scholarly journals The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep

Solid Earth ◽  
2013 ◽  
Vol 4 (2) ◽  
pp. 315-330 ◽  
Author(s):  
J. Bial ◽  
C. A. Trepmann
Keyword(s):  
Plastic Deformation ◽  
Upper Mantle ◽  
Boundary Migration ◽  
High Stress ◽  
Preferred Orientation ◽  
Dislocation Creep ◽  
Polarization Microscopy ◽  
Crystallographic Preferred Orientation ◽  
Fine Grained ◽  
Serpentinized Peridotite

Abstract. We present microfabrics in high-pressure, metamorphic, partly serpentinized peridotite mylonites from the Voltri Massif, in which porphyroclasts and matrix record independent deformation events. The microfabrics are analysed using polarization microscopy and electron microscopy (SEM/EBSD, EMP). The mylonites contain diopside and olivine porphyroclasts originating from the mantle protolith embedded in a fine-grained matrix consisting mainly of antigorite and minor olivine and pyroxene. The porphyroclasts record brittle and crystal-plastic deformation of the peridotite at upper-mantle conditions and differential stresses of a few hundred MPa. After the peridotites became serpentinized, deformation occurred mainly by dissolution–precipitation creep resulting in a pronounced foliation of the antigorite matrix, crenulation cleavages and newly precipitated olivine and pyroxene from the pore fluid at sites of dilation, i.e. in strain shadows next to porphyroclasts and folded fine-grained antigorite layers. Antigorite reveals a pronounced associated shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) with the basal (001) cleavage plane oriented in the foliation plane. In monomineralic antigorite aggregates at sites of stress concentration around porphyroclasts, a characteristically reduced grain size and deflecting CPO as well as sutured grain boundaries indicate also some contribution of crystal-plastic deformation and grain-boundary migration of antigorite. In contrast, the absence of any intragranular deformation features in newly precipitated olivine in strain shadows reveals that stresses were not sufficiently high to allow for significant dislocation creep of olivine at conditions at which antigorite is stable. The porphyroclast microstructures are not associated with the microstructures of the mylonitic matrix, but are inherited from an independent earlier deformation. The porphyroclasts record a high-stress deformation of the peridotite with dislocation creep of olivine in the upper mantle probably related to rifting processes, whereas the serpentinite matrix records dominantly dissolution–precipitation creep and low stresses during subduction and exhumation.

scholarly journals The microstructural record of porphyroclasts and matrix of serpentinite mylonites – from brittle and crystal-plastic deformation to dissolution-precipitation creep

2013 ◽  
Vol 5 (1) ◽  
pp. 365-390 ◽  
Author(s):  
J. Bial ◽  
C. A. Trepmann
Keyword(s):  
Plastic Deformation ◽  
High Pressure ◽  
Upper Mantle ◽  
High Stress ◽  
Oceanic Lithosphere ◽  
Dislocation Creep ◽  
Polarization Microscopy ◽  
Fine Grained ◽  
Dehydration Reactions ◽  
Deformation Features

Abstract. We examine the microfabric development in high-pressure, low-temperature metamorphic serpentinite mylonites exposed in the Erro-Tobbio Unit (Voltri Massif, Italy) using polarization microscopy and electron microscopy (SEM/EBSD, EMP). The mylonites are derived from mantle peridotites, were serpentinized at the ocean floor and underwent high pressure metamorphism during Alpine subduction. They contain diopside and olivine porphyroclasts embedded in a fine-grained matrix essentially consisting of antigorite. The porphyroclasts record brittle and crystal-plastic deformation of the original peridotites in the upper mantle at stresses of a few hundred MPa. After the peridotites became serpentinized, deformation occurred mainly by dissolution-precipitation creep resulting in a foliation with flattened olivine grains at phase boundaries with antigorite, crenulation cleavages and olivine and antigorite aggregates in strain shadows next to porphyroclasts. It is suggested that the fluid was provided by dehydration reactions of antigorite forming olivine and enstatite during subduction and prograde metamorphism. At sites of stress concentration around porphyroclasts antigorite reveals an associated SPO and CPO, characteristically varying grain sizes and sutured grain boundaries, indicating deformation by dislocation creep. Stresses were probably below a few tens of MPa in the serpentinites, which was not sufficiently high to allow for crystal-plastic deformation of olivine at conditions at which antigorite is stable. Accordingly, any intragranular deformation features of the newly precipitated olivine in strain shadows are absent. The porphyroclast microstructures are not associated with the microstructures of the mylonitic matrix, but are inherited from an independent earlier deformation. The porphyroclasts record a high-stress deformation in the upper mantle of the oceanic lithosphere probably related to rifting processes, whereas the antigorite matrix records deformation at low stresses during subduction and exhumation.

