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Pommier, A, Kohlstedt DL, Hansen LN, Mackwell S, Tasaka M, Heidelbach F, Leinenweber K.  2018.  Transport properties of olivine grain boundaries from electrical conductivity experiments. Contributions to Mineralogy and Petrology. 173   10.1007/s00410-018-1468-z   AbstractWebsite

Grain boundary processes contribute significantly to electronic and ionic transports in materials within Earth's interior. We report a novel experimental study of grain boundary conductivity in highly strained olivine aggregates that demonstrates the importance of misorientation angle between adjacent grains on aggregate transport properties. We performed electrical conductivity measurements of melt-free polycrystalline olivine (Fo(90)) samples that had been previously deformed at 1200 degrees C and 0.3 GPa to shear strains up to gamma = 7.3. The electrical conductivity and anisotropy were measured at 2.8 GPa over the temperature range 700-1400 degrees C. We observed that (1) the electrical conductivity of samples with a small grain size (3-6 mu m) and strong crystallographic preferred orientation produced by dynamic recrystallization during large-strain shear deformation is a factor of 10 or more larger than that measured on coarse-grained samples, (2) the sample deformed to the highest strain is the most conductive even though it does not have the smallest grain size, and (3) conductivity is up to a factor of similar to 4 larger in the direction of shear than normal to the shear plane. Based on these results combined with electrical conductivity data for coarse-grained, polycrystalline olivine and for single crystals, we propose that the electrical conductivity of our fine-grained samples is dominated by grain boundary paths. In addition, the electrical anisotropy results from preferential alignment of higher-conductivity grain boundaries associated with the development of a strong crystallographic preferred orientation of the grains.

Pommier, A, Leinenweber K, Kohlstedt DL, Qi C, Garnero EJ, Mackwell SJ, Tyburczy JA.  2015.  Experimental constraints on the electrical anisotropy of the lithosphere-asthenosphere system. Nature. 522:202-+.   10.1038/nature14502   AbstractWebsite

The relative motion of lithospheric plates and underlying mantle produces localized deformation near the lithosphere-asthenosphere boundary(1). The transition from rheologically stronger lithosphere to weaker asthenosphere may result from a small amount of melt or water in the asthenosphere, reducing viscosity(1-3). Either possibility may explain the seismic and electrical anomalies that extend to a depth of about 200 kilometres(4,5). However, the effect of melt on the physical properties of deformed materials at upper-mantle conditions remains poorly constrained(6). 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 lower most 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.

Khan, A, Connolly JAD, Pommier A, Noir J.  2014.  Geophysical evidence for melt in the deep lunar interior and implications for lunar evolution. Journal of Geophysical Research-Planets. 119:2197-2221.   10.1002/2014je004661   AbstractWebsite

Analysis of lunar laser ranging and seismic data has yielded evidence that has been interpreted to indicate a molten zone in the lowermost mantle overlying a fluid core. Such a zone provides strong constraints on models of lunar thermal evolution. Here we determine thermochemical and physical structure of the deep Moon by inverting lunar geophysical data (mean mass and moment of inertia, tidal Love number, and electromagnetic sounding data) in combination with phase-equilibrium computations. Specifically, we assess whether a molten layer is required by the geophysical data. The main conclusion drawn from this study is that a region with high dissipation located deep within the Moon is required to explain the geophysical data. This region is located within the mantle where the solidus is crossed at a depth of approximate to 1200 km (1600 degrees C). Inverted compositions for the partially molten layer (150-200 km thick) are enriched in FeO and TiO2 relative to the surrounding mantle. The melt phase is neutrally buoyant at pressures of similar to 4.5-4.6 GPa but contains less TiO2 (<15 wt %) than the Ti-rich (similar to 16 wt %) melts that produced a set of high-density primitive lunar magmas (density of 3.4 g/cm(3)). Melt densities computed here range from 3.25 to 3.45 g/cm(3) bracketing the density of lunar magmas with moderate-to-high TiO2 contents. Our results are consistent with a model of lunar evolution in which the cumulate pile formed from crystallization of the magma ocean as it overturned, trapping heat-producing elements in the lower mantle.

