Aluminum and Iron Effects on the Electrical Conductivity of the Dense Hydrous Magnesium Silicate Phase E.

The electrical conductivity of pure and Al/Fe‐bearing phase E was measured up to 950 K at 15 GPa using a complex impedance spectroscopy. Pure phase E shows comparable conductivity to that of phase D, and a few orders of magnitude higher than that of phase A and super‐hydrous phase B. Al‐bearing phase E does not exhibit a conductivity difference, while a certain amount of incorporated Fe prominently increases its electrical conductivity by a factor of 4. Unlike the sole substitution 2Al3+→Mg2++Si4+ in phase D and H, H+ is likely involved in the substitution. Proton conduction is the dominant conduction mechanism, while small polaron conduction becomes dominant with increasing Fe content. Phase E in subducted slabs at depth of the upper transition zone cannot explain the high electrical conductivity anomalies beneath the Philippine Sea or Northeast China. Other mechanisms such as dehydration of hydrous minerals is needed to account for them. Plain Language Summary: Some subduction zones exhibit anomalously higher electrical conductivity compared to the average value of mantle, which may be due to the existence of hydrous minerals, of free water from dehydration, or of a small fraction of melt. If it remains a relatively low temperature below the choke point inside the slab during descending, dense hydrous magnesium silicates are potential candidates explaining the high conductivity anomalies. Impedance spectroscopy experiments are effective way to determine the electrical conductivity of the hydrous minerals. Combined with multi‐anvil apparatus, and the results at high pressure and high temperature can be directly applied to Earth's mantle condition. In this study, we synthesized pure and Fe/Al bearing phase E and measured the electrical conductivity. The Fe/Al substitution mechanism and conduction mechanism have been discussed. Applying our conductivity results to the depth of upper transition zone, we found that the high conductivity anomalies cannot be explained by the existence of phase E. Therefore, other mechanisms are needed, such as an existence of free water from dehydration, or an existence of melt. Key Points: The electrical conductivity of pure and Al/Fe‐bearing phase E was measured up to 950 K at 15 GPaIncorporation of Fe prominently increases its electrical conductivity by a factor of 4Phase E in subducted slabs cannot explain the high electrical conductivity anomalies beneath the Philippine Sea or Northeast China [ABSTRACT FROM AUTHOR]