On the Origin of Ultramylonites

Mapping Intimacies ◽

10.5194/egusphere-egu2020-3938 ◽

2020 ◽

Author(s):

Jacques Précigout

Keyword(s):

Grain Size ◽

Grain Boundary ◽

Host Rock ◽

Shear Zones ◽

Smooth Transition ◽

Seismogenic Zone ◽

Fine Grained ◽

Lattice Preferred Orientation ◽

Dislocation Densities ◽

Creep Cavitation

Deformation of lithospheric rocks regularly localizes into high-strain shear zones that include fine-grained ultramylonites. Occurring as quasi-straight layers of intimately mixed phases that often describe sharp transitions with the host rock, these structures may channelize fluid flow[1,2] and could serve as precursors for deep earthquakes[3]. However, although intensively documented, ultramylonites originate from still unknown processes. Here I focus on a mylonitic complex that includes numerous mantle ultramylonites in the Ronda peridotite (Spain). Among them, I was able to highlight one of their precursors that I better describe as a long and straight grain boundary, along which four-grain junctions are observed with randomly oriented grains of olivine and pyroxenes. This precursor starts from a pyroxene porphyroclast and extends to an incipient, weakly undulated ultramylonite, where intimate phase mixing arises with asymmetrical grain size distribution. While the finer grain size locates on one side, describing a sharp &#8211; but continuous &#8211; transition with the host rock, the grain size gradually increases towards the other side, giving rise to a smooth transition. All phases have a very weak lattice preferred orientation (LPO) in the ultramylonite, which strongly differs from the host rock where olivine is highly deformed with evidence of high dislocation densities and a strong LPO. Altogether, these features shed light on the origin of mantle ultramylonites that I attribute to a migrating grain boundary, the sliding of which continuously produces new grains by phase nucleation, probably at the favor of transient four-grain junctions. Nucleated grains then grow and progressively detach from the precursor as it keeps on migrating depending on the dislocation densities in the host rock. Although such an unusual grain boundary remains to be understood in terms of source mechanism, these findings provide new constraints on the appearing and development of ultramylonites.&#160;[1] Fusseis, F., Regenauer-Lieb, K., Liu, J., Hough, R. M. & De Carlo, F. Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones. Nature 459: 974&#8211;977 (2009)[2] Pr&#233;cigout, J., Prigent, C., Palasse, L. & Pochon, A. Water pumping in mantle shear zones. Nat. commun. 8: 15736, https://doi.org/10.1038/ncomms15736 (2017)[3] White, J. C. Paradoxical pseudotachylyte &#8211; Fault melt outside the seismogenic zone. J. Struct. Geol. 38: 11-20 (2012)

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Recrystallization of ice enhances the creep and vulnerability to fracture of ice shelves

10.5194/egusphere-egu21-3266 ◽

2021 ◽

Author(s):

Meghana Ranganathan ◽

Brent Minchew ◽

Colin Meyer ◽

Matej Pec

Keyword(s):

Grain Size ◽

Ice Sheet ◽

Shear Zones ◽

Dislocation Creep ◽

Model Parameters ◽

Localized Deformation ◽

Ice Shelves ◽

Fine Grained ◽

Initiation And Propagation ◽

Rapid Deformation

The initiation and propagation of fractures in floating regions of Antarctica has the potential to destabilize large regions of the ice sheet, leading to significant sea-level rise. While observations have shown rapid, localized deformation and damage in the margins of fast-flowing glaciers, there remain gaps in our understanding of how rapid deformation affects the creep and toughness of ice. Here we derive a model for dynamic recrystallization in ice and other rocks that includes a novel representation of migration recrystallization, which is absent from existing models but is likely to be dominant in warm areas undergoing rapid deformation within the ice sheet. We show that, in regions of elevated strain rate, grain sizes in ice may be larger than expected (~15 mm) due to migration recrystallization, a significant deviation from solid earth studies which find fine-grained rock in shear zones. This may imply that ice in shear margins deforms primarily by dislocation creep, suggesting a flow-law exponent of n=4 in these regions. Further, we find from existing models that this increase in grain size results in a decrease in tensile strength of ice by ~75% in the margins of glaciers. Thus, we expect that this increase in grain size makes the margins of fast-flowing glaciers less viscous and more vulnerable to fracture than we may suppose from standard model parameters.

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Myrmekite and strain weakening in granitoid mylonites

10.5194/se-2018-70 ◽

2018 ◽

Cited By ~ 2

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.

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Grain size and grain boundary character distribution in ultra-fine grained (ECAP) nickel

Materials Science and Engineering A ◽

10.1016/j.msea.2007.11.072 ◽

2008 ◽

Vol 491 (1-2) ◽

pp. 1-7 ◽

Cited By ~ 41

Author(s):

K. Sitarama Raju ◽

M. Ghanashyam Krishna ◽

K.A. Padmanabhan ◽

K. Muraleedharan ◽

N.P. Gurao ◽

...