Reaction-enhanced ductile deformation during carbonation of serpentinized peridotite

2021 ◽  
Author(s):  
Manuel D. Menzel ◽  
Janos L. Urai ◽  
Peter B. Kelemen ◽  
Greg Hirth ◽  
Alexander Schwedt ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Grain Boundary Sliding ◽  
Preferred Orientation ◽  
Ductile Deformation ◽  
Dislocation Creep ◽  
Crystallographic Preferred Orientation ◽  
Serpentinized Peridotite ◽  
Boundary Sliding ◽  
Penetrative Foliation ◽  
Drilling Project

<p>Carbonated serpentinites record carbon fluxes in subduction zones and are a possible natural analogue for carbon capture and storage via mineralization, but the processes by which the reaction of serpentinite to listvenite (magnesite-quartz rocks) goes to completion are not well understood. Large-scale hydration and carbonation of peridotite in the Oman Ophiolite produced massive listvenites, which have been drilled by the ICDP Oman Drilling Project (OmDP, site BT1) [1]. Here we report evidence for localized ductile deformation during serpentinite carbonation in core BT1B, based on observations from optical microscopy, cathodoluminescence microscopy, SEM, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) in segments of the core that lack a brittle overprint after listvenite formation [2].</p><p>Microstructural analysis of the serpentinized peridotite protolith shows a range of microstructures common in serpentinite with local ductile deformation manifested by a shape and crystallographic preferred orientation and kinking of lizardite. Listvenites with ductile deformation microstructures contain a penetrative foliation due to a shape preferred alignment of magnesite spheroids and/or dendritic magnesite, bending around Cr-spinel porphyroclasts. Locally the foliation can be due to aligned dendritic overgrowths on euhedral magnesite grains. Magnesite grains have a weak but consistent crystallographic preferred orientation with the c-axis perpendicular to the foliation, and show high internal misorientations. Locally, the microcrystalline quartz matrix also shows a crystallographic preferred orientation with the c-axes preferentially oriented parallel to the foliation. Folding and ductile transposition of early magnesite veins indicates that carbonation initiated before the ductile deformation stage recorded in listvenites with penetrative foliation. On the other hand, dendritic magnesite overgrowths on folded veins and truncated vein tips suggest that folding likely occurred before complete carbonation, when some serpentine was still present. TEM analysis of magnesite revealed that subgrain boundaries oriented at high angle to the foliation can consist of nano-cracks sealed by inclusion-free magnesite precipitates. High dislocation densities are not evident suggesting that dislocation creep was minor or negligible, in agreement with very low predicted strain rates for magnesite dislocation creep at the low temperatures (100 – 200 °C) of serpentinite carbonation. This points to dissolution-precipitation, possibly in addition to grain boundary sliding, as the main mechanism for the formation of the shape preferred orientation of magnesite. The weak magnesite crystallographic preferred orientation may be explained by a combination of initial growth competition in an anisotropic (sheared) serpentine medium with subsequent preferred dissolution of smaller, less favorably oriented grains. We infer that transient lithostatic pore pressures during listvenite formation promoted ductile deformation in the reacting medium through grain boundary sliding accommodated by dilatant granular flow and dissolution-precipitation. Because the reaction product listvenite is stronger than the reacting mass, deformation may be preferentially partitioned in the reacting mass, locally enhancing transient fluid flow and, thus, the carbonation reaction progress.</p><p>[1] Kelemen et al., 2020. Site BT1: fluid and mass exchange on a subduction zone plate boundary. In: Proceedings of the Oman Drilling Project: College Station, TX</p><p>[2] Menzel et al., 2020, JGR Solid Earth 125(10)</p>

