An experimental test of the viscous anisotropy hypothesis for partially molten rocks

Chemical differentiation of rocky planets occurs by melt segregation away from the region of melting. The mechanics of this process, however, are complex and incompletely understood. In partially molten rocks undergoing shear deformation, melt pockets between grains align coherently in the stress field; it has been hypothesized that this anisotropy in microstructure creates an anisotropy in the viscosity of the aggregate. With the inclusion of anisotropic viscosity, continuum, two-phase-flow models reproduce the emergence and angle of melt-enriched bands that form in laboratory experiments. In the same theoretical context, these models also predict sample-scale melt migration due to a gradient in shear stress. Under torsional deformation, melt is expected to segregate radially inward. Here we present new torsional deformation experiments on partially molten rocks that test this prediction. Microstructural analyses of the distribution of melt and solid reveal a radial gradient in melt fraction, with more melt toward the centre of the cylinder. The extent of this radial melt segregation grows with progressive strain, consistent with theory. The agreement between theoretical prediction and experimental observation provides a validation of this theory, which is critical to understanding the large-scale geodynamic and geochemical evolution of Earth.Comment: 21 pages, 4 figures, 1 table, supplementary inf

Crystallographic Preferred Orientation of Olivine in Sheared Partially Molten Rocks: The Source of the “a-c Switch”

© 2018. American Geophysical Union. All Rights Reserved. To investigate the mechanism that produces the crystallographic preferred orientations (CPO) characteristic of sheared partially molten rocks of mantle composition, we analyzed the microstructures of samples of olivine plus 7% basaltic melt deformed in torsion to shear strains as large as γ = 13.3 Electron backscattered diffraction (EBSD) observations reveal a CPO characterized by a weak a-c girdle in the shear plane that develops by γ = 4. This CPO, which exhibits a slightly stronger alignment of 001 than 100 axes in the shear direction, changes little in both strength and distribution with increasing stress and with increasing strain. Furthermore, it is significantly weaker than the CPO observed for dry, melt-free olivine aggregates. Orientation maps correlated with grain shape measurements from tangential, radial, and transverse sections indicate that olivine grains are longer along 001 axes than along 100 axes and shortest along 010 axes. This morphology is similar to that of olivine grains in a mafic melt. We conclude that the weak a-c girdle observed in sheared partially molten rocks reflects contributions from two processes. Due to their shape-preferred orientation (SPO), grains rotate to align their 001 axes parallel to the flow direction. At the same time, dislocation glide on the (010)100 slip system rotates 100 axes into the flow direction. The presence of this CPO in partially molten regions of the upper mantle significantly impacts the interpretation of seismic anisotropy and kinematics of flow

The impact of water on slip system activity in olivine and the formation of bimodal crystallographic preferred orientations

Crystallographic preferred orientations (CPOs) in olivine are widely used to infer the mechanisms, conditions, and kinematics of deformation of mantle rocks. Recent experiments on water-saturated olivine were the first to produce a complex CPO characterised by bimodal orientation distributions of both [100] and [001] axes and inferred to form by combined activity of (001)[100], (100)[001], and (010)[100] slip. This result potentially provides a new microstructural indicator of deformation in the presence of elevated concentrations of intracrystalline hydrous point defects and has implications for the interpretation of seismic anisotropy. Here, we document a previously unexplained natural example of this CPO type in a xenolith from Lesotho and demonstrate that it too may be explained by elevated concentrations of hydrous point defects. We test and confirm the hypothesis that combined (001)[100], (100)[001], and (010)[100] slip were responsible for formation of this CPO by (1) using high-angular resolution electron backscatter diffraction to precisely characterise the dislocation types present in both the experimental and natural samples and (2) employing visco-plastic self-consistent simulations of CPO evolution to assess the ability of these slip systems to generate the observed CPO. Finally, we utilise calculations based on effective-medium theory to predict the anisotropy of seismic wave velocities arising from the CPO of the xenolith. Maxima in S-wave velocities and anisotropy are parallel to both the shear direction and shear plane normal, whereas maxima in P-wave velocities are oblique to both, adding complexity to interpretation of deformation kinematics from seismic anisotropy.D. Wallis, L.N. Hansen, and A.J. Wilkinson acknowledge support from the Natural Environment Research Council Grant NE/M000966/1. M. Tasaka acknowledges support through a JSPS Research Fellowship for Young Scientists (26-4879) and the Japan Society for the Promotion of Science (16K17832). D.L. Kohlstedt acknowledges support through NASA Grant NNX15AL53G. K.M. Kumamoto acknowledges support through NSF Division of Earth Science grants 1255620 and 1625032