Keyword(s):

Grain Size ◽

Grain Boundary ◽

Grain Boundary Character Distribution ◽

Boundary Character ◽

Fine Grained ◽

Grain Boundary Character ◽

Character Distribution ◽

Boundary Character Distribution

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Effects of resolution on the measurement of grain ‘size’

Mineralogical Magazine ◽

10.1180/minmag.1985.049.353.07 ◽

1985 ◽

Vol 49 (353) ◽

pp. 539-546 ◽

Cited By ~ 5

Author(s):

R. Dearnley

Keyword(s):

Grain Size ◽

Image Analysis ◽

Grain Boundary ◽

Surface Area ◽

Unit Volume ◽

Fractal Dimensions ◽

Size Analysis ◽

Automatic Image Analysis ◽

Fine Grained ◽

Kinetics Of

AbstractMeasurements of fine-grained dolerites by optical automatic image analysis are used to illustrate the effects of magnification and resolution on the values obtained for grain ‘size’, grain boundary length, surface area per unit volume, and other parameters. Within the measured range of optical magnifications (× 26 to × 3571) and resolutions (1.20 × 10−3 cm to 8.50 × 10−6 cm), it is found that the values of all grain parameters estimated by chord size analysis vary with magnification. These results are interpreted in terms of the concepts of ‘fractal dimensions’ introduced by Mandelbrot (1967, 1977). For some comparative purposes the fractal relationships may be of little significance as relative changes of size, surface area, and other parameters can be expressed adequately at given magnification(s). But for many studies, for instance in kinetics of grain growth, the actual diameter or surface area per unit volume is an important dimension. The consequences are disconcerting and suggest that it may be difficult in some instances to specify the ‘true’ measurements of various characteristics of fine-grained aggregates.

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Electrical properties of ultrafine-grained yttria-stabilized zirconia ceramics

Journal of Materials Research ◽

10.1557/jmr.1997.0314 ◽

1997 ◽

Vol 12 (9) ◽

pp. 2374-2380 ◽

Cited By ~ 38

Author(s):

Shusheng Jiang ◽

Walter A. Schulze ◽

Vasantha R. W. Amarakoon ◽

Gregory C. Stangle

Keyword(s):

Grain Size ◽

Oxygen Vacancy ◽

Grain Boundary ◽

Oxygen Vacancies ◽

Synthesis Process ◽

Physical Parameters ◽

Fine Grained ◽

Oxygen Ion ◽

Average Grain Size ◽

Vacancy Concentrations

Nanoparticles of yttria-doped tetragonal zirconia polycrystalline ceramics (Y-TZP) with an average crystallite size of less than 9 nm were prepared by a combustion synthesis process. Dense and fine-grained (<200 nm) Y-TZP ceramics were obtained by fast-firing using temperatures lower than 1400 °C and dwell times of less than 2 min. Impedance spectroscopy was employed to measure conductivities of oxygen vacancies in the grain and the grain boundary of the fine-grained Y-TZP. The relationships between the concentration of the oxygen vacancies in the grain boundary and measurable physical parameters were determined semiquantitatively. The oxygen vacancy concentrations and activation energies for the oxygen-ion conduction in the grain and the grain boundary of the fine-grained Y-TZP were found to be independent of the average grain size in the average grain-size range of 90–200 nm. These experimental results suggest that, in order to retain the abnormally high oxygen vacancy concentrations of the Y-TZP nanoparticles and thus enhance the oxygen-ion conductivity, it may be necessary to decrease the average grain size to approximately 10 nm.

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Relationship between microstructures and resistance in mafic assemblages that deform and transform

Solid Earth ◽

10.5194/se-11-2141-2020 ◽

2020 ◽

Vol 11 (6) ◽

pp. 2141-2167

Author(s):

Nicolas Mansard ◽

Holger Stünitz ◽

Hugues Raimbourg ◽

Jacques Précigout ◽

Alexis Plunder ◽

...

Keyword(s):

