[S-IT20] MAGMA, FLUID TRANSPORT, AND SEISMICITY IN THE EARTH'S INTERIOR

[SIT20-P01] Electrical conductivity in texturally equilibrated fluid-bearing forsterite aggregates at 800ºC and 1 GPa: implications for the high electrical conductivity anomalies in mantle wedges

*Yongsheng HUANG¹, Haihao Guo², Takayuki Nakatani^1,4, Kentaro Uesugi³, Michihiko Nakamura¹, Hans Keppler² (1.Graduate School of Science, Tohoku University, 2.Bayerisches Geoinstitut, University of Bayreuth, 3.Spring-8, Japan Synchrotron Radiation Research Institute (JASRI), 4.Geological Survey of Japan)

Keywords:Electrical conductivity, Brine-bearing forsterite aggregates, Textural equilibrium, X-ray CT, Mantle wedge, Subduction zones

In subduction zones, aqueous fluids released from the subducting slab percolate through the mantle wedge, controlling arc magma genesis and element recycling. Because the presence of fluid significantly enhances the bulk electrical conductivity of rocks, the distribution of fluids in subduction zones may be imaged by magnetotelluric (MT) observations. Recently, a growing number of MT observations have revealed high electrical conductivity anomalies (0.001–1 S/m) in the deep fore-arc crust and mantle wedge in some subduction zones. The precise determination of fluid fraction is essential to understand bulk physical-properties such as rheology and permeability, and the geophysical-state of the mantle. Laboratory-based electrical conductivity measurements are an effective method to estimate the fluid distribution and fraction in a fluid-bearing rock. In this study, the electrical conductivity of texturally equilibrated fluid-bearing forsterite aggregates was for the first time measured for various fluid fractions at a constant salinity of 5.0 wt.% NaCl at 1 GPa and 800 °C. We found that the electrical conductivity nonlinearly increases with increasing fluid fraction, and the data can be well reproduced by the modified Archie’s law model. Three-dimensional (3-D) microstructure of interstitial-pores visualized by the high-resolution synchrotron X-ray computed micro-tomography shows a change in fluid distribution from isolated pockets at fluid fraction of 0.51 vol.% to interconnected networks at fluid fractions of 2.14 vol.% and above due to grain anisotropy and grain size differences, accounting for the nonlinear-increase in the electrical conductivity. The rapid-enhancement of conductivity indicated that there is a threshold fluid fraction between 0.51 and 2.14 vol.% for forming interconnected fluid networks, which is consistent with the 3-D images. Our results provide direct evidence that > 1.0 vol.% aqueous fluid with 5.0 wt.% NaCl is required to explain the high conductivity anomalies above 0.01 S/m detected in deep fore-arc mantle wedges. This work was supported by JSPS Japanese–German Graduate Externship, International Joint Graduate Program in Earth and Environmental Sciences, Tohoku University (GP-EES).