scholarly journals Microstructural Analysis of a Mylonitic Mantle Xenolith Sheared at Laboratory-like Strain Rates from the Edge of the Wyoming Craton

Minerals ◽  
2021 ◽  
Vol 11 (9) ◽  
pp. 995
Author(s):  
Yuval Boneh ◽  
Emily J. Chin ◽  
Greg Hirth
Keyword(s):  
Grain Size ◽  
Time Scales ◽  
Electron Backscatter Diffraction ◽  
Microstructural Analysis ◽  
Optical Microscope ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Strain Rates ◽  
Crystallographic Preferred Orientation ◽  
Experimental Deformation

Combined observations from natural and experimental deformation microstructures are often used to constrain the rheological properties of the upper mantle. However, relating natural and experimental deformation processes typically requires orders of magnitude extrapolation in strain rate due to vastly different time scales between nature and the lab. We examined a sheared peridotite xenolith that was deformed under strain rates comparable to laboratory shearing time scales. Microstructure analysis using an optical microscope and electron backscatter diffraction (EBSD) was done to characterize the bulk crystallographic preferred orientation (CPO), intragrain misorientations, subgrain boundaries, and spatial distribution of grains. We found that the microstructure varied between monophase (olivine) and multiphase (i.e., olivine, pyroxene, and garnet) bands. Olivine grains in the monophase bands had stronger CPO, larger grain size, and higher internal misorientations compared with olivine grains in the multiphase bands. The bulk olivine CPO suggests a dominant (010)[100] and secondary activated (001)[100] that are consistent with the experimentally observed transition of the A to E-types. The bulk CPO and intragrain misorientations of olivine and orthopyroxene suggest that a coarser-grained initial fabric was deformed by dislocation creep coeval with the reduction of grain size due to dynamic recrystallization. Comparing the deformation mechanisms inferred from the microstructure with experimental flow laws indicates that the reduction of grain size in orthopyroxene promotes activation of diffusion creep and suggests a high activation volume for wet orthopyroxene dislocation creep.

scholarly journals Myrmekite and strain weakening in granitoid mylonites

2018 ◽  
Author(s):  
Alberto Ceccato ◽  
Luca Menegon ◽  
Giorgio Pennacchioni ◽  
Luiz Fernando Grafulha Morales
Keyword(s):  
Grain Size ◽  
Heterogeneous Nucleation ◽  
Eastern Alps ◽  
Grain Boundary Sliding ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Phase Mixing ◽  
Fine Grained ◽  
Fine Grain ◽  
Creep Cavitation

Abstract. At mid-crustal conditions, deformation of feldspar is mainly accomplished by a combination of fracturing, dissolution/precipitation and reaction-weakening mechanisms. In particular, K-feldspar is reaction-weakened by formation of strain-induced myrmekite – a fine-grained symplectite of plagioclase and quartz. Here we investigate with EBSD the microstructure of a granodiorite mylonite, developed at 420–460 °C during cooling of the Rieserferner pluton (Eastern Alps), to assess the microstructural processes and the role of weakening associated with myrmekite development. Our analysis shows that the crystallographic orientation of the plagioclase of pristine myrmekite was controlled by that of the replaced K-feldspar. Myrmekite nucleation resulted in both grain size reduction and ordered phase mixing by heterogeneous nucleation of quartz and plagioclase. The fine grain size of sheared myrmekite promoted grain size-sensitive creep mechanisms including fluid-assisted grain boundary sliding in plagioclase, coupled with heterogeneous nucleation of quartz within creep cavitation pores. Flow laws calculated for monomineralic quartz, feldspar, and quartz + plagioclase aggregates (sheared myrmekite), show that during mylonitization at 450 °C, grain-size-sensitive creep in sheared myrmekite accommodated strain rates several orders of magnitude higher than monomineralic quartz layers deforming by dislocation creep. Therefore, diffusion creep and grain size-sensitive processes contributed significantly to bulk rock weakening during mylonitization. Our results have implications for modelling the rheology of the mid-upper continental (felsic) crust.

scholarly journals Dislocation Creep of Olivine and Amphibole in Amphibole Peridotites from Åheim, Norway

