FREQUENCY-DEPENDENT ELECTRICAL-PROPERTIES OF DUNITE AS FUNCTIONS OF TEMPERATURE AND OXYGEN FUGACITY

Using impedance spectroscopy, we have measured the electrical properti es of two dunites and a single crystal olivine sample from 1000 to 120 0-degrees-C as a function of oxygen fugacity (f(O2)). Two conduction m echanisms with resistances that add in series are observed for the dun ites corresponding to grain interior and grain boundary conduction mec hanisms. The conductivities for each mechanism were determined by anal yzing the data using a complex nonlinear least squares fitting routine and the equivalent circuit approach. The grain interiors display a co nductivity dependent on f(O2) to the 1/5.5-1/7 power, consistent with other determinations, and interpreted as indicating small polaron tran sport (Fe(Mg).). The grain boundaries demonstrate a weaker f(O2) depen dence that is dependent on temperature and material. Under certain con ditions the f(O2) dependence of the grain boundary conductivity is neg ative. This result indicates that oxygen ion transport is probably not the dominant grain boundary charge transport mechanism; however, an u nequivocal determination of the grain boundary mechanism has not been achieved. In some dunites the grain boundaries are more conductive tha n the grain interiors; in other dunites they are more resistive than g rain interiors. The grain boundaries do not enhance the total conducti vity of any of the materials of this study but are the controlling mec hanism in some instances. Measurement of the complex electrical respon se at frequencies as low as 10(-4) Hz is required to determine the rol e of grain boundaries on the overall electrical properties of polycrys talline dunite.