Shear Deformation Experiments Research Articles

Deformation experiments of bubble‐ and crystal‐bearing magmas: Rheological and microstructural analysis

Simple shear deformation experiments on three‐phase, hydrous, haplogranitic magmas, composed of quartz crystals (24–65 vol.%), CO2‐rich gas bubbles (9–12 vol.%) and melt in different proportions, were performed with a Paterson‐type rock deformation apparatus. Strain rates from 5 · 10−6 s−1 to 4 · 10−3 s−1 were applied at temperatures between 723 and 1023 K and at pressure of 200 MPa. The results show that the three‐phase suspension rheology is strongly strain rate dependent (non‐Newtonian behavior). Two non‐Newtonian regimes were observed: shear thinning (viscosity decreases with increasing strain rate) and shear thickening (viscosity increases with increasing strain rate). Shear thinning occurs in crystal‐rich magmas (55–65 vol.% crystals; 9–10 vol.% bubbles) as a result of crystal size reduction and shear zoning. Shear thickening prevails in dilute suspensions (24 vol.% crystals; 12 vol.% bubbles), where bubble coalescence and outgassing dominate. At intermediate crystallinity (44 vol.% crystals; 12 vol.% bubbles) both shear thickening and thinning occur. Based on the microstructural observations using synchrotron radiation X‐ray tomographic microscopy, bubbles can develop two different shapes: oblate at low temperature (<873 K) and prolate at high temperature (>873 K). These differences in shape are caused by different conditions of flow: unsteady flow, where the relaxation time of the bubbles is much longer than the timescale of deformation (oblate shapes); steady flow, where bubbles are in their equilibrium deformation state (prolate shapes). Three‐phase magmas are characterized by a rheological behavior that is substantially different with respect to suspensions containing only crystals or only gas bubbles.

High-temperature magnetic fabric development from plastically deformed magnetite in experimental shear zones

Magnetic fabric development has been studied in synthetic magnetite-silicate aggregates in a series of high-temperature simple shear deformation experiments. Samples composed of magnetite grains with a nominal size of 20-40 µm dispersed at 3 wt % in a matrix of plagioclase were deformed between 1000 and 1200 °C with a confining pressure of 300 MPa, shear stresses in the range 10-130 MPa, and shear strains up to γ = 3. We determined that both magnetite and plagioclase were deformed plastically at these conditions. An oblate magnetic fabric ellipsoid develops subparallel to the shear plane and the degree of AMS rapidly increases with strain up to a value of 2.5. Theoretical strain response models of magnetic fabric development were applied to the data to obtain estimated magnetite strains. The shape-preferred orientation of magnetite grains after deformation, determined from reflected light image analysis, was used to calculate independent magnetite strain estimates. These results were then compared with strains estimated from theoretical magnetic anisotropy development. Both strain estimation methods indicate strain partitioning between magnetite and the plagioclase matrix and the results are used to determine approximate viscosity ratios between the two phases at the experimental conditions.

Technical development of simple shear deformation experiments using a deformation-DIA apparatus

Technical developments for simple shear deformation experiments at high pressures were made. The newly designed cell assembly can be compressed by deformation-DIA apparatuses with the MA 6-6 system, which consists of six second-stage tungsten carbide anvils (with a truncated edge length of 5 mm) and the anvil guide. Deformation of samples was barely observed during the compression process, showing that the shear strain of the deformed samples can be measured by the rotation of a strain marker. Simple shear deformation experiments on anhydrous and hydrous olivine aggregates were conducted under upper mantle conditions (pressures of 5.2–7.6 GPa and temperatures of 1 473–1 573 K), and sample deformation with a shear strain of γ=0.8−1.2 was successfully achieved at a shear strain rate of 4.0×10−5−7.5×10−5 s−1. The present study extended the pressure range of simple shear deformation experiments in the deformation-DIA apparatus from 3 GPa in an early study to 7.6 GPa at high temperatures.