Khan, A, Pommier A, Neumann GA, Mosegaard K.  2013.  The lunar moho and the internal structure of the Moon: A geophysical perspective. Tectonophysics. 609:331-352.   10.1016/J.Tecto.2013.02.024   AbstractWebsite

Extraterrestrial seismology saw its advent with the deployment of seismometers during the Apollo missions that were undertaken from July 1969 to December 1972. The Apollo lunar seismic data constitute a unique resource being the only seismic data set which can be used to infer the interior structure of a planetary body besides the Earth. On-going analysis and interpretation of the seismic data continues to provide constraints that help refine lunar origin and evolution. In addition to this, lateral variations in crustal thickness (similar to 0-80 km) are being mapped out at increasing resolution from gravity and topography data that have and continue to be collected with a series of recent lunar orbiter missions. Many of these also carry onboard multi-spectral imaging equipment that is able to map out major-element concentration and surface mineralogy to high precision. These results coupled with improved laboratory-based petrological studies of lunar samples provide important constraints on models for lunar magma ocean evolution, which ultimately determines internal structure. Whereas existing constraints on initial depth of melting and differentiation from quantitative modeling suggested only partial Moon involvement (<500 km depth), more recent models tend to favor a completely molten Moon, although the former cannot be ruled out sensu stricto. Recent geophysical analysis coupled with thermodynamical computations of phase equilibria and physical properties of mantle minerals suggest that the Earth and Moon are compositionally distinct. Continued analysis of ground-based laser ranging data and recent discovery of possible core reflected phases in the Apollo lunar seismic data strengthens the case for a small dense lunar core with a radius of <400 km corresponding to 1-3% of lunar mass. (C) 2013 Elsevier B.V. All rights reserved.

Pommier, A, Evans RL, Key K, Tyburczy JA, Mackwell S, Elsenbeck J.  2013.  Prediction of silicate melt viscosity from electrical conductivity: A model and its geophysical implications. Geochemistry Geophysics Geosystems. 14:1685-1692.   10.1002/ggge.20103   AbstractWebsite

Our knowledge of magma dynamics would be improved if geophysical data could be used to infer rheological constraints in melt-bearing zones. Geophysical images of the Earth's interior provide frozen snapshots of a dynamical system. However, knowledge of a rheological parameter such as viscosity would constrain the time-dependent dynamics of melt bearing zones. We propose a model that relates melt viscosity to electrical conductivity for naturally occurring melt compositions (including H2O) and temperature. Based on laboratory measurements of melt conductivity and viscosity, our model provides a rheological dimension to the interpretation of electromagnetic anomalies caused by melt and partially molten rocks (melt fraction similar to >0.7).

Key, K, Constable S, Liu L, Pommier A.  2013.  Electrical image of passive mantle upwelling beneath the northern East Pacific Rise. Nature. 495:499-502.: Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.   10.1038/nature11932   AbstractWebsite

Melt generated by mantle upwelling is fundamental to the production of new oceanic crust at mid-ocean ridges, yet the forces controlling this process are debated1, 2. Passive-flow models predict symmetric upwelling due to viscous drag from the diverging tectonic plates, but have been challenged by geophysical observations of asymmetric upwelling3, 4, 5 that suggest anomalous mantle pressure and temperature gradients2, 6, 7, and by observations of concentrated upwelling centres8 consistent with active models where buoyancy forces give rise to focused convective flow2. Here we use sea-floor magnetotelluric soundings at the fast-spreading northern East Pacific Rise to image mantle electrical structure to a depth of about 160 kilometres. Our data reveal a symmetric, high-conductivity region at depths of 20–90 kilometres that is consistent with partial melting of passively upwelling mantle9, 10, 11. The triangular region of conductive partial melt matches passive-flow predictions, suggesting that melt focusing to the ridge occurs in the porous melting region rather than along the shallower base of the thermal lithosphere. A deeper conductor observed east of the ridge at a depth of more than 100 kilometres is explained by asymmetric upwelling due to viscous coupling across two nearby transform faults. Significant electrical anisotropy occurs only in the shallowest mantle east of the ridge axis, where high vertical conductivity at depths of 10–20 kilometres indicates localized porous conduits. This suggests that a coincident seismic-velocity anomaly12 is evidence of shallow magma transport channels13, 14 rather than deeper off-axis upwelling. We interpret the mantle electrical structure as evidence that plate-driven passive upwelling dominates this ridge segment, with dynamic forces being negligible.