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Electrical conductivity in texturally equilibrated fluid-bearing forsterite aggregates at 800 ºC and 1 GPa: implications for the high electrical conductivity anomalies in mantle wedges
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  • Yongsheng Huang,
  • Haihao Guo,
  • Takayuki Nakatani,
  • Kentaro Uesugi,
  • Michihiko Nakamura,
  • Hans Keppler
Yongsheng Huang
Tohoku University

Corresponding Author:[email protected]

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Haihao Guo
Bayerisches Geoinstitut
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Takayuki Nakatani
Tohoku University
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Kentaro Uesugi
SPring-8/Japan Synchrotron Radiation Research Institute
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Michihiko Nakamura
Tohoku University
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Hans Keppler
Bayerisches Geoinstitut, University of Bayreuth
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Aqueous fluids are one of the principal agents of chemical transport in Earth´s interior. The precise determination of fluid fractions 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 for estimating the fluid distribution and fraction in a fluid-bearing rock. In this study, the electrical conductivity of texturally equilibrated fluid-bearing forsterite aggregates was measured for the first time with 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. The three-dimensional (3-D) microstructure of the interstitial pores visualized by the high-resolution synchrotron X-ray computed micro-tomography (CT) shows a change in fluid distribution from isolated pockets at a 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 electrical conductivity. The rapid increase in conductivity indicates 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 the presence of > 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.
Apr 2021Published in Journal of Geophysical Research: Solid Earth volume 126 issue 4. 10.1029/2020JB021343