INTERPRETATION OF UPPER-MANTLE ELECTRICAL CONDUCTIVITIES

Since most past attempts to interprete the upper-mantle electrical conductivities derived from induction theory appear content to support temperature distributions calculated with out-dated static earth models, this paper explores the possibility of finding solutions that are compatible with current notions of a dynamic earth. This has necessarily meant abandoning olivine conductivity functions as a basis for the interpretation. The key notion of the heat transport process as a self-regulating mechanism for viscosity suggests that viscosity rather than temperature be used as the principle variable controlling conductivity. Using a fundamental relation, expressed as an inequality, between measured values of conductivity and viscosity, it is shown that consistent interpretations are possible involving average temperatures many hundreds of degrees below the usual estimates. Local conductivity anomalies are attributed to deformational heating which has a pronounced tendency to become spatially concentrated in zones. The conductivity of oceanic mantle as well as its low Q at depths ~100-400 km is thought to be due to an aqueous phase. © 1979.