Minerals ◽  
2021 ◽  
Vol 11 (9) ◽  
pp. 1018
Author(s):  
Sejin Jung ◽  
Takafumi Yamamoto ◽  
Jun-ichi Ando ◽  
Haemyeong Jung
Keyword(s):  
Electron Microscopy ◽  
Slip System ◽  
Electron Backscatter Diffraction ◽  
Microstructural Analysis ◽  
Dislocation Creep ◽  
Transmission Electron ◽  
Electron Backscatter ◽  
Different Types ◽  
Localized Shear Deformation ◽  
Backscatter Diffraction

Amphibole peridotite samples from Åheim, Norway, were analyzed to understand the deformation mechanism and microstructural evolution of olivine and amphibole through the Scandian Orogeny and subsequent exhumation process. Three Åheim amphibole peridotite samples were selected for detailed microstructural analysis. The Åheim amphibole peridotites exhibit porphyroclastic texture, abundant subgrain boundaries in olivine, and the evidence of localized shear deformation in the tremolite-rich layer. Two different types of olivine lattice preferred orientations (LPOs) were observed: B- and A-type LPOs. Electron backscatter diffraction (EBSD) mapping and transmission electron microscopy (TEM) observations revealed that most subgrain boundaries in olivine consist of dislocations with a (001)[100] slip system. The subgrain boundaries in olivine may have resulted from the deformation of olivine with moderate water content. In addition, TEM observations using a thickness-fringe method showed that the free dislocations of olivine with the (010)[100] slip system were dominant in the peridotites. Our data suggest that the subgrain boundaries and free dislocations in olivine represent a product of later-stage deformation associated with the exhumation process. EBSD mapping of the tremolite-rich layer revealed intracrystalline plasticity in amphibole, which can be interpreted as the activation of the (100)[001] slip system.

scholarly journals Myrmekite and strain weakening in granitoid mylonites

Solid Earth ◽  
2018 ◽  
Vol 9 (6) ◽  
pp. 1399-1419 ◽  
Author(s):  
Alberto Ceccato ◽  
Luca Menegon ◽  
Giorgio Pennacchioni ◽  
Luiz Fernando Grafulha Morales
Keyword(s):  
Grain Size ◽  
Heterogeneous Nucleation ◽  
Eastern Alps ◽  
Grain Boundary Sliding ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Electron Backscattered Diffraction ◽  
Fine Grained ◽  
Fine Grain ◽  
Creep Cavitation

Abstract. At mid-crustal conditions, deformation of feldspar is mainly accommodated by a combination of fracturing, dissolution–precipitation, and reaction-weakening mechanisms. In particular, K-feldspar is reaction-weakened by the formation of strain-induced myrmekite – a fine-grained symplectite of plagioclase and quartz. Here we use electron backscattered diffraction to (i) investigate the microstructure of a granodiorite mylonite, developed at  ∼ 450 °C during cooling of the Rieserferner pluton (Eastern Alps); and (ii) assess the microstructural processes and the weakening associated with myrmekite development. Our analysis shows that the crystallographic orientation of plagioclase in pristine myrmekite was controlled by that of the replaced K-feldspar. Myrmekite nucleation resulted in both grain-size reduction and anti-clustered phase mixing by heterogeneous nucleation of quartz and plagioclase. The fine grain size of sheared myrmekite promoted grain-size-sensitive creep mechanisms including fluid-assisted grain boundary sliding in plagioclase, coupled with heterogeneous nucleation of quartz within creep cavitation pores. Flow laws, calculated for monomineralic quartz, feldspar, and quartz + plagioclase aggregates (sheared myrmekite) during deformation at 450 °C, show that grain-size-sensitive creep in sheared myrmekite accommodated strain rates several orders of magnitude higher than monomineralic quartz layers deforming by dislocation creep. Therefore, diffusion creep and grain-size-sensitive processes contributed significantly to bulk rock weakening during mylonitization. Our results have implications for modelling the rheology of the felsic middle crust.

Microstructure Modeling of Dynamically Recrystallized Grain Size of Sintered Al–4 wt % B4C Composite During Hot Upsetting

2017 ◽  
Vol 140 (2) ◽  
Author(s):  
R. Seetharam ◽  
S. Kanmani Subbu ◽  
M. J. Davidson
Keyword(s):  
Grain Size ◽  
Size Control ◽  
Absolute Error ◽  
Optical Microscope ◽  
Strain Rates ◽  
Hollomon Parameter ◽  
Microstructure Modeling ◽  
Grain Size Control ◽  
And Performance ◽  
Recrystallized Grain Size

