Linking electrical anomalies in the upper mantle to petrological and geochemical heterogeneities

TL;DR

This paper explores how electrical anomalies in Earth's upper mantle are linked to variations in its composition and chemistry.

Contribution

The study identifies specific mantle materials that can cause electrical anomalies under certain conditions.

Findings

Four materials—olivine, certain lithologies, partial melt, and aqueous fluids—can cause detectable electrical anomalies.

Hydrous minerals and non-silicate minerals like graphite are unlikely to cause mantle electrical anomalies.

Mantle heterogeneities lead to heterogeneous electrical structures detectable through geophysical surveys.

Abstract

The electrical structure of Earth’s interior, resolved by using geophysical surveys, is key to understanding its composition, dynamics and relevant properties. Electrically anomalous zones in the upper mantle have been frequently observed, yet the origin remains debated. The geophysically imaged electrical anomalies cannot be properly interpreted if the constraints on the mantle materials from petrological and geochemical surveys and mineral physics experiments are not combined. Studying mantle samples has revealed widespread heterogeneities in their mineral constituents, elemental compositions and thermodynamic properties, in addition to the local occurrence of melts and fluids. The heterogeneities are macroscale, ranging on the levels of meters to kilometers. Four conductive candidates have been identified for the electrical anomalies by using laboratory experiments under mantle conditions, including olivine owing to its oxidized state (but not water), lithologies (such as pyroxenites, eclogites and phlogopite-bearing assemblages due to enriched Fe, water and/or F), partial melt and aqueous fluids. Such materials are able to cause electrical anomalies in a variety of settings that are geophysically detectable, if connected forms, rational fractions and/or suitable temperature and redox states are spatially maintained along certain direction(s). Hydrous minerals except phlogopite (within their stability fields) and non-silicate minerals such as graphite, sulfides and carbonates are usually hard to produce mantle electrical anomalies. Mantle macroscale heterogeneities cause heterogeneous electrical structures. Geophysically imaged electrical anomalies in the upper mantle are intimately related to its petrological and geochemical evolution.

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