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Experimental constraints on the electrical anisotropy of the lithosphere–asthenosphere system

Nature volume 522, pages 202–206 (2015)Cite this article

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Abstract

The relative motion of lithospheric plates and underlying mantle produces localized deformation near the lithosphere–asthenosphere boundary1. The transition from rheologically stronger lithosphere to weaker asthenosphere may result from a small amount of melt or water in the asthenosphere, reducing viscosity1,2,3. Either possibility may explain the seismic and electrical anomalies that extend to a depth of about 200 kilometres4,5. However, the effect of melt on the physical properties of deformed materials at upper-mantle conditions remains poorly constrained6. Here we present electrical anisotropy measurements at high temperatures and quasi-hydrostatic pressures of about three gigapascals on previously deformed olivine aggregates and sheared partially molten rocks. For all samples, electrical conductivity is highest when parallel to the direction of prior deformation. The conductivity of highly sheared olivine samples is ten times greater in the shear direction than for undeformed samples. At temperatures above 900 degrees Celsius, a deformed solid matrix with nearly isotropic melt distribution has an electrical anisotropy factor less than five. To obtain higher electrical anisotropy (up to a factor of 100), we propose an experimentally based model in which layers of sheared olivine are alternated with layers of sheared olivine plus MORB or of pure melt. Conductivities are up to 100 times greater in the shear direction than when perpendicular to the shear direction and reproduce stress-driven alignment of the melt. Our experimental results and the model reproduce mantle conductivity–depth profiles for melt-bearing geological contexts. The field data are best fitted by an electrically anisotropic asthenosphere overlain by an isotropic, high-conductivity lowermost lithosphere. The high conductivity could arise from partial melting associated with localized deformation resulting from differential plate velocities relative to the mantle, with subsequent upward melt percolation from the asthenosphere.

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Figure 1: Anisotropic conductivity models obtained from inversion of electromagnetic data in different geological contexts.
Figure 2: Electrical conductivity–temperature diagrams of experimental results.
Figure 3: Comparison between the electrical anisotropy of deformed materials measured in laboratory experiments and that determined from field measurements.
Figure 4: Cross-section portrayal of the electrical conductivity of the uppermost mantle in a melt-bearing context.

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Acknowledgements

This work benefited from funding by the NSF through Cooperative Studies of the Earth’s Deep Interior (CSEDI) projects number 1461594 to the University of California San Diego, number 1265395 to Arizona State University, and number 1265428 to the University of Minnesota. This research was partially supported by COMPRES (the Consortium for Materials Properties Research in Earth Sciences) under National Science Foundation (NSF) Cooperative Agreement EAR 11-57758 (cell assembly development project). We thank A. von der Handt (University of Minnesota) and H. Cathey (Arizona State University) for their assistance with the electron microprobe and scanning electron microscope imaging, respectively. A.P. appreciates help with calibration experiments from staff and scientists at Bayerisches GeoInstitut (BGI). This paper is Lunar and Planetary Institute (LPI) contribution number 1840.

Author information

Authors and Affiliations

  1. University of California San Diego, Scripps Institution of Oceanography, Institute of Geophysics and Planetary Physics, La Jolla, 92093, California, USA

    Anne Pommier

  2. School of Earth and Space Exploration, Arizona State University, Tempe, 85287, Arizona, USA

    Anne Pommier, Edward J. Garnero & James A. Tyburczy

  3. Department of Chemistry and Biochemistry, Arizona State University, Tempe, 85287, Arizona, USA

    Kurt Leinenweber

  4. Department of Earth Sciences, University of Minnesota, Minneapolis, 55455, Minnesota, USA

    David L. Kohlstedt & Chao Qi

  5. Lunar and Planetary Institute, Universities Space Research Association, Houston, 77058, Texas, USA

    Stephen J. Mackwell

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Contributions

A.P. proposed and led the project, performed the conductivity experiments and the electron microprobe analyses, interpreted the results, wrote the first draft and made the figures. K.L. and A.P. developed the conductivity setup at Arizona State University. D.L.K. and C.Q. synthesized the deformed starting materials and provided some of the scanning electron microscope images. E.J.G., S.J.M. and D.L.K. contributed to the data interpretation and discussion. All authors commented on the manuscript and provided editorial assistance.

Corresponding author

Correspondence to Anne Pommier.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Experimental protocol.

a, Starting material preparation. Deformed material is synthesized in a gas-medium apparatus at 300 MPa (left), and its outer part is extracted for electrical measurements in the multi-anvil apparatus at about 3 GPa (right). Two orientations of the sample are considered, leading to vertical (blue) and tangential (purple) electrical measurements. b, Cross-section of the electrical conductivity cell (14/8 multi-anvil assembly, that is, the corner-truncation edge length is 8 mm and the pressure media edge length is 14 mm). Both electrodes are made of W–Re thermocouple wire with one electrode also serving as a thermocouple. c, Example of a complex impedance spectrum (real part Z′ versus imaginary part Z′′) for a sheared sample of olivine + 5 vol% MORB sample at 750 °C and approximately 3 GPa. The intersection between the response of the sample (blue dots, each dot corresponding to one frequency) with the real axis corresponds to the electrical resistance of the sample. The corresponding electrical conductivity value is obtained using the geometric factor (the surface of the electrode divided by thickness of the sample).

Extended Data Figure 2 Texture and melt geometry.

a, Back-scattered electron image of sheared sample PT0683-2 after electrical measurements, showing that deformation-induced melt texture was preserved during the experiment in the multi-anvil apparatus. b, Back-scattered electron image of sheared sample PT0756-2 after electrical measurements, illustrating the location of melt amongst the olivine grains. c, Starting material PT0683 showing the presence of melt-rich bands. d, Back-scattered electron image of sheared sample PT0683-1HT after electrical measurements at high temperature (up to 1,573 K). The absence of pronounced melt-rich bands suggests a loss in structural anisotropy, attributed to the effect of high temperature. These observations are consistent with electrical data that showed a noticeable decrease in electrical anisotropy with increasing temperature. e, Back-scattered electron image of sheared sample PT0683-1LT after electrical measurements at low temperature (<1,073 K), showing several light-coloured zones identified as melt-rich bands. The rectangle corresponds to the location of image f below. f, Zoom on melt-rich bands in sample PT0683-1LT. Melt is also present between the melt-rich zones as pockets amongst the olivine grains. g, Map of sodium distribution in sample PT0742-2 after electrical measurements. Colours correspond to the number of counts. Warm colours (reddish) correspond to high sodium concentration and are interpreted as pockets of carbonatite melt between olivine grains.

Extended Data Table 1 Description of the starting materials.
Full size table
Extended Data Table 2 Electron microprobe analyses of most of the run products
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Extended Data Table 3 Electrical results for polycrystalline olivine materials
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Extended Data Table 4 Electrical results for melt-bearing materials
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Extended Data Table 5 Uncertainties on electrical conductivity values for the experiment on sample PT0683-2
Full size table

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Pommier, A., Leinenweber, K., Kohlstedt, D. et al. Experimental constraints on the electrical anisotropy of the lithosphere–asthenosphere system. Nature 522, 202–206 (2015). https://doi.org/10.1038/nature14502

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