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Orientation maps for sample 0609 generated from EBSD data. Regions of a BSE image obtained around a bead (shown as the dim background) are overlain by orientation maps to indicate the positions of the associated pole figures. The orientation maps are gray-scale coded by the shape factor of each grain. Step size is 1 μm. Data are indexed as olivine, Fo90. During the reconstruction of grains, un-indexed points (melt, cpx and pores) are removed as noise. The slip directions projected in the observed plane (pink lines) are reconstructed with respect to the directions of b-plane ((010) plane), each averaged from an area of 50 × 50 μm. The length of each pink line is proportional to the magnitude of the J index. Pole figures are shown for selected regions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Source publication
Deformation of a two-phase, solid-melt rock containing rigid particles
results in the formation of pressure shadows as melt flows from regions
in relative compression to those in relative tension coupled with a
counter flux of solid. To investigate this compaction-decompaction
process, samples fabricated from fine-grained San Carlos olivine plus 10...
Contexts in source publication
Context 1
... CPOs characterized by EBSD provide the directions of flow of the solid phase around a bead, as in Fig. 6. Pole figures indicate moderate CPOs (multiples of uniform distribution (MUD) < 5 in the pole figures), with strongly clustered vertical b-axes but weakly girdled a-and c-axes. The girdled a-and c-axes suggest a combi- nation of a-type ((010) [100]) and b-type ((010) [001]) fabrics, but in both cases, the b-plane is the slip plane. The ...
Context 2
... of uniform distribution (MUD) < 5 in the pole figures), with strongly clustered vertical b-axes but weakly girdled a-and c-axes. The girdled a-and c-axes suggest a combi- nation of a-type ((010) [100]) and b-type ((010) [001]) fabrics, but in both cases, the b-plane is the slip plane. The slip directions for olivine grains (pink lines in Fig. 6, with lengths proportional to the strength of fabric) are generated from the traces of the b-planes. Then the flow field of the solid matrix featuring only the flow di- rections is ...
Context 3
... in our samples, but melt flux does not provide further comparison with melt distribution (i.e., accumulation of flux) in samples deformed in the laboratory. The orientations of the flow field determined from CPOs provide a measure of the matrix velocity directions. A comparison between the pink lines in the region to the bottom of the bead in Fig. 6 with the flow lines in Fig. 9(a) reveals that the flow directions and the matrix velocity data have the same trend in the compaction re- gion. However, due to the rotation of the bead in the deformation experiments, the similarity breaks down in the decompaction re- gion. It should also be noticed that the analytical results are given ...
Citations
... In the X-Z sections, S-C fabric along with most of the asymmetric shadow zones indicate a top-to-South sense of shear. A combined microstructural and EBSD analyses on the shadow zones provide information about the slip and flow patterns related to the deformation (e.g., Qi et al., 2013;Barbosa et al., 2019). The aim of the current ongoing study is to-(i) check the fate of porphyroclasts with varying orientations and aspect ratios in the shear zone, (ii) understand the mechanism of formation of the recrystallised grains. ...
This is a MERGED PDF of the Banner, Schedule, Group Photo and Abstract Volume related to TecTask Workshop “STRUCTURAL GEOLOGY IN THE 21st CENTURY” held from 26-28 February 2020 in the Department of Geology & Geophysics, Indian Institute of Technology Kharagpur (INDIA)
... Kinematic indicators are remarkable features used in structural geology to provide a general indication of the movement direction inside a rock body, using a simple morphological observation in most cases (Pabst, 1931;Elliott, 1972;Ramsay & Huber, 1987;Taboada et al., 1990;Urai et al., 1991;Berton et al., 2011;Qi et al., 2013). In shear and fault zones, these structures are the main source of data about the shear systems, which can be interpreted as motion at different scales (Fairbairn, 1950;Strömgård, 1973;Choukroune et al., 1987;Daniel et al., 2003;Han et al., 2010;Mukherjee, 2017). ...
Kinematic indicators, including certain strain fringes, represent an important group of structures related to the progressive deformation in rocks. The evolution of these fibrous textures can be explained by the combination of multiple mechanisms of deformation and fluid flow, mainly controlled by the orientation of the strain field and the morphology of the grains. In general, the observations are done with an optical microscope and compared with computational models of growth. This work proposes a combination of crystallographic and cathodoluminescence data obtained in rocks from banded iron formations of the Iron Quadrangle in Brazil to represent an example of how complementary analytical techniques can be useful to understand geological problems. The chosen sample exhibits a strain fringe structure of quartz around a clast of magnetite partially transformed into goethite and hematite. Through the crystallographic data it was possible to identify the grain boundary morphology and domains of low deformation areas. On the other hand, the cathodoluminescence signal evidenced the occurrence of grains with a higher concentration of crystalline defects.