High-stress crystal plasticity of amphibole and quartz in pseudotachylyte-bearing gneisses and dislocation creep in concordant quartz-rich layers from the Silvretta basal thrust (Austria)

2021 ◽  
Author(s):  
Lisa Marie Brückner ◽  
Claudia A. Trepmann
Keyword(s):  
Crystal Plasticity ◽  
Rotation Axis ◽  
High Stress ◽  
Polarized Light ◽  
Dislocation Creep ◽  
Crystallographic Preferred Orientation ◽  
Dislocation Glide ◽  
Kink Bands ◽  
Mechanical Twins ◽  
Quartz Grains

<p>Pseudotachylyte-bearing amphibole-rich gneisses with concordant quartz-rich layers from the base of the Silvretta nappe, Austria, are analyzed by polarized light microscopy, scanning electron microscopy and electron back scattered diffraction. Amphibole grains show microfractures, undulatory extinction, deformation lamellae, kink bands, mechanical twins and locally recrystallized grains restricted to sites of high strain, e.g. along microshear zones and twin boundaries. The twins are characterized by a twin plane parallel to (-101), a rotation axis parallel to [101] and a misorientation angle of 178°. The (-101) amphibole twins document the high differential stresses during crystal plasticity coeval with pseudotachylyte formation, given their high critical resolved shear stress of 200 MPa. Directly at the contact to twinned amphibole within the gneisses, quartz grains commonly show subbasal deformation lamellae, short-wavelength undulatory extinction and cleavage cracks mostly parallel to {10-11} rhombohedral planes that are decorated by recrystallized grains with a diameter of < 10 µm. The small recrystallized grains show a crystallographic preferred orientation (CPO) that is controlled by the orientation of the host grains. This quartz microstructure consistently indicates high-stress crystal plasticity of quartz concurrent with high-stress crystal plasticity of amphibole and pseudotachylyte formation.</p><p>Quartz-rich layers (>90% quartz) concordant to the foliation of the gneisses commonly show evidence of dynamic recrystallization in the regime of dislocation creep. The recrystallized grain microstructure is mostly homogenous without a gradient towards the lithological contact to the amphibole-rich gneisses. Locally, however, a gradient of decreasing strain towards the contact can be observed as indicated by a decreasing number of recrystallized grains. Close to the contact, quartz grains are coarse with long axes of a few mm. A core-and-mantle structure, where recrystallized grains surround a few hundred µm wide and mm-long porphyroclasts, is occurring in transition towards an almost completely recrystallized microstructure. The recrystallized grains show a CPO indicating rhombohedral <a> dislocation glide. Recrystallized grains are isometric and subgrains in porphyroclasts are of similar shape and size, indicating dynamic subgrain rotation recrystallization. Stresses on the order of hundred MPa are suggested by the diameter of recrystallized grains of in average about 10 µm. Locally, the recrystallized quartz aggregate is affected by subsequent low-temperature plasticity, as indicated by shear fractures offsetting the recrystallized microstructure. The missing or decreasing strain gradients of dislocation creep within the quartz-rich layers towards the amphibole-rich gneisses indicate that dislocation creep in the quartz-rich layers cannot be responsible for transferring high stresses required for high-stress crystal-plasticity of quartz and amphibole as well as pseudotachylyte-formation and suggest that dislocation creep of quartz represents an independent earlier stage of deformation.</p>

scholarly journals Effect of confining pressure on the strength of wet quartzite revisited

2020 ◽  
Author(s):  
Lucille Nègre ◽  
Holger Stünitz ◽  
Hugues Raimbourg ◽  
Jacques Précigout ◽  
Petr Jeřábek ◽  
...  
Keyword(s):  
Plastic Deformation ◽  
High Pressure ◽  
Grain Boundaries ◽  
Solid Earth ◽  
Constant Strain Rate ◽  
Dislocation Creep ◽  
Temperature Deformation ◽  
Diffusion Creep ◽  
Fine Grained ◽  
Bulk Strain