Strength of single-crystal orthopyroxene under lithospheric conditions

Creep strength of oriented orthopyroxene single crystals was investigated via shear deformation experiments under lithospheric conditions [P (pressure) = 1.3 GPa and T (temperature) = 973–1,373 K]. For the A-orientation (shear direction [001] on (100) plane), the samples have transformed completely to clinoenstatite and much of the deformation occurred after transformation. In contrast, for the B-orientation (shear direction [001] on (010) plane), samples remained orthoenstatite and deformation occurred through dislocation motion in orthoenstatite. The strength of orthopyroxene with these orientations is smaller than for olivine aggregates under all experimental conditions. Flow of the B-orientation samples is described by a power-law, and the pre-exponential constant, the apparent activation energy, and the stress exponent are determined to be A = 10−9.5 s−1·MPa−4.2, Q = 114 kJ/mol and n = 4.2. However, for the A-orientation, the results cannot be fit by a single flow law and we obtained the following: A = 108.9 s−1·MPa−3.0, Q = 459 kJ/mol and n = 3.0 at high temperatures (≥1,173 K), and A = 10−27.4 s−1·MPa−14.3, Q = 296 kJ/mol and n = 14.3 at low temperatures (<1,173 K). The stress exponent for the low-temperature regime is high, suggesting that deformation involves some processes where the activation energy decreases with stress such as the Peierls mechanism. Our study shows that orthopyroxene with these orientations is significantly weaker than olivine under the lithospheric conditions suggesting that orthopyroxene may reduce the strength of the lithosphere, although the extent to which orthopyroxene weakens the lithosphere depends on its orientation and connectivity.

Shear deformation of polycrystalline wadsleyite up to 2100 K at 14–17 GPa using a rotational Drickamer apparatus (RDA)

Shear deformation experiments on polycrystalline wadsleyite (water content, ∼200–2200 H/106 Si) have been conducted at 14.4–17.0 GPa, 1690–2100 K, and strain rates of 2.6–16 × 10−5 s−1 using a rotational Drickamer apparatus (RDA) at a synchrotron facility. The stress was measured from the orientation dependence of lattice spacing for the (013), (211), (141), (240) and (244) planes. On the basis of the mechanical and microstructural observations, we infer that deformation occurs by exponential creep through the Peierls mechanism at relatively low temperatures of 1690–2030 K. However, a sample deformed at the temperature of 2100 K showed significant grain‐size reduction, and most of small grains are dislocation‐free, although sub‐boundaries were observed in some grains in the sample. We interpret these observations as evidence for dynamic recrystallization and that diffusion creep (and grain boundary sliding) plays an important role after dynamic recrystallization caused by power law creep. Consequently, the strength observed in the high‐temperature conditions determined by the present study provides an important constraint on strength of diffusion creep and a lower limit for that of the power law dislocation creep. We conclude that the strength of wadsleyite in the power law dislocation creep is higher than or comparable to that of olivine and the strength of wadsleyite in the Peierls regime is similar to that of olivine.

Shear deformation experiments on vesicular rhyolite: Implications for brittle fracturing, degassing, and compaction of magmas in volcanic conduits

Shear‐induced brittle fractures in vesicular magmas are thought to be possible pathways for open‐system degassing that controls the explosivity of volcanic eruptions. To investigate the detailed processes involved in the shear‐induced degassing, we performed torsional deformation experiments on columnar rhyolites with water contents and averaged vesicularities of 0.4–0.5 wt % and 20–41 vol %, respectively, at temperatures of 780–930°C and strain rates of &lt;0.03 s−1. The experimental conditions, such as viscosity and vesicularity of the sample, strain rate, and total strain, simulated those of typical natural dehydrated rhyolites in shallow volcanic conduits. At relatively high temperatures (≥855°C), the columnar samples were homogeneously twisted and deformed. On the other hand, at temperatures of ≤830°C, the deformation was localized and finally resulted in brittle failure into a column and a disk, followed by a slip at the fractured interface. This slip prevented further brittle failure and shear‐induced bubble coalescence in the remaining parts of the sample. A permeable fragmented zone was formed near the interface, while in the rest of the sample, the bubbles remained isolated and permeability did not increase. From this, we infer that a single event of magma fracturing may enhance open‐system degassing locally near a fracture, but repeated fracturing and healing processes are necessary for the effective degassing of the entire magma that leads to nonexplosive eruptions.

