High Electrical Conductivity of Olivine at Oxidizing Conditions of the Shallow Mantle and Geophysical Implications

High electrical conductivity in the shallow mantle has long been recognized by electromagnetic depth soundings, but the origin remains debated. Various candidates, of which the two most popular are water enrichment in olivine (the dominant mineral in the upper mantle) and partial melt, have been proposed to explain the electrical anomalies. These models nearly exclusively assume a very low conductivity for dry or water‐poor olivine. Here, we show by experimental work that, under well‐controlled conditions of pressure, temperature, oxygen fugacity (fO2), and water inventory, the conductivity of olivine is in fact insensitive to water content at mantle temperature (e.g., >1,000°C), although the enhancement is substantial at relatively low temperature. In particular, the conductivity of olivine at temperature beyond ∼1,000°C is strikingly high, about 0.01–0.1 S/m, at the oxidizing conditions of the shallow mantle. The high conductivity is comparable to the geophysically measured high conductivity in many conductive regions of the shallow mantle. The results provide a new framework for understanding the electrical anomalies and electrical structure of the shallow mantle. Plain Language Summary Highly conductive zones at ∼50–150‐km depths in the shallow mantle have long been geophysically resolved, but the origin remains debated. It is commonly assumed that the electrical conductivity of dry and water‐poor olivine is very low, so that the imaged electrical anomalies are caused by other conductive candidates. By carefully controlling the various factors that may affect the electrical conductivity of olivine, the dominant mineral in the upper mantle that determines largely the electrical structure, we provide experimental evidence that the conductivity of olivine is actually much higher at the oxidizing conditions of the shallow mantle than what has been assumed previously. This offers new insights into the electrical structure and dynamics of the shallow mantle. Key Points Electrical conductivity of olivine at well‐controlled conditions of pressure, temperature, redox state, and water inventory High electrical conductivity of olivine at oxidizing conditions of the shallow mantle A new framework for understanding electrical anomalies in the shallow mantle