the lower mantle is sluggish, whereas flow in the upper mantle-transition zone is time-dependent. The decrease in k_rad as Fe/(Fe + Mg) increases beyond 0.1 may be connected with formation of lower mantle, thermo-chemical plumes through positive feedback.     

Electric and dielectric properties of forsterite between 400 and 900°C

The electrical impedance of a sintered forsterite sample containing 1 mol percent SiO₂ in excess has been studied as a function of frequency (1–10⁵ Hz) and temperature (400–900°C). The electrical properties are strongly frequency-dependent, requiring both conduction and displacement current for their interpretation. The electrical conductivity is thermally activated with a mean activation energy of 1.15 eV. It is interpreted as being ionic and caused by migration of magnesium ions via a vacancy mechanism. Dielectric data deduced from impedance measurements vary as ω^n?1 (0 < n < 1) at constant temperature. The value of n is different below and above a critical frequency ω_c, which is thermally activated and interpreted as the jump frequency of the migrating species.     

Investigating plant root effects on soil electrical conductivity: An integrated field monitoring and statistical modelling approach

Plants have been shown to affect soil water content and temperature. Previous studies were conducted mainly in forestry and agricultural soils, where conditions of soil and vegetation are different from those in an urban landscape. In an urban landscape, the influence of plant roots on electrical conductivity, soil water content and temperature is still not clear. This study aims to investigate the effects of soil water content and temperature on electrical conductivity in vegetated soils through an integrated field monitoring and computational modelling approach. A new relationship between soil electrical conductivity and water content as well as temperature is proposed. Field monitoring was conducted in both vegetated (tree species) and bare soils. The monitoring included measurements of soil water content, soil temperature and soil electrical conductivity. This was followed by response surface regression modelling. Measured results show that soil temperature at shallow depths was lower in vegetated soil than that in the bare soil. This observation was also consistent with the higher soil water content and hence, higher electrical conductivity under tree canopy. The model developed could predict nonlinear relationships between electrical conductivity and soil temperature and water content. Uncertainty analysis indicated normal distribution for electrical conductivity under variation of soil temperature and water content. © 2018 John Wiley & Sons, Ltd.     

Negative pressure effect on the electrical conductivity of San Carlos olivine and its implication to the electrical structure in the upper mantle

Pressure effect on the electrical conductivity of San Carlos olivine was investigated by the newly installed electrical conductivity measurement system at China University of Geosciences. Electrical conductivity of San Carlos olivine aggregates was measured up to 12 GPa and 1475 K using the Walker-type multi-anvil apparatus equipped with eight WC cubes as the second-stage anvils. The pressure generation against applied load for the experimental assemblage was examined by phase transition of Bi,quartz, forsterite under different P-T conditions. To check the data validity of this new system, electrical conductivities of the serpentinites and talc samples were measured. The results are consistent with the published data of the same samples. Electrical conductivity(σ) of the San Carlos olivine aggregates and temperature(T) satisfy the Arrhenian formula: σ=σ0exp[.(ΔE+PΔV)/kT].The pre-exponential factor(σ0), activation energy(ΔE) and activation volume(ΔV) yield value of 7.74 S/m, 0.85 eV and 0.94cm3/mol, respectively. Electrical conductivities of the San Carlos olivine aggregates decline with increasing pressure at same temperatures. The negative pressure effect can be interpreted by strain energy model of defect energy together with the lattice deformation. In addition, the electrical conductivity-depth 1-D profile of the upper mantle was constructed based on our results and some assumptions. The calculated profile is concordant with the geophysical observation at the depth of 180–350 km beneath Europe, which indicates that the upper mantle beneath Europe might be dry.     

Early Permian Tarim Large Igneous Province in northwest China

Tarim Large Igneous Province (TLIP) is the second Late Paleozoic LIPs in China after the recognition of Emeishan LIP, and is a hot research topic in geosciences. On the basis of the analysis of research history about TLIP, this paper summarizes the research result during last twenty years and suggests the key research area in the future. The residual distribution range of TLIP is up to 250000 km², and the largest residual thickness is 780 m. The eruption of basalt happened during 290–288 Ma and belongs to LIPs magmatic event with fast eruption of magma. The lithological units of the TLIP include basalt, diabase, layered intrusive rock, breccia pipe mica-olivine pyroxenite, olivine pyroxenite, gabbro, ultramafic dyke, quartz syenite, quartz syenite porphyry and bimodal dyke. The basalt and diabase of TLIP exhibit OIB-like trace element patterns and enrichment of LILE and HFSE, and mainly belong to high TiO₂ series. There is an obvious difference in isotope among the basalt from Keping and the basalt and dibase from the northern Tarim Basin. The basalt from Keping with negative ? _Nd and high REE value derives from enriched mantle, and the diabase and basalt from the northern Tarim Basin with positive ? _Nd and low REE value are related to depleted mantle. The crust uplifting in the Early Permian and the development of picrite and large scale dyke and formation of large scale V-Ti-Magnetite deposit in Wajilitag area support the view that the TLIP is related to mantle plume. The TLIP has a temporal-spatial relationship with Permian basic to ultra-basic igneous rock, which is distributed widely in Central Asia, and they represent a tectono-magmatic event with very important geodynamic setting. This paper also suggests that the deep geological process, the relation with mantle plume, mineralization, the relation with environmental change and biological evolution, and the geodynamics of the TLIP will be the key research topics in the future.     