Shear deformation of dry polycrystalline olivine under deep upper mantle conditions using a rotational Drickamer apparatus (RDA)

Shear deformation experiments on dry hot-pressed polycrystalline San Carlos olivine have been conducted at 4.9–9.6 GPa, 1300–1870 K and strain rates of 0.6–7.4 × 10 −5 s −1 using a rotational Drickamer apparatus (RDA) at a synchrotron facility. The stress was measured from the orientational dependence of lattice spacing for the (1 3 0), (1 3 1), (1 1 2), (1 2 2), (1 4 0) and (2 4 1) planes, as well as from the dislocation densities. Based on the mechanical and microstructural observations, we infer that deformation occurs by power-law creep involving dislocation glide and climb under high temperature conditions, whereas deformation is due to exponential creep through the Peierls mechanism at relatively low temperatures. In both regimes the strength of olivine at steady-state deformation at these pressures is much larger than those at lower pressures. The activation volumes V* is estimated to be ∼15–20 × 10 −6 m 3/mol for the power-law creep. Combined with the previous results under wet conditions, we present the recommended values of flow law parameters on olivine for power-law creep constrained by high-pressure deformation experiments.

Low-temperature, high-stress deformation of olivine under water-saturated conditions

Low-temperature high-stress rheology of olivine was investigated under water-saturated conditions by large strain shear deformation experiments using the Griggs-type apparatus. Samples at T = 1273–1373 K and P = 2 GPa under water-saturated conditions deformed under a significantly lower applied stress than expected from the power-law relation and the strain rate increases more rapidly with stress. These observations suggest that an exponential flow law operates under the present experimental conditions, in which the stress dependence of activation enthalpy is characterized by the Peierls stress. The Peierls stress is determined between 1.6 and 2.9 GPa depending on the choice of the form of stress dependence of the activation enthalpy. These values are significantly smaller than the Peierls stress under dry conditions (9.1 GPa). We suggest that water (hydrogen) incorporated in the olivine lattices decreases the Peierls stress, and hence the rate of deformation in the exponential creep is significantly enhanced by the presence of water. The reduction of the Peierls stress by water results in the highly anisotropic weakening effect of water that causes the fabric transitions in olivine. The transition between the power-law and the exponential creep occurs at relatively low stress ∼100 MPa at a geologically relevant strain rate of 10 −15 s −1 when temperature is lower than 1273 K and under water-rich conditions. Hence, the exponential creep might be dominated in olivine-rich rocks at low-temperature and high-stress under water-rich conditions, and such regions could be widespread in the subduction zone upper mantle where the cold lithospheric plates sink into the mantle.

Geodynamic Significance of Seismic Anisotropy of the Upper Mantle: New Insights from Laboratory Studies

Seismic anisotropy is caused mainly by the lattice-preferred orientation of anisotropic minerals. Major breakthroughs have occurred in the study of lattice-preferred orientation in olivine during the past ∼10 years through large-strain, shear deformation experiments at high pressures. The role of water as well as stress, temperature, pressure, and partial melting has been addressed. The influence of water is large, and new results require major modifications to the geodynamic interpretation of seismic anisotropy in tectonically active regions such as subduction zones, asthenosphere, and plumes. The main effect of partial melting on deformation fabrics is through the redistribution of water, not through a change in deformation geometry. A combination of new experimental results with seismological observations provides new insights into the distribution of water associated with plume-asthenosphere interactions, formation of the oceanic lithosphere, and subduction. However, large uncertainties remain regarding the role of pressure and the deformation fabrics at low stress conditions.

