Electrical conductivity of olivine, a dunite, and the mantle : Constable, S; Duba, A J geophys res V95, NB5, May 1990, P6967–6978

Abstract

Laboratory studies of the electrical conductivity of rocks and minerals are vital to the interpretation of electromagnetic soundings of the Earth's mantle. To date, the most reliable data have been collected from single crystals. We have extended these studies with electrical conductivity measurements on a dunite from North Carolina, in the temperature range of 600o-1200oC and under controlled oxygen fugacity. Observations of conductivity as a function of oxygen fugacity and temperature demonstrate that conduction in the dunite is indistinguishable from conduction in single olivine crystals. Thus the common practice of exaggerating the single-crystal conductivities to account for conduction by grain boundary phases in the mantle is unnecessary. Because the dunite conductivity is consistent with that published for single crystals under similar conditions, we have made a combined analysis of these data. Conductivity as a function of temperature between 600 o and 1450oC displays three conduction mechanisms whose activation energies may be recovered by nonlinear least squares fitting, yielding activation e ergies of 0.21 42.56 x 10 -19 J (0.13 41.60 eV) below 720oC, 2.56 40.02 x 10 -19 J (1.60 40.01 eV) between 720oC and 1500oC and 11.46 40.90 x 10 -19 J (7.16 40.56 eV) above 1500oC. The behavior of conductivity as a function of oxygen fugacity is well explained by a model in which an fo2-independent population of charge carriers is supplemented at high oxygen fugacities with a population that is proportional to fo20'3. This parametrization produces a clear correlation f the fo2 dependent term with iron content, which is otherwise obscured by variations inconductivity among olivines.