Electrical Conductivity of Talc Dehydration at High Pressures and Temperatures: Implications for High‐Conductivity Anomalies in Subduction Zones

• Published in: Journal of geophysical research. Solid earth 2020-10, Vol.125 (10), p.n/a
• Main Authors: Wang, Libing, Wang, Duojun, Shen, Kewei
• Format: Article
• Language: English
• Subjects: Analytical methods; Anomalies; Cations; Dehydration; Electrical conductivity; Electrical resistivity; Geophysics; high pressure and temperature; Inflection points; Ion migration; Minerals; Pressure; Pressure effects; Silica; silica deposition; Silicon dioxide; Slabs; Spectroscopy; Subduction; Subduction (geology); subduction zone; Subduction zones; Talc; Temperature
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2020JB020091

The dehydration of hydrous minerals is one of the causes of high‐conductivity anomalies in subduction zones. To determine the origin of these anomalies, the trade‐off between the dehydration and conduction mechanisms of hydrous minerals at high pressures and temperatures should be clarified. Talc is a typical hydrous mineral in hot subduction zones, and previous studies may have underestimated its contribution to high‐conductivity anomalies. We report the new electrical conductivity results for talc, which were measured at 1.0–4.0 GPa and 523–1293 K using impedance spectroscopy. The pressure effect on conductivity of talc is obvious in the different heating stages. The pressure decreased the conductivity prior to dehydration and remarkably increased the conductivity during dehydration. A sharp conductivity increase was observed beyond the dehydration temperature, and the maximum conductivity was 0.1 S/m. The increase in conductivity associated with a high activation energy of 284.5 ± 11.8 kJ/mol and an activation volume of −6.2 ± 0.6 cm3/mol was attributed to an inhomogeneous dehydration model involving cation migration. The talc dehydration temperatures at different pressures derived from the conductivity inflection points are 1023–1093 K. The increased electrical conductivity produced by talc ongoing dehydration provides an explanation for the high‐conductivity anomalies observed at deep depths in hot subduction zone. The silica‐rich fluid released by talc may contribute to the silica deposition in plate interface and induce the high‐conductivity anomalies observed at shallow depths in the hot subduction zones. Plain Language Summary Geophysical observations have revealed many anomalously high electrical conductivity regions in subduction zones. The dehydration of hydrous minerals may induce high‐conductivity anomalies in subduction zones, but how the dehydration affects the electrical conductivity remains unclear. We report new results of high‐pressure conductivity measurements on talc at 1.0–4.0 GPa and up to 1293 K. We found a sharp conductivity increase above the dehydration temperature, and the maximum conductivity was ~0.1 S/m. We also found the increase in conductivity was attributed to cations migration. The experimental results were analyzed to interpret the geophysical observations in subduction zones. We conclude that dehydration provides an explanation for the high‐conductivity anomalies observed in some hot subduction zones. These results wi