Effects of temperature and sliding rate on frictional strength of granite

Layers of artificial granite gouge have been deformed on saw-cut granite surfaces inclined 30° to the sample axes. Samples were deformed at a constant confining pressure of 250 MPa and temperatures of 22 to 845°C. The velocity dependence of the steady-state coefficient of friction (^ss) was determined by comparing sliding strengths at different sliding rates. The results of these measurements are consistent with those reported bySolberg andByerlee (1984) at room temperature andStesky (1975) between 300 and 400°C. Stesky found that the slip-rate dependence of (^ss) increased above 400°C. In the present study, however, the velocity dependence of (^ss) was nearly independent of temperature.     

福建紫金山外围东留花岗斑岩体地质特征及成矿作用分析

福建紫金山及其外围地区分布的浅成-超浅成斑岩体与成矿关切密切.紫金山外围东留花岗斑岩体位于福建省武平县境内,花岗斑岩体内外接触带普遍发育强烈的矿化蚀变带.通过对东留花岗斑岩体进行野外地质调查及全岩地球化学分析,结果表明东留花岗斑岩体具有过铝质A型花岗岩的特征：富硅、钾、铁,贫钙、钠、镁,A/CNK值大于1.1,分异指数高,富集HFSE、Ga、LREE、Y（Ce）,亏损Ba、Sr、P、Ti,具有强烈的铕负异常.东留花岗斑岩体作为矿化蚀变岩石的母岩,可能为矿化作用提供热源和成矿物质,具有寻找锡多金属矿的前景.     

Electrical conductivity of molten FeNiSC core mix

The electrical conductivity of liquid (Fe₉₀Ni₁₀)₃S₂ saturated with 2.6 weight percent carbon averages 2.7·10⁵ mho/m at 1000°C and zero pressure. This may imply a slightly lower electrical conductivity for the earth's core than that obtained by extrapolating the properties of pure liquid iron and solid iron alloys to core pressures and temperatures. Although a sulphur-rich core would have a smaller proportion of sulphur, the effect of lowering the sulphur content of the FeNiSC liquid to about 15 weight percent would be unlikely to increase the conductivity above 5·10⁵ mho/m.     

Origin of High Electrical Conductivity in the Lower Continental Crust: A Review

Electromagnetic measurements have demonstrated that the lower continental crust has remarkable electrical anomalies of high conductivity and electrical anisotropy on a global scale (probably with some local exceptions), but their origin is a long-standing and controversial problem. Typical electrical properties of the lower continental crust include: (1) the electrical conductivity is usually 10⁻⁴ to 10⁻¹ S/m; (2) the overlying shallow crust and underlying upper mantle are in most cases less conductive; (3) the electrical conductivity is statistically much higher in Phanerozoic than in Precambrian areas; (4) horizontal anisotropy has been resolved in many areas; and (5) in some regions there appear to be correlations between high electrical conductivity and other physical properties such as seismic reflections. The explanation based on conduction by interconnected, highly conductive phases such as fluids, melts, or graphite films in grain boundary zones has various problems in accounting for geophysically resolved electrical conductivity and other chemical and physical properties of the lower crust. The lower continental crust is dominated by mafic granulites (in particular beneath stable regions), with nominally anhydrous clinopyroxene, orthopyroxene, and plagioclase as the main assemblages, and the prevailing temperatures are mostly 700–1,000°C as estimated from xenolith data, surface heat flow, and seismic imaging. Pyroxenes have significantly higher Fe content in the lower crust than in the upper mantle (peridotites), and plagioclase has higher Na content in the lower crust than in the shallow crust (granites). Minerals in the lower continental crust generally contain trace amounts of water as H-related point defects, from less than 100 to more than 1,000 ppm H₂O (by weight), with concentrations usually higher than those in the upper mantle. Observations of xenolith granulites captured by volcano-related eruptions indicate that the lower continental crust is characterized by alternating pyroxene-rich and plagioclase-rich layers. Experimental studies on typical lower crustal minerals have shown that their electrical conductivity can be significantly enhanced by the higher contents of Fe (for pyroxenes), Na (for plagioclase), and water (for all minerals) at thermodynamic conditions corresponding to the lower continental crust, e.g., to levels comparable to those measured by geophysical field surveys. Preferred orientation of hydrous plagioclase, e.g., due to ductile flow in the deep crust, and alternating mineral fabrics of pyroxene-rich and plagioclase-rich layers can lead to substantial anisotropy of electrical conductivity. Electrical conductivity properties in many regions of the lower continental crust, especially beneath stable areas, can mostly be accounted for by solid-state conduction due to the major constituents; other special, additional conduction mechanisms due to grain boundary phases are not strictly necessary.     