Grain size control of any engineering metal is very important in the hot upsetting process. Generally, the grain size directly controls the mechanical properties and performance of the material. Al–B4C composite finds extensive applications in nuclear industries, defense, and electronic industries. Therefore, the aim of this work is to study the dynamic recrystallization (DRX) behavior of Al–4 wt % B4C composite during the hot upsetting test. Experimental work was performed on sintered Al–4 wt % B4C preforms at various initial relative density (IRD) values of 80%, 85%, and 90%, and over the temperature range of 300–500 °C and strain rates range of 0.1–0.3 s−1. The DRXed grain size of Al–4 wt % B4C preforms for IRDes, and temperatures and strain rates were evaluated by using an optical microscope. The activation energy (Q) and Zener–Hollomon parameter of sintered Al–4 wt % B4C preforms were calculated for various deformation conditions and IRDes. The mathematical models of DRX were developed as a function of Zener–Hollomon parameter for various IRDes to predict the DRXed grain size. It was found that the DRXed grain size decreases with increasing Zener–Hollomon parameter. Verification tests were done between the measured and predicted DRXed grain size for various IRDes, and absolute and mean absolute error was found to be 9.92% and 8.58%, respectively.

Grain Refinement during Superplastic Deformation of Coarse-Grained Al-Mg-Cu Alloys

2006 ◽  
Vol 509 ◽  
pp. 75-80 ◽  
Author(s):  
M.I. Cruz-Palacios ◽  
D. Hernández-Silva ◽  
L.A. Barrales-Mora ◽  
M.A. García-Bernal
Keyword(s):  
Grain Size ◽  
Strain Rate ◽  
Grain Refinement ◽  
Strain Rate Sensitivity ◽  
Rate Sensitivity ◽  
Mg Alloy ◽  
Coarse Grained ◽  
Dislocation Creep ◽  
Rolled Plate ◽  
Strain Rates

In the present study the superplastic behavior of Al-6%Mg–0.5%Cu and Al–8%Mg– 0.5%Cu in a coarse grain size condition has been studied. The alloys are melted in an electrical furnace under argon atmosphere. The ingots (25 mm thick) are homogenized at 400 °C during 72 h and then rolled at 430 °C to a thickness of 5 mm. The mean grain size after rolling is 55 µm for the 6%Mg alloy and 61 µm for the 8%Mg alloy. Tensile test specimens are machined from the rolled plate in the rolling direction. Strain-rate-change tests at temperatures between 300 and 450 °C and strain rates between 1x10-4 and 1x10-1 s-1 are carried out to determine the strain rate sensitivity of the flow stress. Finally, elongation to failure tests are conducted at temperatures and strain rates where the alloys show a high strain rate sensitivity. Elongations higher than 390 % are obtained for the 8%Mg alloy. It is observed that the grip regions of the deformed samples show coarser grains than the regions near to the fracture surface. This means that grain refinement takes place during deformation, suggesting that the principal deformation mechanism is dislocation creep.

scholarly journals Anisotropic diffusion creep in postperovskite provides a new model for deformation at the core−mantle boundary

2019 ◽  
Vol 116 (52) ◽  
pp. 26389-26393
Author(s):  
David P. Dobson ◽  
Alexander Lindsay-Scott ◽  
Simon A. Hunt ◽  
Edward Bailey ◽  
Ian G. Wood ◽  
...  
Keyword(s):  
Anisotropic Diffusion ◽  
Seismic Anisotropy ◽  
Shear Zones ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Chemical Diffusivity ◽  
Crystallographic Preferred Orientation ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core

The lowermost portion of Earth’s mantle (D″) above the core−mantle boundary shows anomalous seismic features, such as strong seismic anisotropy, related to the properties of the main mineral MgSiO3postperovskite. But, after over a decade of investigations, the seismic observations still cannot be explained simply by flow models which assume dislocation creep in postperovskite. We have investigated the chemical diffusivity of perovskite and postperovskite phases by experiment and ab initio simulation, and derive equations for the observed anisotropic diffusion creep. There is excellent agreement between experiments and simulations for both phases in all of the chemical systems studied. Single-crystal diffusivity in postperovskite displays at least 3 orders of magnitude of anisotropy by experiment and simulation (Da= 1,000Db;Db≈Dc) in zinc fluoride, and an even more extreme anisotropy is predicted (Da= 10,000Dc;Dc= 10,000Db) in the natural MgSiO3system. Anisotropic chemical diffusivity results in anisotropic diffusion creep, texture generation, and a strain-weakening rheology. The results for MgSiO3postperovskite strongly imply that regions within the D″ region of Earth dominated by postperovskite will 1) be substantially weaker than regions dominated by perovskite and 2) develop a strain-induced crystallographic-preferred orientation with strain-weakening rheology. This leads to strain localization and the possibility to bring regions with significantly varying textures into close proximity by strain on narrow shear zones. Anisotropic diffusion creep therefore provides an attractive alternative explanation for the complexity in observed seismic anisotropy and the rapid lateral changes in seismic velocities in D″.