An experimental and numerical study of surface tension-driven melt flow

To determine the role of surface tension-driven melt migration in planetary bodies, we investigated the effect of static annealing on the evolution of melt-rich bands in partially molten samples. In shear deformation experiments, deviatoric stress causes melt to segregate; when the stress is removed, surface tension causes the melt to relax back to a homogeneous distribution. Samples composed of 76 vol.% olivine + 20 vol.% chromite + 4 vol.% MORB were deformed to shear strains of ~ 3.5 at 1523 K, 300 MPa and shear stresses of 20 to 55 MPa. After deformation, the samples were statically annealed for 0, 10, or 100 h. During annealing, melt transport driven by surface tension occurs, but takes place much more slowly than flow driven by deviatoric stress. Finite difference numerical simulations were performed of surface tension-driven melt flow resisted by viscous deformation of the olivine matrix. These models best reproduce the distribution of melt in the annealed samples when the solid viscosity η s = 1.7 ± 0.5 × 10 12 Pa s with n = 2.4 ± 0.3 and b = 9000 ± 1900 in the expression for permeability κ = ϕ nd 2 / b where d is grain size. The large value of b compared with estimates from geometrical models is probably due to clogging of the melt tubes by the secondary solid phase (chromite). Redistribution of melt by surface tension is likely to be the dominant process in small (~ 10 km radius) planetesimals in the absence of convection or impact-induced deformation. However, this redistribution process is sufficiently slow that large bodies of localized melt (magma chambers) are likely to develop.

Effects of water and iron content on the rheological contrast between garnet and olivine

The effects of water and iron content on the relative creep strengths of garnet and olivine were investigated by shear deformation experiments. Garnet and olivine samples were sandwiched together between alumina pistons in a simple shear geometry and were deformed at P = 1–2 GPa, T = 1473 K and strain rates ranging from 10 −5 to 10 −3 s −1 using a Griggs-type solid-medium apparatus. The stress- and strain-rate relation, as well as the deformation microstructures including lattice-preferred orientation and dynamic recrystallization, indicates that the deformation by dislocation creep. The creep tests show that the Fe-rich garnet (Alm 67Prp 29Grs 3) was slightly weaker than olivine (Fo90), whereas the Mg-rich garnet (Alm 19Prp 68Grs 12) was significantly stronger than olivine under dry conditions. The wet experiments show that the creep rate of the Mg-rich garnet is more sensitive to water than olivine; the water fugacity exponent on strain rate was estimated to be ∼2.4 for garnet and ∼1.2 for olivine, and the Mg-rich garnet becomes weaker than olivine in a water-rich environment. The experimental results show that the rheological contrast between garnet and olivine depends strongly on water content and to a lesser degree on Fe content. Consequently, the geodynamic behavior of geochemical reservoirs can be sensitive to their chemical environments in the upper mantle.

Deformation-enhanced reaction in experimentally deformed plagioclase-olivine aggregates

The effects of deformation on the kinetics of the net-transfer reaction anorthite + forsterite → cpx + opx + spinel ± gt were studied using static and shear deformation experiments. Experiments were performed on dry anorthite-olivine (An92–Fo93) samples at 900°C and pressures between 1,000 and 1,600 MPa in a Griggs apparatus. Deformed (‘non-hydrostatic’) and undeformed (‘static’) samples are compared in terms of phase petrology, reaction rate and reaction mechanisms. Anorthite + olivine reactions are diffusion-controlled as seen from reaction rim structures. In undeformed samples, delayed reaction onset and low reaction rates demonstrate sluggish nucleation of reaction products and slow rates of diffusion at dry conditions, even at 700–900 MPa confining pressure overstepping. The reaction rate is enhanced in deformed An–Fo samples. The higher rate is mainly attributed to a combination of high stresses and viscous deformation processes of the reactants and products, which cause an increase in the nucleation rate of products. The results imply that viscous deformation processes alone can be responsible for the initiation and localisation of metamorphic reactions in dry rocks in the absence of fluid infiltration.