Microstructure effects on the phase transition behavior of a prototypical quantum material

Materials with insulator metal transitions promise advanced functionalities for future information technology. Patterning on the microscale is key for miniaturized functional devices, but material properties may vary spatially across microstructures. Characterization of these miniaturized devices requires electronic structure probes with sufficient spatial resolution to understand the influence of structure size and shape on functional properties. The present study demonstrates the use of imaging soft X ray absorption spectroscopy with a spatial resolution better than 2 amp; 956;m to study the insulator metal transition in vanadium dioxide thin film microstructures. This novel technique reveals that the transition temperature for the conversion from insulating to metallic vanadium dioxide is lowered by 1.2 K 0.4 K close to the structure edges compared to the center. Facilitated strain release during the phase transition is discussed as origin of the observed behavior. The experimental approach enables a detailed understanding of how the electronic properties of quantum materials depend on their patterning at the micrometer scal

Velocity–conductivity relationships for mantle mineral assemblages in Archean cratonic lithosphere based on a review of laboratory data and Hashin–Shtrikman extremal bounds

Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Lithos 109 (2009): 131-143, doi:10.1016/j.lithos.2008.10.014.Can mineral physics and mixing theories explain field observations of seismic velocity and electrical conductivity, and is there an advantage to combining seismological and electromagnetic techniques? These two questions are at the heart of this paper. Using phenomologically-derived state equations for individual minerals coupled with multi-phase, Hashin-Shtrikman extremal-bound theory we derive the likely shear and compressional velocities and electrical conductivity at three depths, 100 km, 150 km and 200 km, beneath the central part of the Slave craton and beneath the Kimberley region of the Kaapvaal craton based on known petrologically-observed mineral abundances and magnesium numbers, combined with estimates of temperatures and pressures. We demonstrate that there are measurable differences between the physical properties of the two lithospheres for the upper depths, primarily due to the different ambient temperature, but that differences in velocity are negligibly small at 200 km. We also show that there is an advantage to combining seismic and electromagnetic data, given that conductivity is exponentially dependent on temperature whereas the shear and bulk moduli have only a linear dependence in cratonic lithospheric rocks. Focussing on a known discontinuity between harzburgite-dominated and lherzolitic mantle in the Slave craton at a depth of about 160 km, we demonstrate that the amplitude of compressional (P) wave to shear (S) wave conversions would be very weak, and so explanations for the seismological (receiver function) observations must either appeal to effects we have not considered (perhaps anisotropy), or imply that the laboratory data require further refinement