Grain Size ◽

Shear Strain ◽

Phase Distribution ◽

Shear Bands ◽

Constant Strain Rate ◽

Peak Stress ◽

Confining Pressure ◽

Shear Zones ◽

Reaction Products ◽

Fine Grained

Abstract. Syn-kinematic mineral reactions play an important role for the mechanical properties of polymineralic rocks. Mineral reactions (i.e., nucleation of new phases) may lead to grain size reduction, producing fine-grained polymineralic mixtures, which have a strongly reduced viscosity because of the activation of grain-size-sensitive deformation processes. In order to study the effect of deformation–reaction feedback(s) on sample strength, we performed rock deformation experiments on “wet” assemblages of mafic compositions in a Griggs-type solid-medium deformation apparatus. Shear strain was applied at constant strain rate (10−5 s−1) and constant confining pressure (1 GPa) with temperatures ranging from 800 to 900 ∘C. At low shear strain, the assemblages that react faster are significantly weaker than the ones that react more slowly, demonstrating that reaction progress has a first-order control on rock strength. With increasing strain, we document two contrasting microstructural scenarios: (1) the development of a single throughgoing high-strain zone of well-mixed, fine-grained aggregates, associated with a significant weakening after peak stress, and (2) the development of partially connected, nearly monomineralic shear bands without major weakening. The lack of weakening is caused by the absence of interconnected well-mixed aggregates of fine-grained reaction products. The nature of the reaction products, and hence the intensity of the mechanical weakening, is controlled by the microstructures of the reaction products to a large extent, e.g., the amount of amphibole and the phase distribution of reaction products. The samples with the largest amount of amphibole exhibit a larger grain size and show less weakening. In addition to their implications for the deformation of natural shear zones, our findings demonstrate that the feedback between deformation and mineral reactions can lead to large differences in mechanical strength, even at relatively small initial differences in mineral composition.

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Formation of Ultramylonites in an Upper Mantle Shear Zone, Erro-Tobbio, Italy

Minerals ◽

10.3390/min11101036 ◽

2021 ◽

Vol 11 (10) ◽

pp. 1036

Author(s):

Jolien Linckens ◽

Sören Tholen

Keyword(s):

Grain Size ◽

Shear Zone ◽

Upper Mantle ◽

Microstructural Analysis ◽

Shear Zones ◽

Geodynamic Setting ◽

Mixing Processes ◽

Phase Mixing ◽

Fine Grained ◽

Order Of Magnitude

Deformation in the upper mantle is localized in shear zones. In order to localize strain, weakening has to occur, which can be achieved by a reduction in grain size. In order for grains to remain small and preserve shear zones, phases have to mix. Phase mixing leads to dragging or pinning of grain boundaries which slows down or halts grain growth. Multiple phase mixing processes have been suggested to be important during shear zone evolution. The importance of a phase mixing process depends on the geodynamic setting. This study presents detailed microstructural analysis of spinel bearing shear zones from the Erro-Tobbio peridotite (Italy) that formed during pre-alpine rifting. The first stage of deformation occurred under melt-free conditions, during which clinopyroxene and olivine porphyroclasts dynamically recrystallized. With ongoing extension, silica-undersaturated melt percolated through the shear zones and reacted with the clinopyroxene neoblasts, forming olivine–clinopyroxene layers. Furthermore, the melt reacted with orthopyroxene porphyroclasts, forming fine-grained polymineralic layers (ultramylonites) adjacent to the porphyroclasts. Strain rates in these layers are estimated to be about an order of magnitude faster than within the olivine-rich matrix. This study demonstrates the importance of melt-rock reactions for grain size reduction, phase mixing and strain localization in these shear zones.

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Myrmekite and strain weakening in granitoid mylonites

Solid Earth ◽

10.5194/se-9-1399-2018 ◽

2018 ◽

Vol 9 (6) ◽

pp. 1399-1419 ◽

Cited By ~ 6

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.

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Development of interconnected fine-grained polyphase networks during progressive exhumation of a shear zone

10.5194/egusphere-egu2020-12715 ◽

2020 ◽

Author(s):

Alexander Lusk ◽

John Platt

Keyword(s):

Grain Size ◽

Grain Boundary ◽

Shear Zone ◽

Strain Localization ◽

Cooling System ◽

Boundary Migration ◽

Grain Boundary Sliding ◽

Fine Grained ◽

Subgrain Rotation ◽

Boundary Sliding

Present exposure of the ductile Caledonian retrowedge in northwestern Scotland records the evolution of a shear zone that was exhuming while actively deforming, providing a natural laboratory to study strain localization in a progressively cooling system. Examination of rocks from two detailed transects across this region consistently show a transition from microstructures that are dominated by interconnected phyllosilicate networks in a quartz-rich matrix with feldspar porphyroclasts, to interconnected fine-grained regions of mixed quartz + phyllosilicate + feldspar. These polyphase regions are demonstrably weaker than surrounding quartz layers and likely deform by grain-size sensitive mechanisms including diffusion-accommodated grain boundary sliding.Microstructures characterized by a quartz-rich matrix and interconnected phyllosilicates undergo quartz recrystallization by high temperature grain boundary migration and are dominated by prism a slip. In contrast, fine-grained polyphase microstructures record quartz recrystallization dominated by subgrain rotation and activation of rhomb a and basal a slip systems. We propose transient hardening occurs in quartz-dominated regions as quartz with a strong Y-axis maximum undergoes the switch from prism a easy slip to basal a easy slip during cooling, and thus partitions strain into interconnected phyllosilicate layers. In response, interconnected phyllosilicate layers undergo mechanical comminution, becoming increasingly mixed by grain-size sensitive creep processes to form polyphase layers as they accommodate an increased proportion of strain. This transition from quartz-rich matrix with phyllosilicate interconnected weak layers to fine-grained, polyphase weak layers could be of first-order importance in strain localization within polyphase mylonitic and ultramylonitic rocks.

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