Superplastic Behaviors of Casting AZ31 Magnesium Alloy

2007 ◽  
Vol 551-552 ◽  
pp. 237-240
Author(s):  
Hong Bo Li ◽  
J. Zhao ◽  
Jun Ting Luo ◽  
M. Hang
Keyword(s):  
Grain Size ◽  
Magnesium Alloy ◽  
Deformation Behavior ◽  
Superplastic Deformation ◽  
Optical Microscope ◽  
Az31 Magnesium Alloy ◽  
Uniaxial Tensile ◽  
Strain Rates ◽  
Tensile Experiment ◽  
Uniaxial Tensile Experiment

The superplasticity of magnesium alloy is important in industrial application. However the superplastic deformation of casting magnesium alloy is hard to be realized. In this paper, the stress–strain behaviors of casting AZ31 magnesium alloy with various strain rates at different deformation temperatures were investigated. The alloy was tested in the tensile condition with initial grain size of 25μm. It was found that the elongation of the alloy at 400°C with ε& = 4.25×10-4 s-1 is almost 200%. According to the results of uniaxial tensile experiment, the alloy exhibited superplastic deformation behavior with the slow stain rate in a temperature range of 350 to 450°C. The microstructures deformed and undeformed samples were observed with aid of optical microscope.

scholarly journals Self-Consistent Grain Size Evolution controls Strain Localization during Rifting

2021 ◽  
Author(s):  
Jonas Ruh ◽  
Leif Tokle ◽  
Whitney Behr
Keyword(s):  
Grain Size ◽  
Upper Mantle ◽  
Numerical Models ◽  
Shear Zones ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Plate Boundaries ◽  
Mantle Dynamics ◽  
Size Evolution ◽  
Dominant Deformation Mechanism

Abstract Geodynamic numerical models often employ solely grain-size-independent dislocation creep to describe upper mantle dynamics. However, observations from nature and rock deformation experiments suggest that shear zones can transition to a grain-size-dependent creep mechanism due to dynamic grain size evolution, with important implications for the overall strength of plate boundaries. We apply a two-dimensional thermo-mechanical numerical model with a composite diffusion-dislocation creep rheology coupled to a dynamic grain size evolution model based on the paleowattmeter. Results indicate average olivine grain sizes of 3–12 cm for the upper mantle below the LAB, while in the lithosphere grain size ranges from 0.3–3 mm at the Moho to 6–15 cm at the LAB. Such a grain size distribution results in dislocation creep being the dominant deformation mechanism in the upper mantle. However, deformation-related grain size reduction below 100 μm activates diffusion creep along lithospheric-scale shear zones during rifting, affecting the overall strength of tectonic plate boundaries.

scholarly journals On the self-regulating effect of grain size evolution in mantle convection models: application to thermochemical piles

Solid Earth ◽  
2020 ◽  
Vol 11 (3) ◽  
pp. 959-982 ◽  
Author(s):  
Jana Schierjott ◽  
Antoine Rozel ◽  
Paul Tackley
Keyword(s):  
Grain Size ◽  
Recovery Time ◽  
Subduction Zones ◽  
Shear Velocity ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Lower Shear ◽  
Size Evolution ◽  
Dynamic Recrystallisation ◽  
Dependent Viscosity