Electrical conductivities of two granite samples in southern Tibet and their geophysical implications

There are clear differences in the electrical conductivities of the crustal granites of the Qinghai-Tibet Plateau.Because these granites are among the major rock types on the Qinghai-Tibet Plateau, it is very important to detect the electrical conductivity of granites under high temperatures and pressures to study the electrical conductivity structure of this area. Using impedance spectroscopy at a frequency range of 10.1–106 Hz, the electrical conductivity of the muscovite-granite collected from Yadong was investigated at a confining pressure of 1.0 GPa and temperatures ranging from 577 to 996 K, while the electrical conductivity of the biotite-granite collected from Lhasa was investigated at a pressure of 1.0 GPa and temperatures ranging from587 to 1382 K. The calculated activation enthalpies of the Yadong muscovite-granite sample is 0.92 eV in the low-temperature range(577–919 K) and 2.16 eV in the high-temperature range(919–996 K). The activation enthalpies of the Lhasa biotite-granite sample is 0.48 eV in the low-temperature range(587–990 K) and 2.06 eV in the high-temperature range(990–1382 K). The change in the activation enthalpies of the granites at different temperature ranges may be associated with the dehydration of the two samples. The electrical conductivities of the granite samples obtained in the laboratory using impedance spectroscopy correspond well with field observations conducted near the sampling points, both in terms of the actual conductivity values and the observed variations between the low-temperature and high-temperature regimes. This correlation of laboratory and field conductivities indicates that the conductivities of the crustal rocks in the two regions closely correspond to granite conductivities.We calculated the electrical conductivities of muscovite-granite and biotite-granite samples using the effective medium and HS boundary models. When applied to the crustal rocks of southern Tibet, the results of the geophysical conductivity profiles lie within the range of laboratory data. Thus, the electrical characteristics of the crustal rocks underlying the southern Qinghai-Tibet Plateau can largely be attributed to granites, with the large changes to high conductivities at increasing depths resulting from the dehydration of crustal rocks with granitic compositions.     

Frequency dependent electrical properties of minerals and partial-melts

The resistance to current flow of minerals and partial-melts is a frequency dependent electrical property. Measurements of the frequency dependent electrical impedance of single crystal olivine, polycrystalline olivine, dunites, metapelites, and partial-melts, between 10^–4 and 10⁵ Hz, when plotted in the complex impedance plane, reveal arcs that correspond to different conduction mechanisms in the material being studied. In polycrystalline materials, two impedance arcs related to material properties (as opposed to electrode properties or electrode-sample interactions) are observed. Each impedance arc is activated over a distinct range of frequency, that is, the mechanisms occur in series. Based on experiments comparing single and polycrystalline impedance spectra, experiments on samples with different electrode configurations, and on samples of varying dimension, the mechanisms responsible for these impedance arcs are interpreted as grain interior conduction ( _gi ), grain boundary conduction (in polycrystalline materials; _gb ), and sample-electrode interface effects, from highest to lowest frequency, respectively. Impedance spectra of natural dunitic rocks reveal analogous behavior, that is, _gb and _gi add in series. The grain boundaries do not enhance the conductivity of any of the materials studied (a direct result of the observed series electrical behavior) and, under certain conditions, limit the total conductivity of the grain interior-grain boundary system. By examining the frequency dependence of the electrical properties of partial-melts, it is possible to gain information about microstructure and the distribution of the melt phase and to determine the conditions under which the presence of melt enhances the total conductivity. Impedance spectra of olivine-basalt partial-melts indicate that at least two conduction mechanisms occur in series over the frequency range 10^–4-10⁵ Hz, similar to the observed electrical response of melt-absent polycrystalline materials. In a sample containing isolated melt pockets the intermediate frequency grain boundary impedance arc is modified by the presence of melt indicating series conduction behavior. In a sample with an interconnected melt phase the high frequency grain interior impedance arc is modified by the melt phase, indicating the initiation of parallel conduction behavior. Because field EM response versus frequency curves are used to derive conductivity versus depth profiles, it is important to perform laboratory experiments to understand the frequency-dependent electrical behavior of Earth materials. Activation energies determined from studies that measure conductivity at a single frequency may be erroneous because of the shift of the dominant conduction mechanism with frequency as temperature is varied.