Origin of seismic anisotropy in the D″ layer inferred from shear deformation experiments on post-perovskite phase

Seismic anisotropy is one of the significant features in the D″ layer of the Earth and is thought to be derived from the lattice preferred orientation (LPO) of constituent materials or shape preferred orientation (SPO) of heterogeneous materials such as melt and inclusions. Recent experimental and theoretical studies strongly suggest that the D″ layer consists mainly of a MgSiO 3 post-perovskite phase together with ferro-periclase. To understand the anisotropy in the D″ layer, we have conducted a series of simple shear deformation experiments at high temperature and pressure on polycrystalline CaIrO 3 as an analogue of MgSiO 3 and measured the LPO of the post-perovskite phase. Crystallographic orientation analysis of the deformed post-perovskite phase showed strong LPOs with the dominant slip system being [100](010). Calculation of the elastic wave velocities considering the effect of LPOs of post-perovskite and ferro-periclase showed as azimuthal and polarization anisotropies in the horizontal shear plane where the velocity of horizontally polarized S-wave is considerably faster than that of vertically polarized S-wave. Thus, the seismic anisotropy observed in the D″ layer can be reasonably explained by the LPO of the mixture of post-perovskite and ferro-periclase, where the LPO may result from the horizontal shear flow generated by the mantle convection.

Effect of water and stress on the lattice-preferred orientation of olivine

The influence of water and stress on the lattice-preferred orientation (LPO) of olivine aggregates was investigated through large strain, shear deformation experiments at high pressures and temperatures ( P = 0.5–2.1 GPa, T = 1470–1570 K) under both water-poor and water-rich conditions. The specimens are hot-pressed synthetic olivine aggregates or single crystals of olivine. Water was supplied to the sample by decomposition of a mixture of talc and brucite. Deformation experiments were conducted up to γ (shear strain) ∼ 6 using the Griggs apparatus where water fugacity was up to ∼ 13 GPa at the pressure of 2 GPa. The water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at P = 0.5–2 GPa was ∼ 400–1200 ppm H/Si. Several new types of LPO in olivine are found depending on water content and stress. Samples deformed in water-poor conditions show a conventional LPO of olivine where the olivine [100] axis is subparallel to the shear direction, the (010) plane subparallel to the shear plane (type-A). However, we identified three new types (type-B, C, and E) of LPO of olivine depending on the water content and stress. The type-B LPO of olivine which was found at relatively high stress and/or under moderate to high water content conditions is characterized by the olivine [001] axis subparallel to the shear direction, the (010) plane subparallel to the shear plane. The type-C LPO which was found at low stress and under water-rich conditions is characterized by the olivine [001] axis subparallel to the shear direction, the (100) plane subparallel to the shear plane. The type-E LPO which was found under low stress and moderate water content is characterized by the olivine [100] axis subparallel to the shear direction, the (001) plane subparallel to the shear plane. Observations by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) show that the dislocations in water-poor samples (type-A) are curved and both b = [100] and b = [001] dislocations have a similar population. Numerous subgrains are seen in water-poor samples in backscattered electron images. In contrast, water-rich samples (both type-B and type-C) contain mostly b = [001] dislocations and dislocations are straight and sub-grain boundaries are rare compared to those in water-poor samples. These observations suggest that (1) dominant slip systems in olivine change with water fugacity (and stress) and (2) grain boundary migration is enhanced in the presence of water. Seismic anisotropy corresponding to the fabrics under water-rich condition is significantly different from that under water-poor condition. Consequently, the relationship between seismic anisotropy and flow geometry in water-rich regions is expected to be different from that in water-poor regions in which type-A fabric dominates (i.e., the lithosphere). A few cases are discussed including anisotropy in the subduction zone and in the deep upper mantle.