... Melt pockets occur throughout the samples with sizes varying from <1 to ∼15 μm in olivine + alkali basalt and olivine + albite samples (Figures 1a and 1b) and up to ∼30 μm in olivine + Li-silicate samples ( Figure 1c). In terms of dihedral angle, the microstructures of all three types of samples are similar to those of previously studied partially molten olivine aggregates (e.g., Cooper & Kohlstedt, 1986;Hirth & Kohlstedt, 1995;Qi et al., 2013;Renner et al., 2003;von Bargen & Waff, 1986;Waff & Bulau, 1979). Most of the melt occupies triple junctions and grain boundaries of olivine, with no melt-preferred orientation (MPO). ...
To investigate the influence of compaction length on radial melt segregation during torsional shear deformation of partially molten rocks, experiments were performed on samples composed of olivine plus ∼7 vol.% of either an albite, alkali basalt, or lithium silicate melt. These three melts cover a range of three orders of magnitude in viscosity, yielding samples that vary by approximately two orders of magnitude in compaction length. Samples were deformed in torsion at 1,473 K and 300 MPa in constant strain rate experiments to outer-radius shear strains of up to 14.3. Radial melt segregation occurred toward the axial center in all three types of samples that were sheared to γ(R) ≥ 4. At the same strain, samples with the largest compaction length exhibited the highest segregation rate, while samples with intermediate and smallest compaction lengths exhibited similar segregation rates. The experimental observations qualitatively agree with previously published results from two-phase flow theory for base-state melt segregation with anisotropic viscosity; specifically, the segregation rate for radial melt segregation increases with increasing compaction length. However, quantitatively, the segregation rate in experiments is smaller than the rate predicted by simulations for the same compaction length. This discrepancy may, for example, reflect the difference in rheological behavior between that observed in our experiments (non-Newtonian, dislocation-accommodated creep) and that incorporated into the numerical models (Newtonian, diffusion-accommodated creep). Our results thus provide a baseline for testing current and future models of two-phase flow, particularly as applied to understanding melt migration, segregation, and extraction from Earth's deeper interior.
... The growth-rate of melt bands also depends on the ratio of bulk to shear viscosity. The rheological laws here suggest the bulk viscosity is comparable in magnitude to the shear viscosity, as also recently argued by Alisic et al. (2016) on the basis of a comparison of numerical models of melt bands with the laboratory experiments by Qi et al. (2013). ...
Partially molten materials resist shearing and compaction. This resistance is described by a fourth-rank effective viscosity tensor. When the tensor is isotropic, two scalars determine the resistance: an effective shear and an effective bulk viscosity. Here, calculations are presented of the effective viscosity tensor during diffusion creep for a 2D tiling of hexagonal unit cells and a 3D tessellation of tetrakaidecahedrons (truncated octahedrons). The geometry of the melt is determined by assuming textural equilibrium. The viscosity tensor for the 2D tiling is isotropic, but that for the 3D tessellation is anisotropic. Two parameters control the effect of melt on the viscosity tensor: the porosity and the dihedral angle. Calculations for both Nabarro-Herring (volume diffusion) and Coble (surface diffusion) creep are presented. For Nabarro-Herring creep the bulk viscosity becomes singular as the porosity vanishes. This singularity is logarithmic, a weaker singularity than typically assumed in geodynamic models. The presence of a small amount of melt (0.1% porosity) causes the effective shear viscosity to approximately halve. For Coble creep, previous modelling work has argued that a very small amount of melt may lead to a substantial, factor of 5, drop in the shear viscosity. Here, a much smaller, factor of 1.4, drop is obtained for tetrakaidecahedrons. Owing to a Cauchy relation symmetry, the Coble creep bulk viscosity is a constant multiple of the shear viscosity when melt is present.
... In the Earth's lower crust, viscous deformation of multiphase rocks can often be associated with characteristic structures on the microscopic scale. Deformation of these rocks, consisting of several different mineral phases, leads to the establishment of pockets of multiple weak mineral phases embedded in a substrate consisting of stronger mineral phases (Handy, Wissing & Streit 1999;Holyoke & Tullis 2006;Qi, Zhao & Kohlstedt 2013). As the deformation of these rocks facilitates the motion of overlying tectonic plates, the interaction between these pockets of weakness plays an important role in establishing the effective strength of the rocks ( Handy et al. 1999). ...