<p>The ability of water to enhance plastic deformation of a quartz aggregate has been experimentally demonstrated during the sixties (e.g. Griggs and Blacic 1965), however the processes involved are still questioned. Notably the processes combining the effect of water and pressure during the deformation are still not completely understood. Pressure strongly influences the strength of fine-grained (3.6 - 4.9 µm) wet quartz aggregates (Kronenberg and Tullis 1984), where diffusion creep operates (Fukuda et al. 2018) but its effect on coarser-grained material expected to deform only by dislocation creep is not well constrained. To re-assess the effect of pressure on quartz crystal plastic deformation, natural wet quartzite samples from the Tana quarry in northern Norway (grain size ≈ 150 µm) have been deformed using a Griggs-type apparatus at varying confining pressures (from 0.6 to 2.0 GPa). All the samples with 0.1 wt. % H<sub>2</sub>O added were shortened coaxially up to 30% strain at constant strain rate (≈10<sup>-6</sup> s<sup>-1</sup>) and temperature (900°C).</p><p>All mechanical records show that quartzite flow stresses decrease systematically with increasing pressure. These results allow to determine the strength of quartzite as a function of water fugacity, such as introduced in the flow law by Kohlstedt et al. (1995) to account for both pressure and water effects. In our case, the fugacity coefficient is m≈1 when using a stress exponent of n=2.</p><p>Microstructure and image analyses of samples reveal that the bulk strain results mainly from crystal plastic deformation of original grains whereas the recrystallization processes are limited volumetrically (less than 5%) and restricted to the boundaries of original grains. Deformation is not strongly partitioned into recrystallized domains compared to flattened original grains. Optical and SEM-cathodoluminescence images revealed the presence of cracks in conjunction with recrystallization (even for high-pressure samples) and associated chemical/fluid interaction, but the cracks do not contribute significantly to the bulk strain of the samples.</p><p>In order to determine the amount of water used for the deformation and the redistribution of H<sub>2</sub>O during deformation, the H<sub>2</sub>O content of the quartzite has been calculated from FTIR (Fourier Transform InfraRed spectroscopy) measurements for both, grain interiors and grain boundaries. The H<sub>2</sub>O concentrations decrease inside grains with the onset of deformation with respect to the starting material. H<sub>2</sub>O is primarily stored in the grain boundary region. There is no systematic correlation with pressure. Thus, pressure dependence of H<sub>2</sub>O weakening is not restricted to fine-grained materials at high pressure and temperature. Deformation redistributes water from the grain interiors to their grain boundaries.</p><p>References:<br>Fukuda, J., Holyoke III, C.W., and Kronenberg, A.K. (2018). J. Geophysical Res.: Solid Earth, 123(6), 4676-4696.<br>Griggs, D. T., and Blacic J. D. (1965). Science, 147(3655), 292‑295.<br>Kohlstedt, D. L., Evans B., and Mackwell S. J. (1995). J. Geophysical Res.: Solid Earth, 100(B9), 17587-17602.<br>Kronenberg, A. K., and Tullis J. (1984). J. Geophysical Res.: Solid Earth, 89(B6), 4281‑4297.</p>

scholarly journals Characterizing the microstructural anisotropy of fine-grained phyllosilicate-rich rocks

2021 ◽  
Vol 1 ◽  
pp. 69-70
Author(s):  
Rebecca Kühn ◽  
Michael Stipp ◽  
Bernd Leiss
Keyword(s):  
Physical Properties ◽  
Texture Analysis ◽  
High Energy ◽  
Preferred Orientation ◽  
Low Grade ◽  
X Ray Diffraction ◽  
Crystallographic Preferred Orientation ◽  
X Ray ◽  
Fine Grained ◽  
Microstructural Anisotropy