Effect of temperature on the B- to C-type olivine fabric transition and implication for flow pattern in subduction zones

The effect of temperature on the B- to C-type fabric transition in olivine was investigated by large-strain shear deformation experiments. A new series of deformation experiments was conducted at 1273–1373 K, 2.0 GPa and strain-rates ranging from 3.8 × 10 −4 to 3.2 × 10 −5 s −1 under water-saturated conditions, and combined with previous results at 1473 K, the influence of temperature on deformation fabrics was investigated. Two types of olivine fabrics were identified, one in which the olivine [0 0 1] axis is subparallel to the shear direction and the (0 1 0) plane is parallel to the shear plane (B-type), and another one in which the [0 0 1] axis is parallel to the shear direction and the (1 0 0) plane is parallel to the shear plane (C-type). The B-type olivine fabric occurs at higher stress than the C-type, but the stress magnitude at which the B- to C-type fabric transition occurs decreases significantly with decreasing temperature. The temperature dependence of the transition stress is parameterized using a range of exponential flow law that is constrained by the experimental study. When the experimental results are extrapolated to lower temperatures, the fabric transition between B- and C-type is predicted to occur at ∼1 MPa and ∼1000 K. This prediction is consistent with the olivine fabrics in naturally deformed peridotites found in plate-convergent regions. The present study indicates that when some amounts of water (>200 ppm H/Si) are present, the B-type fabric will dominate in low temperature regions of the upper mantle whereas the C-type (or E-type) fabric will dominate at high temperatures. The complex seismic anisotropy in subduction zones can be explained by changes in the dominant type of olivine fabrics caused by spatial variations in physical and chemical variables, such as temperature and water content rather than by complexities in mantle wedge flow pattern.

Lattice preferred orientation in deformed polycrystalline (Mg,Fe)O and implications for seismic anisotropy in D″

Magnesiowüstite [(Mg,Fe)O] is an important constituent of the lower mantle, probably occupying about 20–25% of its volume. Laboratory and theoretical studies have shown this mineral to be highly elastically anisotropic at lower mantle pressures and temperatures. Thus, strain-induced formation of lattice preferred orientation (LPO) in magnesiowüstite is a candidate mechanism for the origin of anisotropic structure in D″. Although observations of seismic anisotropy within D″ are robust, both the occurrence and the style of that anisotropy are spatially variable. Two hypotheses have been offered to explain the observations of D″ anisotropy: LPO of intrinsically anisotropic minerals, or shape preferred orientation (SPO), perhaps in the form of horizontal layering or oriented inclusions. To investigate the first hypothesis, we performed confined simple shear deformation experiments in the dislocation creep regime using a gas-medium deformation apparatus over a range of compositions: the MgO and FeO endmembers and three intermediate compositions. Samples were deformed at 1273–1473 K, at a confining pressure of 300 MPa, to large shear strains ( γ = 3.5–4.5) using deformation pistons cut at 45°. After deformation, the LPO was measured by electron backscatter diffraction (EBSD). The LPO produced varied for differing compositions, indicating the activity on individual slip systems and/or the nature of grain boundary migration in (Mg,Fe)O are affected by composition and/or homologous temperature. We predicted seismic anisotropy from the measured LPOs and theoretically determined single-crystal elastic constants. Anisotropic behavior predicted from LPO agrees well with observations of D″ anisotropy, so the LPO hypothesis appears to satisfy the seismological constraints. Our calculated anisotropy patterns suggest that if D″ anisotropy is due to LPO of (Mg,Fe)O, then azimuthal variations in anisotropy in the horizontal plane should be present. Such azimuthal variations are not generally predicted for SPO-type hypotheses, and this may provide a means for distinguishing the cause of D″ anisotropy.

