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1.
Determination of the controlling nucleation and recrystallisation mechanisms from a samples microstructure are essential for understanding how the microstructure formed and evolved through time. The aim of our research was to apply a quantified analytical approach to the identification of the controlling nucleation, recrystallisation and microstructural modification mechanisms. We used electron backscatter diffraction to quantify the microstructures of naturally deformed quartz-rich rocks which were deformed at various temperature and pressure conditions. Our results show that ratios of the recrystallised grain size to the subgrain size with values less than 1 (0.5–0.7 in the data presented here) suggest bulge nucleation, whereas ratios of ∼1 suggest subgrain rotation nucleation. Other supporting evidence for subgrain rotation nucleation is an increase in misorientation from the centre of an original protolith ‘parent’ grain to the edge. All samples show evidence for modification of the microstructure due to grain boundary sliding including increased misorientation angles between grains and movement of recrystallised grains between parent grains. By systematically analysing sample microstructures it is possible to separate out evidence to determine the controlling nucleation and recrystallisation mechanisms, as well as being able to identify microstructure modification mechanisms. Using microstructural quantification via EBSD allows a systematic methodology to analyse samples from any location from an objective viewpoint.  相似文献   

2.
Large biotite grains, up to 1 metre across, in a pegmatite near Broken Hill, Australia, have been heterogeneously deformed and partly recrystallized in zones of relatively high strain, probably under conditions of the lower amphibolite facies. Many of the new aggregates (which consist mainly of more magnesian biotite, with muscovite, and less abundant ilmenite and albite) have a mica preferred orientation. Some of the oriented mica aggregates have grown in kink-like deformation zones (some of which appear to have involved fracturing) and others have grown in dilatation zones (growth of mica probably keeping pace with the opening of the zones). The shapes and preferred orientation of new mica grains appear to be due to varying contributions by (a) mechanical rotation of slices cleaved parallel to (001), (b) coaxial nucleation and/or growth of new mica on rotated portions of deformed biotite, and (c) preferred nucleation and/or growth of new grains in directions of minimum mechanical constraint and maximum transport of chemical components. This preferred growth can also explain the observed high degree of elongation of the oriented mica grains.  相似文献   

3.
Deformation experiments have been carried out to investigate the effect of dynamic recrystallisation on crystallographic preferred orientation (CPO) development. Cylindrical samples of natural single crystals of quartz were axially deformed together with 1 vol.% of added water and 20 mg of Mn2O3 powder in a Griggs solid medium deformation apparatus in different crystallographic orientations with compression direction: (i) parallel to <c>, (ii) at 45° to <c> and 45° to <a> and (iii) parallel to <a>. The experiments were performed at a temperature of 800 °C, a confining pressure of 1.2 GPa, a strain rate of  10− 6 s− 1, to bulk finite strains of  14–36%. The deformed samples were analysed in detail using optical microscopy, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Two different microstructural domains were distinguished in the deformed samples: (i) domains with undulatory extinction and deformation lamellae, and (ii) domains with new recrystallised grains. Within the domains of undulatory extinction, crystal-plastic deformation caused gradual rotations of the crystal lattice up to  30° away from the host orientation. New recrystallised grains show a strong CPO with c-axis maxima at  45° to the compression direction. This is the case in all experiments, irrespective of the initial crystallographic orientation. The results show that c-axes are not continuously rotated towards the new maxima. The new grains thus developed through a mechanism different from subgrain rotation recrystallisation. New grains have a subeuhedral shape and numerous microcavities, voids, fluid channels and fluid inclusions at their grain boundaries. No host control is recorded in misorientation axes across their large angle grain boundaries. New grains might have been created by nucleation from solution in the μm-scale voids and microfractures. The CPO most likely developed due to preferred growth of the freshly precipitated grains with orientations suitable for intracrystalline deformation at the imposed experimental conditions.  相似文献   

4.
Cordierite — (Mg,Fe)2Al4Si5O18 — occurs as porphyroclasts within metapelitic and metavolcanic rocks from the Kemiö-Orijärvi belt, SW Finland. After crystallisation the cordierites have been deformed at temperatures between 550–825° C and pressures of 3–5 kbar. Optical microscopy reveals the following deformation-induced microstructures: a bimodal size distribution between host, 0.3 to 4.0 mm, and recrystallised (new) grains, 0.1 to 0.5 mm; the intracrystalline defect-structures of host grains yield undulatory extinction, subgrains and some twinning. Recrystallised grains are optically strain free. Grain and subgrain boundaries are generally straight and parallel to crystallographic low-index planes. Orientation distribution diagrams for host and recrystallised grains yield similar fabric diagrams, i.e. [010] perpendicular to foliation -S-, [001] and [100] parallel to S and [001] parallel to lineation -L-. The fabric diagrams indicate that [001] (010) is the dominant slip system. Transmission electron microscopy reveals straight free dislocations, glide and climb loops, minor {130} and {110} microtwins, isolated nodal points and dislocation walls. Contrast analyses yield Burgers vector b = [001] being dominant and b = [100] subordinate. Climb loops consist of 〈c〉-dislocations that are dissociated in (001) planes, glide loops are defined by [100] [010] and [001] (100). The cordierite microstructures have been interpreted to be generated by dislocation creep. The dominant recrystallisation mechanism is thought to be subgrain rotation subsequently followed by minor grain or twin-band boundary migration.  相似文献   

