FREQUENCY-DEPENDENT ELECTRICAL-PROPERTIES OF MINERALS AND PARTIAL-MELTS

The resistance to current flow of minerals and partial-melts is a freq uency dependent electrical property. Measurements of the frequency dep endent electrical impedance of single crystal olivine, polycrystalline olivine, dunites, metapelites, and partial-melts, between 10(-4) and 10(5) Hz, when plotted in the complex impedance plane, reveal arcs tha t correspond to different conduction mechanisms in the material being studied. In polycrystalline materials, two impedance arcs related to m aterial properties (as opposed to electrode properties or electrode-sa mple 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 configuration s, and on samples of varying dimension, the mechanisms responsible for these impedance arcs are interpreted as grain interior conduction (si gma(gi)), grain boundary conduction (in polycrystalline materials; sig ma(gb)), and sample-electrode interface effects, from highest to lowes t frequency, respectively. Impedance spectra of natural dunitic rocks reveal analogous behavior. that is, sigma(gb) and sigma(gi) add in ser ies. The grain boundaries do not enhance the conductivity of any of th e 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 frequen cy dependence of the electrical properties of partial-melts, it is pos sible to gain information about microstructure and the distribution of the melt phase and to determine the conditions under which the presen ce of melt enhances the total conductivity. Impedance spectra of olivi ne-basalt partial-melts indicate that at least two conduction mechanis ms occur in series over the frequency range 10(-4)-10(5) Hz, similar t o the observed electrical response of melt-absent polycrystalline mate rials. In a sample containing isolated melt pockets the intermediate f requency grain boundary impedance arc is modified by the presence of m elt indicating series conduction behavior. In a sample with an interco nnected melt phase the high frequency grain interior impedance arc is modified by the melt phase, indicating the initiation of parallel cond uction 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 e rroneous because of the shift of the dominant conduction mechanism wit h frequency as temperature is varied.