Numerical simulation of the graphite effect on the electrical conductivity of the upper mantle

Abstract

SUMMARY The origin of high-conductivity anomalies in the deep Earth is one of the hot issues in geoscience research. The presence of graphite is a possible reason, but the effects of the volume fraction and geometry of graphite on the high-conductivity anomaly in the upper mantle are still controversial. Based on the possible morphology of graphite in the uppermost mantle, graphite–olivine–orthopyroxene assemblage models are constructed and their conductivities are calculated by a finite-element method. The results show that when graphite is isolated from each other, temperature is the main factor that leads to a change in electrical conductivity. When graphite is in contact with each other, increases in the diameter/thickness ratio (β), orientation arrangement along the direction of conduction, and a mixture of powdered and disc-shaped graphite can significantly cause an increase in electrical conductivity. We found that a threshold value of graphite content exists at which the model conductivity suddenly increases. The threshold decreases significantly with increasing β. A model with β larger than 25 could explain the high-conductivity anomaly in the upper mantle.