Heterogeneity of Electrical Conductivity in the Oceanic Upper Mantle

Abstract

We discuss conductivity heterogeneities of the oceanic upper mantle using available experimental data. The activation energy of the polaron conduction in olivine, wadsleyite, and ringwoodite is similar, at 1.4–1.6 eV. The ionic conduction is significant in olivine, but not in wadsleyite and ringwoodite. Its activation energy is much larger than that of the small polaron conduction (7–12 eV). The proton conductions have a smaller activation energy than the small polaron conduction (less than 1 eV) and are negligible at high temperatures in the depleted MORB mantle . The effects of the secondary minerals are negligible. No significant conductivity jump is associated with the olivine–wadsleyite transition. Volatile components greatly increase conductivity of basaltic melt. The anisotropy in both intrinsic and proton conditions in olivine is small. Sheared, partially molten peridotite can show conductivity anisotropy. The high conductivity below mid-oceanic ridges could be caused by partial melting. Conductivity at several locations suggests a melt fraction of the order of 0.1 vol.%, whereas that under the East Pacific Rise at 9°N suggests one of 15 vol.%. Lithosphere has low conductivity, which should be primarily due to its low temperature. However, the conductivity is too high, judging from the temperature structure and intrinsic conduction of olivine. The circulation of water does not provide enough explanation. The high-conductivity layer at the top of the asthenosphere is not a ubiquitous feature of the mantle—it is relatively limited to regions under young plates. The associated conductivity anisotropy suggests its partial melting origin. The conductivity in the mantle transition zone can be explained by the intrinsic conduction of wadsleyite and ringwoodite. Estimations of water content in the transition zone are largely affected by the uncertainty of geophysical modeling. The MT studies do not detect mantle plumes, although the seismic studies show the presence of low-velocity zones. The conductivity anomalies, whose origins are not understood, are observed under the southern Philippine Sea and the broad region north of Hawaii.