Constraints from the dehydration of antigorite on high-conductivity anomalies in subduction zones

Abstract

Regions with high electrical conductivities in subduction zones have attracted a great deal of attention. Determining the exact origin of these anomalies could provide critical information about the water storage and cycling processes during subduction. Antigorite is the most important hydrous mineral within deep subduction zones. The dehydration of antigorite is believed to cause high-conductivity anomalies. To date, the effects of dehydration on the electrical conductivity of antigorite remain poorly understood. Here, we report new measurements of the electrical conductivity of both natural and hot-pressed antigorite at pressures of 4 and 3 GPa, respectively, and at temperatures reaching 1073 K. We observed significantly enhanced conductivities when the antigorite was heated to temperatures beyond its thermodynamic stability field. Sharp increases in the electrical conductivity occurred at approximately 848 and 898 K following the decomposition of antigorite to forsterite, enstatite and aqueous fluids. High electrical conductivities reaching 1 S/m can be explained by the presence of an interconnected network of conductive aqueous fluids. Based on these results for the electrical conductivity of antigorite, we conclude that high-conductivity regions associated with subduction zones can be attributed to dehydration-induced fluids and the formation of interconnected networks of aqueous fluids during the dehydration of antigorite.

Effect of chemical composition on the electrical conductivity of gneiss at high temperatures and pressures

“…Aside from the chemical composition, other available alternative causes for high conductivity anomalies can be considered, such as water in nominally anhydrous minerals (Wang et al, 2006;Yang, 2011;Karato, 2009, 2014a), interconnected saline (or aqueous) fluids (Hashim et al, 2013;Shimojuku et al, 2014;Sinmyo and Keppler, 2017;Guo et al, 2015;Li et al, 2018), partial melting (Wei et al, 2001;Maumus et al, 2005;Gaillard et al, 2008;Ferri et al, 2013;Laumonier et al, 2015Laumonier et al, , 2017Ghosh and Karki, 2017), interconnected secondary high conductivity phases (e.g., FeS, Fe 3 O 4 ; Jones et al, 2005;Bagdassarov et al, 2009;Padilha et al, 2015), dehydration of hydrous minerals (Wang et al, 2012(Wang et al, , 2017Manthilake et al, 2015Manthilake et al, , 2016Hu et al, 2017;Sun et al, 2017a, b;Chen et al, 2018) and graphite films on mineral grain boundaries (Freund, 2003;Pous et al, 2004;Chen et al, 2017). In consideration of the similar formation conduction and geotectonic environments, the Himalaya-Tibetan orogenic system was compared with the Dabie-Sulu UHPM belt and explained high electrical conductivity anomalies.…”

Response to reviewer 1

“…A regional metamorphic ultrahigh-pressure belt for the Dabie-Sulu orogen is a complexly giant geotectonic unit in central-eastern China. Geophysical exploration results confirmed that a large number of high conductivity anomalies have been observed in metamorphic belts (Xiao et al, 2007;Wannamaker et al, 2009;Zeng et al, 2015). Metamorphic rocks (e.g., slate, schist, gneiss, granulite and eclogite) with different degrees of metamorphism play an important role Published by Copernicus Publications on behalf of the European Geosciences Union.…”

“…Aside from the chemical composition, other available alternative causes for high conductivity anomalies can be considered, such as water in nominally anhydrous minerals (Wang et al, 2006;Yang, 2011;Karato, 2009, 2014a), interconnected saline (or aqueous) fluids (Hashim et al, 2013;Shimojuku et al, 2014;Sinmyo and Keppler, 2017;Guo et al, 2015;Li et al, 2018), partial melting (Wei et al, 2001;Maumus et al, 2005;Gaillard et al, 2008;Ferri et al, 2013;Laumonier et al, 2015Laumonier et al, , 2017Ghosh and Karki, 2017), interconnected secondary high conductivity phases (e.g., FeS, Fe 3 O 4 ; Jones et al, 2005;Bagdassarov et al, 2009;Padilha et al, 2015), dehydration of hydrous minerals (Wang et al, 2012(Wang et al, , 2017Manthilake et al, 2015Manthilake et al, , 2016Hu et al, 2017;Sun et al, 2017a, b;Chen et al, 2018) and graphite films on mineral grain boundaries (Freund, 2003;Pous et al, 2004;Chen et al, 2017). In consideration of the similar formation conduction and geotectonic environments, the Himalaya-Tibetan orogenic system was compared with the Dabie-Sulu UHPM belt and explained high electrical conductivity anomalies.…”

Effect of temperature, pressure and chemical composition on the electrical conductivity of granulite and geophysical implications

“…This result indicates that the calcium and alkali ions were not the main charge carriers in the granulite samples. Previous studies have proposed that hydrogenrelated defect or small polaron conduction is the dominant conduction mechanism for many iron-bearing hydrous minerals and rocks, e.g., talc, chlorite, mudstone, epidote, phyllite, amphibole-bearing rocks, and serpentine (Zhu et al, 1999;Guo et al, 2011;Wang et al, 2012;Manthilake et al, 2016;Hu et al, 2017;Sun et al, 2017aSun et al, , 2017bWang et al, 2017). ΔH for iron-bearing hydrous minerals and rocks with hydrogen-related conduction is~0.60-0.80 eV (Guo et al, 2011;Hu et al, 2017), and~0.55-0.80 eV for iron-bearing hydrous minerals and rocks with a small polaron conduction (Wang et al, 2012;Manthilake et al, 2016;Sun et al, 2017b).…”