Abstract. Seismic studies show two antipodal regions of lower shear velocity at the core–mantle boundary (CMB) called large low-shear-velocity provinces (LLSVPs). They are thought to be thermally and chemically distinct and therefore might have a different density and viscosity than the ambient mantle. Employing a composite rheology, using both diffusion and dislocation creep, we investigate the influence of grain size evolution on the dynamics of thermochemical piles in evolutionary geodynamic models. We consider a primordial layer and a time-dependent basalt production at the surface to dynamically form the present-day chemical heterogeneities, similar to earlier studies, e.g. by Nakagawa and Tackley (2014). Our results show that, relative to the ambient mantle, grain size is higher inside the piles, but, due to the high temperature at the CMB, the viscosity is not remarkably different from ambient mantle viscosity. We further find that although the average viscosity of the detected piles is buffered by both grain size and temperature, the viscosity is influenced predominantly by grain size. In the ambient mantle, however, depending on the convection regime, viscosity can also be predominantly controlled by temperature. All pile properties, except for temperature, show a self-regulating behaviour: although grain size and viscosity decrease when downwellings or overturns occur, these properties quickly recover and return to values prior to the downwelling. We compute the necessary recovery time and find that it takes approximately 400 Myr for the properties to recover after a resurfacing event. Extrapolating to Earth values, we estimate a much smaller recovery time. We observe that dynamic recrystallisation counteracts grain growth inside the piles when downwellings form. Venus-type resurfacing episodes reduce the grain size in piles and ambient mantle to a few millimetres. More continuous mobile-lid-type downwellings limit the grain size to a centimetre. Consequently, we find that grain-size-dependent viscosity does not increase the resistance of thermochemical piles to downgoing slabs. Mostly, piles deform in grain-size-sensitive diffusion creep, but they are not stiff enough to counteract the force of downwellings. Hence, we conclude that the location of subduction zones could be responsible for the location and stability of the thermochemical piles of the Earth because of dynamic recrystallisation.

scholarly journals Stagnant-lid convection with diffusion and dislocation creep rheology: Influence of a non-evolving grain size

2019 ◽  
Vol 220 (1) ◽  
pp. 18-36
Author(s):  
Falko Schulz ◽  
Nicola Tosi ◽  
Ana-Catalina Plesa ◽  
Doris Breuer
Keyword(s):  
Grain Size ◽  
Nusselt Number ◽  
Rayleigh Number ◽  
Flow Pattern ◽  
Numerical Models ◽  
Activation Parameters ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Linear Diffusion ◽  
Average Grain Size

SUMMARY Heat transfer in one-plate planets is governed by mantle convection beneath the stagnant lid. Newtonian diffusion creep and non-Newtonian dislocation creep are the main mechanisms controlling large-scale mantle deformation. Diffusion creep strongly depends on the grain size (d), which in turn controls the relative importance of the two mechanisms. However, dislocation creep is usually neglected in numerical models of convection in planetary mantles. These mostly assume linear diffusion creep rheologies, often based on reduced activation parameters (compared to experimental values) that are thought to mimic the effects of dislocation creep and, as a side benefit, also ease the convergence of linear solvers. Assuming Mars-like parameters, we investigated the influence of a non-evolving grain size on Rayleigh–Bénard convection in the stagnant lid regime. In contrast to previous studies based on the Frank–Kamentskii approximation, we used Arrhenius laws for diffusion and dislocation creep—including temperature as well as pressure dependence—based on experimental measurements of olivine deformation. For d ≲ 2.5 mm, convection is dominated by diffusion creep. We observed an approximately equal partitioning between the two mechanisms for d ≈ 5 mm, while dislocation creep dominates for d ≳ 8 mm. Independent estimates of an average grain size of few mm up to 1 cm or more for present-day Mars suggest thus that dislocation creep plays an important role and possibly dominates the deformation. Mimicking dislocation creep convection using an effective linear rheology with reduced activation parameters, as often done in simulations of convection and thermal evolution of Mars, has significant limitations. Although it is possible to mimic mean temperature, mean lid thickness and Nusselt number, there are important differences in the flow pattern, root mean square velocity, and lid shape. The latter in particular affects the amount and distribution of partial melt, suggesting that care should be taken upon predicting the evolution of crust production when using simplified rheologies. The heat transport efficiency expressed in terms of the Nusselt number as a function of the Rayleigh number is thought to depend on the deformation mechanisms at play. We show that the relative volume in which dislocation creep dominates has nearly no influence on the Nusselt–Rayleigh scaling relation when a mixed rheology is used. In contrast, the flow pattern influences the Nusselt number more strongly. We derived a scaling law for the Nusselt number based on the mean lid thickness (〈L〉) and on the effective Rayleigh number (Raeff) obtained by suitably averaging the viscosity beneath the stagnant lid. We found that the Nusselt number follows the scaling $\mathrm{Nu} = 0.37 \langle L \rangle ^{-0.666} \mathrm{Ra}_{\mathrm{eff}}^{0.071}$ regardless of the deformation mechanism.

Sign in / Sign up

Export Citation Format

Share Document