Frequency dependent electrical properties of dunite as functions of temperature and oxygen fugacity

Using impedance spectroscopy, we have measured the electrical properties of two dunites and a single crystal olivine sample from 1000 to 1200° C as a function of oxygen fugacity (f_o2). Two conduction mechanisms with resistances that add in series are observed for the dunites corresponding to grain interior and grain boundary conduction mechanisms. The conductivities for each mechanism were determined by analyzing the data using a complex nonlinear least squares fitting routine and the equivalent circuit approach. The grain interiors display a conductivity dependent on f_o2 to the 1/5.5-1/7 power, consistent with other determinations, and interpreted as indicating small polaron transport (Fe_Mg^·). The grain boundaries demonstrate a weaker f_o2 dependence that is dependent on temperature and material. Under certain conditions the f_o2 dependence of the grain boundary conductivity is negative. This result indicates that oxygen ion transport is probably not the dominant grain boundary charge transport mechanism; however, an unequivocal determination of the grain boundary mechanism has not been achieved. In some dunites the grain boundaries are more conductive than the grain interiors; in other dunites they are more resistive than grain interiors. The grain boundaries do not enhance the total conductivity of any of the materials of this study but are the controlling mechanism in some instances. Measurement of the complex electrical response at frequencies as low as 10^-4 Hz is required to determine the role of grain boundaries on the overall electrical properties of polycrystalline dunite.