Evaluating Shear Rigidity of Woven Fabrics

A new shear tester of woven fabrics based on the trellis shear model is described. The tester can be used to carry out shear deformation experiments under quite low tensile stresses. The values of shear rigidity tested on the new tester were compared with those tested on the uniaxial tensile tester and the KES (Kawabata’s Evaluation System) instrument. The results confirm that the tester is more feasible and effective for measuring shear behavior in a natural state. This study establishes a foundation for constructing a universal instrument that can be used to measure shear deformation under any conditions.

Reaction-induced weakening of plagioclase–olivine composites

The localisation of strain into natural ductile shear zones is often associated with the occurrence of metamorphic reactions. In order to study the effects of solid–solid mineral reactions on plastic deformation of rocks, we have investigated the shear deformation of plagioclase–olivine composites during the reaction plagioclase + olivine → orthopyroxene + clinopyroxene + spinel (± garnet). Microstructures of plagioclase–olivine composites were studied after shear deformation experiments in a Griggs apparatus. Experiments were performed on anorthite–forsterite (An–Fo) and labradorite–forsterite (Lab–Fo) composites at 900 °C, confining pressures between 1000–1600 MPa and with constant shear strain rates of ∼5 × 10 −5 s −1. In absence of reaction, Lab–Fo composites are stronger than pure olivine and labradorite end-members that deform with a high temperature plasticity mechanism. Lab–Fo composites strain–harden due to the inhibition of extensive recrystallisation by interphase boundaries. In An–Fo composites, the reaction induces strain weakening by a switch from dislocation creep to grain size sensitive deformation mechanisms through the development of fine-grained (size < 0.5 μm) polyphase reaction products. Interconnecting layers of reaction products accommodate most of the applied strain by grain size sensitive creep. Recovery processes are pronounced during syndeformational reaction: original anorthite and olivine dynamically recrystallise by subgrain rotation and bulging recrystallisation. Presumably, the dynamic recrystallisation is caused by reduced stress conditions and partitioning of strain and strain rates between the new reaction products and the relict An–Fo grains. The results of our experiments are in good agreement with natural observations of shear localisation in the lower crust and upper mantle, and imply that anhydrous mineral reactions can be important causes for localisation of deformation.

Shear deformation experiments of forsterite at 11 GPa - 1400C in the multianvil apparatus

Synthetic forsterite samples were shear-deformed at 11 GPa, 1400°C in the multianvil apparatus. The deformation microstructures have been characterised by SEM, EBSD, X-ray diffraction peak broadening and strain anisotropy analysis, and TEM. Different time durations have been characterised with a view to follow the evolution of strain and stress in high-pressure deformation experiments. A high density of [001] dislocations is introduced during pressurization at room temperature although no significant macroscopic shear or crystal preferred orientations are induced at this stage. The deviatoric stress is probably on the order of 1.5 GPa. Heating at 1400°C leads to a rapid decrease of the density of these dislocations. The shear deformation at high-temperature leads to measurable strain and development of crystal preferred orientations after one hour. Stress and strain-rate continue to decrease with time, such that eight hour experiments exhibit microstructures where recovery is apparent. At this stage, the stress level is estimated at ca. 100 MPa from dislocation density measurements. Crystal preferred orientations and TEM characterisation show that glide of [001] dislocations on (100) or (010) is the dominant deformation mechanism. Further investigation is needed to determine whether inhibition of [100] glide in these experiments is due to the role of water or whether a physical effect of pressure is also contributing.

Comparison of Single Crystal Simple Shear Deformation Experiments with Crystal Plasticity Finite Element Simulations

The crystal plasticity finite element method is an ideal approach to simulate the mechanics, texture, and elastic‐plastic anisotropy of materials. Experimental validation of such simulations requires to measure these quantities without the disturbing influence of friction known for instance from channel‐die experiments. This work presents results obtained form simple shear deformation experiments of aluminium single crystals together with corresponding crystal plasticity finite element simulations.