Rheology of Diabase: Implications for Tectonics on Venus and Mars

Two important goals of our experimental investigation of the rheological behavior of diabase rocks were: (1) to determine flow laws describing their creep behavior over wide ranges of temperature, stress and strain rate and (2) to develop an understanding of the physical mechanisms by which these rocks flow under laboratory conditions. With this basis, a primary objective then was to construct constitutive equations that can be used to extrapolate from laboratory to planetary conditions. We specifically studied the rheological properties of both natural rock samples and synthetic aggregates. The former provided constraints for geologic systems, while the latter defined the relative contributions of the constituent mineral phases and avoided the influence of glass/melt found in natural samples. In addition, partially molten samples of crustal rock composition were deformed in shear to large strains (greater than 200%) important in crustal environments. The results of this research yielded essential rheological properties essential for models of crustal deformation on terrestrial planets, specifically Venus and Mars, as well as on the geodynamical evolution of these planets. Over the past three years, we also completed our investigation of the creep behavior of water ice with applications to the glaciers, ice sheets and icy satellites. Constitutive equations were determined that describe flow over a wide ranged of stress, strain rate, grain size and temperature. In the case of ice, three creep regimes were delineate. Extrapolation demonstrates that dislocation glide and grain boundary sliding processes dominate flow in ice I under planetary conditions and that diffusion creep is not an important deformation mechanism either in the laboratory or on icy satellites. These results have already been incorporated by other investigators into models describing, for example, the thickness and stability of the ice shell on Europa and to unravel long-standing discrepancies between field observations on glaciers and laboratory results

Electromigration and chemical diffusion in titanium carbide

The ionic charge and lattice diffusion of carbon in titanium carbide have been investigated through studies of diffusion under the influences of kan applied dc-electric field and of a chemical potential. The electron microprobe was used to measure the diffusion concentration profiles. Electromigration studies of carbon in titanium carbide yielded a positive effective charge which increases from +0.4 at l8250 C to possibly as high as +2.8 at 2480oC. The observed sign and temperature dependence of/the effective charge are interpreted as indicating that during the diffusion jump the average carbon-ion (charge seen by the electric field is positive and that the whole window is increasing with respect to the lielectron winduu with increasing temperature. It is suggested that this positive charge supports the prediction of Lye's energy-band calculation that charge is transferred from titanium to carbon electronic states but still is consistent with qualitative results of recent ESCA and electron mapping experiments that show that the carbon atom is negative at its equilibrium site. The chemical diffusivity in single crystals of titanium carbide was determined for 19200 C ~ T ~ 2328°C using the Boltzmann-Matano method of analysis. The experimental chemical diffusivity increases with decreasing carbon vacancy concentration and has an average value of 220 exp(-97.7!RT) cm !sec. A concentration-dependent diffusivity with an average value of 300 exp(-99~3/RT) cm2/sec is calculated from Sarian's tracer diffusivity data and DePoorter's semi-theoretical model for activity. The present work shows that the chemical diffusivity in titanium carbide depends on concentration through the tracer activation energy but not through the pre-exponential factor. A comparison between these single crystal results and diffusivities reported for chemical diffusion in layer growth samples demonstrates that the latter represent short-circuit enhanced diffusion. Also~ the present work substantiates the results of Sarian for tracer diffusion in single crystals of titanium carbide, and it connects the polycrystalline layer-growth diffusivities with the single crystal tracer diffusivity results reported in the literature.U of I OnlyThesi

Optical micrographs of olivine + melt samples deformed in torsion

An excel spreadsheet summarizes the experimental conditions of the samples included in this data set, and also includes figures that plot the azimuthally averaged, normalized melt fraction vs. the radius for the starting material and each experimental condition. For each sample (with the exception of experiment PT0767), the following data are included. 1. Tiles of 8-bit gray-scale optical microscopic images of 1000x1000 pixels. 1 pixel = 0.32 micron. There is ~20% overlapping between each tile. 2. A txt file registering the relative coordinates of each tile. 3. A black and white image with melt being white processed from the tiles and assembled by their coordinates. 1 pixel = 0.32 micron. 4. A mosaic microscopic image with lower resolution. In the file name, "OL" stands for optical light. 1 pixel = 1.3 micron. For experiment PT0767, no high resolution images were obtained for this sample. For experiment PT0817, two files with relative coordinates exist, with one overlapping row that can be used to stitch the two halves together.This data set contains high-resolution micrographs of the transverse sections of partially molten samples deformed in torsion. These micrographs present the distribution of melt induced by deformation, which is a test of the melt segregation processes predicted by the two-phase flow theory incorporating viscous anisotropy.NS