Spin Transition of Ferric Iron in Silicate Glasses Inferred by High‐Pressure Electrical Conductivity Measurements

Silicate melt has been proposed to exist near the base of the mantle and has been invoked to explain seismic and electrical conductivity anomalies observed near the core-mantle boundary; however, its presence and stability under lowermost-mantle conditions remain uncertain. Here we report high-pressure electrical conductivity measurements of Fe2+- and Fe3+-bearing pyroxene glasses (Fe3+/ΣFe ≈ 0.5), used as analogs of silicate melts, up to megabar pressures at room temperature. At lower pressures, conductivity increases with pressure and iron content, consistent with electron-hole hopping between Fe2+ and Fe3+. Above 77–85 GPa, all samples show a marked conductivity decrease, suggesting a pressure-induced spin transition of Fe3+. Previous studies have suggested that lower-mantle silicate melts may be enriched in Fe3+, and iron spin-state changes may modify iron partitioning between silicate melts and coexisting crystalline phases. Therefore, the Fe3+ spin transition inferred here may affect the evolution of deep-mantle melts through pressure-dependent changes in iron partitioning behavior.