Abstract. The physical properties of claystones, shales, and slates are highly dependent on the alignment of phyllosilicate minerals. With increasing overburdening, the shape and the crystallographic preferred orientation of these minerals are affected by uniaxial shortening as well as tectonic processes including recrystallization under elevated pressure and temperature conditions. The microstructural anisotropy expressed mainly by the alignment of phyllosilicates significantly predetermines the orientation of fractures, hence the shear strength and stability of clay-rich sediments and rocks. A quantitative analysis of phyllosilicate alignment is therefore essential to evaluate the properties and the mechanical behavior of these rocks. This can be carried out by analyzing the crystallographic preferred orientation (texture). Although texture analysis is a common tool in geosciences, it becomes more difficult in fine-grained rocks owing to for example particle size, heterogeneity, the polyphase composition, and difficulties in sample preparation. Methods such as electron backscatter diffraction, neutron diffraction, or laboratory X-ray diffraction are restricted with respect to preparation artifacts, sampling size and statistics, water content, etc. To overcome these issues, we successfully apply high-energy X-ray diffraction as available at synchrotron research facilities, e.g., at the German Electron Synchrotron Facility (DESY) in Hamburg, Germany, or the European Synchrotron Research Facility (ESRF) in Grenoble, France. In combination with Rietveld refinement we analyze the bulk texture of phyllosilicate-rich rocks. Here we present the results of texture analysis from a wide range of these rocks: Pleistocene poorly consolidated mud (rocks), affected only by sedimentation and burial; more highly consolidated but tectonically largely unaffected Jurassic claystone from the Opalinus Formation of the Swabian Alb; Carboniferous shales from the Harz mountains representing low-grade metamorphic and deformed rocks. Our methodical approach to quantifying the microstructural anisotropy using texture analysis in fine-grained rocks allows for the quantification of physical properties resulting from the alignment of phyllosilicates. Furthermore, it enables the prediction of direction-dependent mechanical strength, which is crucial for the establishment of long-term repositories for radioactive waste in shales and claystones.

scholarly journals Kinking facilitates grain nucleation and modifies crystallographic preferred orientations during high-stress ice deformation

2021 ◽  
Author(s):  
Sheng Fan ◽  
David Prior ◽  
Travis Hager ◽  
Andrew Cross ◽  
David Goldsby ◽  
...  
Keyword(s):  
Aspect Ratio ◽  
Dynamic Recrystallization ◽  
Boundary Migration ◽  
High Stress ◽  
Kink Band ◽  
Coarse Grained ◽  
Compression Axis ◽  
Crystallographic Preferred Orientation ◽  
Kink Bands ◽  
Subgrain Rotation

Kinking can accommodate significant amounts of strain during crystal plastic deformation under relatively large stresses and may influence the mechanical properties of cold planetary cryosphere. To better understand the origins, mechanisms, and microstructural effects of kinking, we present detailed microstructural analyses of coarse-grained ice (~1300 µm) deformed under uniaxial compression at -30°C. Microstructural data are generated using cryogenic electron backscattered diffraction (cryo-EBSD). Deformed samples have bimodal grain size distributions, with thin and elongated (aspect ratio ≥ 4) kink domains that develop within, or at the tips of, remnant original grains (≥ 300 µm, aspect ratio < 4). Small, equiaxed subgrains also develop along margins of remnant grains. Moreover, many remnant grains are surrounded by fine-grained mantles of small, recrystallized grains (< 300 µm, aspect ratio < 4). Together, these observations indicate that grain nucleation is facilitated by both kinking and dynamic recrystallization (via subgrain rotation). Low- (< 10°) and high-angle (mostly > 10°, many > 20°) kink bands within remnant grains have misorientation axes that lie predominantly within the basal plane. Moreover, previous studies suggest the kinematics of kinking and subgrain rotation should be fundamentally the same. Therefore, progressive kinking and subgrain rotation should be crystallographically controlled, with rotation around fixed misorientation axes. Furthermore, the c-axes of most kink domains are oriented sub-perpendicular to the sample compression axis, indicating a tight correlation between kinking and the development of crystallographic preferred orientation. Kink band densities are the highest within remnant grains that have basal planes sub-parallel to the compression axis (i.e., c-axes perpendicular to the compression axis)—these data are inconsistent with models suggesting that, if kinking is the only strain-accommodating process, there should be higher kink band densities within grains that have basal planes oblique to the compression axis (for low kink-host misorientation angles, e.g., ≤ 20°, as in this study). One way to rationalize this inconsistency between kink models and experimental observations is that kinking and dynamic recrystallization are both active during deformation, but their relative activities depend on the crystallographic orientations of grains. For grains with basal planes sub-parallel to the compression axis, strain-induced GBM is inhibited, and large intragranular strain incompatibilities can be relaxed via kinking, when other processes such as subgrain rotation recrystallization are insufficient. For grains with basal planes oblique to the compression axis, strain-induced grain boundary migration (GBM) might be efficient enough to relax the strain incompatibility via selective growth of these grains, and kinking is therefore less important. For grains with basal planes sub-perpendicular to the compression axis, kink bands are seldom observed—for these grains, the minimum shear stress required for kinking exceeds the applied compressive stress, such that kinks cannot nucleate.