This article presents a framework for building analytical solutions for coupled flow in two interacting multiphase domains. The coupled system consists of a multiphase sphere embedded in a multiphase substrate. Each of these domains consists of an interconnected, load-bearing, creeping matrix phase and an inviscid, interstitial fluid phase. This article outlines techniques for building analytical solutions for velocity, pressure and compaction within each domain, subject to boundary conditions of continuity of matrix velocity, normal traction, normal pressure gradient, and compaction at the interface between the two domains. The solutions, valid over a short period of time in the limit of small fluid fraction, are strongly dependent on the ratio of shear viscosities between the matrix phase in the sphere and the matrix phase in the substrate. Compaction and pressure drop across the interface, evaluated at the poles and the equator, are strongly dependent on the ratio of matrix shear viscosities in the two domains. When deformed under a pure shear deformation, the magnitude of flow within the sphere rapidly decreases with an increase in this ratio until it reaches a value of ${\sim}80$ , after which the velocity within the sphere becomes relatively insensitive to the increase in the viscosity ratio.
... After achieving the target strain, each sample was cooled rapidly (2 K/s) to 1,300 K under the same torque as imposed at the end of the deformation experiment to preserve the deformation-produced microstructure and then cooled to room temperature with no torque applied. After deformation, samples have an average melt fraction between 0.04 and 0.06 (e.g., Qi et al., 2013, ...
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.
... In the laboratory, melt channelization due to stress in high-pressure, high-temperatures (HP-HT) experiments has been well documented (e.g. Holtzman et al., 2003;Holtzman & Kohlstedt, 2007;Kohlstedt et al., 2010;King et al., 2010;Qi et al., 2013). However, channelization due to reaction at HP-HT conditions has received much less attention from experimentalists (e.g. ...
Melt extraction from partially molten regions of the mantle occurs along high-permeability pathways. Melt-rock reactions can lead to the formation of high-permeability channels due to a positive feedback between melt flow and reaction. To study this process, we performed a series of Darcy-type experiments in which a cylinder of partially molten rock sandwiched between a melt source and a porous sink was annealed at high pressures (P ¼ 300 MPa) and high temperatures (T ¼ 1200 or 1250 C) under a controlled pressure gradient (oP/ox ¼ 0-100 MPa mm-1) for up to 5 h. The partially molten rock was formed from 50:50 mixtures of olivine (Ol) and clinopyroxene (Cpx) plus 4, 10 or 20 vol. % of alkali basalt. The melt source was a disk of alkali basalt undersaturated in silica with respect to the partially molten rock, and the sink was a disk of porous alumina. During an experiment , melt from the source dissolved Cpx in the partially molten rock and precipitated Ol, thereby forming a Cpx-free reaction layer at the interface between the melt source and the partially molten rock. The melt fraction as well as the grain size in the reaction layer increased significantly compared with that present in the starting material, confirming that the reaction increased the local permeability of the partially molten rock, one of the prerequisites for the reaction infiltration instability process to operate. In experiments carried out under a small pressure gradient (and hence slow melt flow velocity), the reaction layer remained roughly planar and no channels developed. However, if the melt flow velocity by porous flow exceeded $0Á1 mm s-1 , the reaction layer locally protruded into the partially molten rock forming finger-like, melt-rich channels. The morphology and spacing of the channels depended on the initial melt fraction. In a partially molten rock with 20 vol. % melt, multiple, voluminous channels with an elliptical core of pure melt developed. At lower melt contents, fewer and thinner channels formed. Our experiments demonstrate that melt-rock reactions can lead to melt channelization in mantle lithologies, consistent with general predictions of the reaction infiltration instability theory.
... For example, based on field and microstructural observations, B-type fabrics were associated with deformation by dislocation-accommodated grain boundary sliding [Pr ecigout and Hirth, 2014], and with topotactic growth of olivine during dehydration breakdown of antigorite [Nagaya et al., 2014]. In laboratory experiments, B-type fabrics have been documented for high temperature/low stress conditions in the presence of melt [Holtzman et al., 2003;Kohlstedt and Holtzman, 2009;Qi et al., 2013], dry conditions at high confining pressures [Couvy et al., 2004;Jung et al., 2009;Ohuchi et al., 2011], during diffusion creep in the presence of orthopyroxene [Sundberg and Cooper, 2008], and during dislocation creep under high water contents and high stress magnitudes [Jung and Karato, 2001a;Jung et al., 2006]. The NVF samples show a strong correlation between hydration and the progressive development of B-type LPO with increasing strain. ...
