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Experimental constraints on the electrical anisotropy of the lithosphere–asthenosphere system
| Content Provider | Semantic Scholar |
|---|---|
| Author | Pommier, Anne Leinenweber, Kurt Kohlstedt, David Lee Qi, Chao Garnero, Edward J. Mackwell, Stephen J. Tyburczy, James A. |
| Copyright Year | 2015 |
| Abstract | The relative motion of lithospheric plates and underlying mantle produces localized deformation near the lithosphere–asthenosphere boundary. The transition from rheologically stronger lithosphere to weaker asthenosphere may result from a small amount of melt or water in the asthenosphere, reducing viscosity. Either possibility may explain the seismic and electrical anomalies that extend to a depth of about 200 kilometres. However, the effect of melt on the physical properties of deformed materials at upper-mantle conditions remains poorly constrained. 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 themantle, with subsequent upwardmelt percolation from the asthenosphere. Electromagnetic profiles of the lithosphere–asthenosphere system reveal zones of high electrical conductivity and electrical anisotropy, which vary with depth (Fig. 1). High conductivity can be attributed to several factors, including the presence of an interconnected fluid phase. Regions of electrical anisotropy are usually attributed tomantle deformation that can result from the motion of rigid lithospheric plates relative to the underlying convecting mantle. Experimental investigations under controlled laboratory conditions allow a direct assessment of the effect of deformation and chemistry on electrical conductivity, an important step in investigating the dynamic coupling of tectonic plates with the underlying mantle. The current laboratory-derived database of electrical anisotropy of mantle materials consists principally of measurements of electrical conductivity s for different crystallographic orientations of dry and hydrous olivine single crystals. Only one set of measurements has been made for s of melt-bearing olivine aggregates during torsion, and these experiments were performed at low crustal pressure (0.3 GPa) and only to low shear strain (c, 0.5–1), limiting the formation of noticeable melt bands. Recently, new electrical measurements on melt 1 olivine aggregates were performed during simple shear at 3 GPa, but only small strains (c , 1.8) were reached. Here we report the results of laboratory experiments at asthenospheric pressure (about 3GPa) and on samples previously deformed to high strains (c < 9). These experiments were designed to investigate electrical anisotropy in deformedmantlematerials, in order to develop an electrical model of the upper mantle to be compared with field results. The electrical anisotropy of mantle materials was investigated by measuring the electrical conductivity of previously deformed sam- |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://garnero.asu.edu/publications/g101_Pommier,etal_Nature2015.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