Microstructure and Magnetic Properties of CoCrTa Thin Films

1985 ◽  
Vol 54 ◽  
Author(s):  
C. J. Robinson ◽  
J. K. Howard
Keyword(s):  
Magnetic Properties ◽  
Perpendicular Magnetic Anisotropy ◽  
Preferred Orientation ◽  
X Ray Diffraction ◽  
Crystallographic Preferred Orientation ◽  
Fine Grained ◽  
Sputtered Films ◽  
Transmission Electron ◽  
Growth Features ◽  
Microstructure And Magnetic Properties

ABSTRACTThe addition of Ta into thin sputtered films of CoCr greatly affects the microstructure and magnetic properties. Grain size, crystallographic preferred orientation and hysteresis parameters have been studied using transmission electron microscopy, electron diffraction, X-ray diffraction, polar Kerr effect and vibrating sample magnetometry techniques. Crystallographic preferred orientation is enhanced and is accompanied by increased perpendicular magnetic anisotropy. An extremely fine-grained microstructure is produced giving rise to films which show no morphological growth features. Thus the observed magnetocrystalline anisotropy is not attributed to columnar growth. Orientation in these films can be further increased by using under-layers of non-ferromagnetic CoCrTa alloys. The mechanism by which this occurs is discussed.

scholarly journals Brittle grain size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

2015 ◽  
Vol 7 (4) ◽  
pp. 2953-2998
Author(s):  
G. Viegas ◽  
L. Menegon ◽  
C. J. Archanjo
Keyword(s):  
Grain Size ◽  
Shear Zone ◽  
Strain Localization ◽  
Shear Bands ◽  
Preferred Orientation ◽  
Size Reduction ◽  
Diffusion Creep ◽  
Crystallographic Preferred Orientation ◽  
Fine Grained

Abstract. The Pernambuco shear zone (northeastern Brazil) is a large-scale strike-slip fault that, in its eastern segment, deforms granitoids at mid-crustal conditions. Initially coarse (> 50 μm) grained feldspar porphyroclasts are intensively fractured and reduced to an ultrafine-grained mixture consisting of plagioclase and K-feldspar grains (~ < 15 μm in size) localized in C' shear bands. Detailed microstructural observations and EBSD analysis do not show evidence of intracrystalline plasticity in feldspar porphyroclasts and/or fluid-assisted replacement reactions. Quartz occurs either as thick (~ 1–2 mm) monomineralic bands or as thin ribbons dispersed in the feldspathic mixture. The microstructure and c axis crystallographic preferred orientation are similar in the thick monomineralic band and in the thin ribbons, and suggest dominant subgrain rotation recrystallization and activity of prism ⟨a⟩ and rhomb ⟨a⟩ slip systems. However, the grain size in monophase recrystallized domains decreases when moving from the transposed veins to the thin ribbons embedded in the feldspathic C' bands (14 μm vs. 5 μm, respectively). The fine-grained feldspar mixture has a weak crystallographic preferred orientation interpreted as the result of oriented growth during diffusion creep, as well as the same composition as the fractured porphyroclasts, suggesting that it generated by mechanical fragmentation of rigid porphyroclasts with a negligible role of chemical disequilibrium. Assuming that the C' shear bands deformed under constant stress conditions, the polyphase feldspathic aggregate would have deformed at a strain rate one order of magnitude faster than the monophase quartz ribbons. Overall, our dataset indicates that feldspar underwent a brittle-viscous transition while quartz was deforming via crystal plasticity. The resulting rock microstructure consists of a two-phase rheological mixture (fine-grained feldspars and recrystallized quartz) in which the polyphase feldspathic material localized much of the strain. Extensive grain-size reduction and weakening of feldspars is attained in the East Pernambuco mylonites mainly via fracturing under relatively fluid-absent conditions which would trigger a switch to diffusion creep and further strain localization without a prominent role of metamorphic reactions.

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