Laramide crustal deformation in the Rocky Mountains of the west-central United States is often considered to relate to a narrow segment of shallow subduction of the Farallon slab, but there is no consensus as to how deformation along the slab-mantle lithosphere interface was accommodated. Here we investigate deformation in mantle rocks associated with hydration and shear above the flat slab at its contact with the base of the North American plate. The rocks we focus on are deformed, hydrated, ultramafic inclusions hosted within diatremes of the Navajo Volcanic Field in the central Colorado Plateau that erupted during the waning stages of the Laramide orogeny. We document a range of deformation textures, including granular peridotites, porphyroclastic peridotites, mylonites, and cataclasites, which we interpret to reflect different proximities to a slab-mantle-interface shear zone. Mineral assemblages and chemistries constrain deformation to hydrous conditions in the temperature range ~550-750 C. Despite the presence of hydrous phyllosilicates in modal percentages of up to 30%, deformation was dominated by dislocation creep in olivine. The mylonites exhibit an uncommon lattice preferred orientation (LPO) in olivine, known as B-type LPO in which the a-axes are aligned perpendicular to the flow direction. The low temperature, hydrated setting in which these fabrics formed are consistent with laboratory experiments that indicate B-type LPOs form under conditions of high stress and high water contents; furthermore, the mantle wedge context of these LPOs is consistent with observations of trench-parallel anisotropy in the mantle wedge above many modern subduction zones. Differential stress magnitudes in the mylonitic rocks estimated using paleopiezometry range from 290 to 444 MPa, and calculated effective viscosities using a wet olivine flow law are on the order of 10^19-10^23 Pa s. The high stress magnitudes, high effective viscosities and high strains recorded in these rocks are consistent with models that invoke significant basal shear tractions as contributing to Laramide uplift and contraction in the continental interior.
... Indeed, the unique deformation of partial melts likely controls flow and strain focussing at both convergent and divergent plate boundaries (e.g., Spiegelman and McKenzie, 1987;Katz, 2008;Gerya and Meilick, 2011;Gerya, 2013). In particular, sheared partial melts have been demonstrated in laboratory experiments (Daines and Kohlstedt, 1997;Holtzman et al., 2003;King et al., 2010;Qi et al., 2013) to develop narrow melt bands at shallow angles (∼20 • ) to the direction of motion. Such melt banding may provide high-permeability pathways that strongly influence the transport of melt to the Earth's surface (Kohlstedt and Holtzman, 2009). ...
The deformation of partially molten mantle in tectonic environments can lead to exotic structures, which potentially affect both melt and plate-boundary focussing. Examples of such structures are found in laboratory deformation experiments on partially molten rocks. Simple-shear and torsion experiments demonstrate the formation of concentrated melt bands at angles of around 20° to the shear plane. The melt bands form in the experiments with widths of a few to tens of microns, and a band spacing roughly an order of magnitude larger. Existing compaction theories, however, cannot predict this band width structure, let alone any mode selection, since they infer the fastest growing instability to occur for wavelengths or bands of vanishing width. Here, we propose that surface tension in the mixture, especially on a diffuse interface in the limit of sharp melt-fraction gradients, can mitigate the instability at vanishing wavelength and thus permit mode selection for finite-width bands. Indeed, the expected weak capillary forces on the diffuse interface lead to predicted mode selection at the melt-band widths observed in the experiments.
... One means of addressing questions on the dynamics of melt segregation and transport is by comparison of simulations with laboratory experiments on partially molten rocks subjected to forced deformation. A recent experimental study with significant potential in this regard is reported by Qi et al. (2013). Following on the torsional deformation experiments of King et al. (2010), Qi et al. (2013) modified the basic experiment by including rigid, spherical beads within the partially molten rock that is undergoing deformation. ...
... A recent experimental study with significant potential in this regard is reported by Qi et al. (2013). Following on the torsional deformation experiments of King et al. (2010), Qi et al. (2013) modified the basic experiment by including rigid, spherical beads within the partially molten rock that is undergoing deformation. They find that pressure shadows around the bead are expressed as variations in melt fraction there. ...
... This allows us to capture the threedimensional scaling of compaction around a sphere, which differs from the two-dimensional scaling around a cylinder (Rudge, 2014). Moreover, the simulations presented here provide a more realistic comparison to the results of laboratory experiments (Qi et al., 2013). These new simulations with ∼ 7×10 6 degrees of freedom would be impossible without an advanced, new preconditioning method for the equations of magma dynamics that has been recently developed (Rhebergen et al., 2015). ...
The processes that are involved in migration and extraction of melt from the
mantle are not yet fully understood. Gaining a better understanding of material
properties of partially molten rock could help shed light on the behavior of
melt on larger scales in the mantle. In this study, we simulate
three-dimensional torsional deformation of a partially molten rock that
contains a rigid, spherical inclusion. We compare the computed porosity
patterns to those found in recent laboratory experiments. The laboratory
experiments show emergence of melt-rich bands throughout the rock sample, and
pressure shadows around the inclusion. The numerical model displays similar
melt-rich bands only for a small bulk-to-shear-viscosity ratio (five or less).
The results are consistent with earlier two-dimensional numerical simulations,
however we show that it is easier to form melt-rich bands in three dimensions
compared to two. The addition of strain-rate dependence of the viscosity causes
a distinct change in the shape of pressure shadows around the inclusion. This
change in shape presents an opportunity for experimentalists to identify the
strain-rate dependence and therefore the dominant deformation mechanism in
torsion experiments with inclusions.
