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Electrical conductivities of molten Hawaiian tholeiite and Crater Lake andesite were measured between 1200°C and 1400°C at atmospheric pressure and at pressures up to 17and 25 kbar, respectively. Isobaric plots of log σ versus 1/T (σ is electrical conductivity) are linear, with the exception of the zero pressure tholeiite melt data. Conductivities decrease with increasing pressure in both melts, with the andesitic melt exhibiting a greater pressure dependence. Between 5 and 10 kbar, abrupt decreases in the slopes of isothermal log σ versus P plots (i.e., decreases in activation volume) are observed for both rock melts. This discontinuity probably reflects changes in melt structure, as opposed to changes in conduction mechanism. In each pressure range, the data for each rock melt can be described reasonably well by an equation of the form σ = σ′0 exp[−(Ea + PΔV′σ)/kT], where σ′0 is a preexponential constant, Ea is the activation energy, and ΔV′σ is the activation volume. A qualitative model involving depolymerization of the melt with increased pressure leading to increased efficiency of packing can explain the observed discontinuity in activation volume as well as the observed pressure dependences of other melt physical properties such as viscosity and density. Conductivity versus melt fraction curves for partially molten peridotite are reevaluated using high pressure tholeiitic melt conductivities and crystalline conductivity values recently determined by other workers. Minimum melt fraction estimates of 5–10% are required to explain upper mantle regions of anomalously high electrical conductivity in terms of a partial melting hypothesis.

Abstract. The electrical conductivity of a partial melt is influenced by many factors, including melt conductivity, crystalline conductivity, and melt fraction, each of which is influenced by temperature. We have performed measurements of bulk conductivity as a function of temperature of an Fo80-basalt partial melt between 684 ø and 1244øC at controlled oxygen fugacity. Melt fraction and composition variations with temperature calculated using MELTS [Ghiorso and Sack, 1995] indicate that the effect on melt conductivity of changing melt composition is balanced by changes in temperature (T). Thus bulk conductivity as a function of T or melt fraction in this system can be calculated assuming a constant melt conductivity. The bulk conductivity is well mod-

In this paper we examine the mechanical and thermodynamic criteria which are necessary for equilibrium to exist in partial melts that are subject to hydrostatic stress. Existing theory satisfactorily demonstrates (1) that crystal‐liquid interfaces will have no sharp edges and will approach constant curvature and (2) that the liquid will form a continuously interconnected network by distributing itself in channels along intergranular edge intersections provided that the liquid wetting angle is less than 60°. However, the full textural implications of the thermodynamics have apparently not been realized previously. Currently accepted theoretical developments have been extended to show how equilibrium melt distribution varies with wetting angle. We find that contrary to a widely accepted view a homogeneous fluid phase cannot completely wet intergranular faces, regardless of the exact wetting angle. As a result the intercrystalline faces will remain in mechanical contact throughout the assemblage even when it is characterized by a 0° dihedral angle.

The complex electrical properties of poly crystalline San Carlos olivine compacts were measured over the range of frequency l0−4–104Hz from 800° to 1400°C under controlled oxygen fugacity. The impedance data display a strong frequency dependence that is evidenced most clearly when the results are displayed in the complex impedance plane. A parameterized model of the frequency dependent electrical response using equivalent electrical circuits is presented. Two distinct conduction mechanisms of the sample are observed: grain interior and grain boundary conduction. Each occurs over a different range of frequency. The resistance of each mechanism adds in series resulting in a lower total DC conductivity for polycrystalline olivine than for either mechanism separately. The total DC conductivity is dominated by the grain interior conductivity above 1200°C, whereas the grain boundary conductivity has the strongest influence below 1000° C. Impedance spectra of natural dunite samples exhibit a similar type of frequency dependence. The grain interior conductivity displays a change in slope at 1344°C and has activation energies of 1.45 eV (800°–1344°C) and 4.87 eV (>1344°C). The grain boundary conductivity has an activation energy of 2.47 eV. In these cases, the ƒO2 for each experimental run was controlled at that of the wustite‐magnetite oxygen buffer. Experiments on samples with different grain sizes reveal no dependence of DC conductivity on grain size for either mechanism, although the relaxation time and real relative permittivity of the grain boundary mechanism are dependent on grain size. Because of the electrical response observed at low frequencies, care must be taken in the inversion of electromagnetic field observations using laboratory measurements made in the kilohertz range since they may not be the same as DC measurements. Impedance measurements must be performed over a range of relatively low frequencies to assess the role of grain boundaries on the overall electrical response of polycrystalline materials.