5.
The microstructures of two contrasting garnet grains are mapped using automated electron backscatter diffraction. In both cases there is a very strong crystallographic preferred orientation, with measurements clustered round a single dominant orientation. Each garnet grain is divided into domains with similar orientations, limited by boundaries with misorientations of 2° or more. In both samples most of misorientation angles measured across orientation domain boundaries are significantly lower than those measured between random pairs of orientation domains. One sample is a deformed garnet that shows considerable distortion within the domains. Lines of orientation measurements within domains and across domain boundaries show small circle dispersions around rational crystallographic axes. The domain boundaries are likely to be subgrain boundaries formed by dislocation creep and recovery. The second sample is a porphyroblast in which the domains have no internal distortion and the orientation domain boundaries have random misorientation axes. These boundaries probably formed by coalescence of originally separate garnets. We suggest that misorientations across these boundaries were reduced by physical relative rotations driven by boundary energy. The data illustrate the potential of orientation maps and misorientation analysis in microstructural studies of any crystalline material.  相似文献   

6.
Recrystallization of perthites in granulite facies (T = 700–730 °C, P = 0.65–0.8 GPa) shear zones in mangerite-charnockite rocks from Lofoten (Norway) is localized along intracrystalline bands parallel to fractures. Fracturing preferentially occurred along the cleavage planes (010) and (001). EBSD analysis of perthite porphyroclasts indicates a very low degree of internal misorientation (within 5°) and the lack of recovery features. Recrystallized grains show coarsening with increasing width of the bands, and chemical changes with respect to the host grains. Crystallographic orientation of the new grains does not show a host-control relation to the parent perthite grains. In summary, the microstructure and CPO data consistently indicate intragranular recrystallization by nucleation and growth from fractured grains. Perthite porphyroclasts are surrounded by a matrix of recrystallized plagioclase + K-feldspar ± amphibole ± biotite. There is extensive evidence of syndeformational nucleation of new phases and of phase boundary migration in the matrix, with plagioclase grains forming bulges and protrusions towards K-feldspar. The spatial distribution of K-feldspar and plagioclase in the recrystallized matrix is characterized by the predominance of phase boundaries over grain boundaries. All these observations are consistent with diffusion creep as the dominant deformation mechanism in the matrix, associated with grain boundary sliding. Accordingly, recrystallized plagioclase and K-feldspar show a very weak crystallographic preferred orientation, which is interpreted in terms of oriented growth during diffusion creep. Fracturing of perthites promoted extensive grain size reduction, recrystallization, fluid infiltration, and operation of grain-size sensitive creep, resulting in strain localization.  相似文献   

7.
Hydrolytic weakening in quartz has been extensively demonstrated by experimental deformation of single crystals and aggregates. This paper describes the deformation and recrystallization microstructures and preferred orientations of quartz in a mylonite zone separating granulite facies (0.2% H2O) from amphibolite facies (1.0% H2O) acid gneisses. The transition from slightly deformed country rock on both sides to the ultimate product of mylonitization (a phyllite) is described and the following major differences are noted:
1. (1) The strain prior to recrystallization is higher on the granulite side.
2. (2) Misorientations across deformation-band boundaries are much higher on the granulite side.
3. (3) Subgrains and new grains are considerably smaller at the same stage of recrystallization on the granulite side.
4. (4) Preferred orientation of [0001] developes more rapidly with respect to strain on the amphibolite side.
5. (5) There is a closer orientation relationship between host and recrystallized (new) grains on the granulite side.
The microstructures and preferred-orientation development on both sides are related to concurrent ductile deformation, dynamic recovery and recrystallization processes. The differences between the two sides is attributed to the difference in bulk H2O content of the rocks and a resultant difference in strain rate. The suggested effect of water on recovery processes is favoured over its possible role in slip processes.  相似文献   

8.
In this study several grain-scale microstructures are presented that are thought to demonstrate the migration direction of once-mobile grain boundaries in a naturally deformed quartzite. An analysis is presented of the sense of migration of the boundaries, and the characteristics of the patterns of relative grain growth and shrinkage. Grain-boundary migration seems to be correlated with the relative crystallographic orientations of neighbouring grains for the quartz—quartz grain boundaries, and the pattern of preferred grain growth is roughly symmetrical about the mica foliation plane.  相似文献   

9.
Comparison of observed microstructures and deduced physical parameters of natural deformations with those encountered in superplastic metals permits identification of superplastic behaviour in rocks. From microstructural evidence, superplastic flow of feldspar is inferred to have occurred in the deformation of a granite from Miéville, Aiguilles-Rouges massif, Switzerland. The original igneous orthoclase and oligoclase phenocrysts have both recrystallised to fine-grained albite which has deformed by superplastic flow. This change in crystal chemistry was associated with a large negative change in free energy, enabling recrystallised grains of a very small size to be stable under the prevailing conditions, thereby producing the pre-requisite conditions for superplastic behaviour.  相似文献   

10.
The microstructure of a quartzite experimentally deformed and partially recrystallised at 900 °C, 1.2 GPa confining pressure and strain rate 10−6/s was investigated using orientation contrast and electron backscatter diffraction (EBSD). Boundaries between misoriented domains (grains or subgrains) were determined by image analysis of orientation contrast images. In each domain, EBSD measurements gave the complete quartz lattice orientation and enabled calculation of misorientation angles across every domain boundary. Results are analysed in terms of the boundary density, which for any range of misorientations is the boundary length for that range divided by image area. This allows a more direct comparison of misorientation statistics between different parts of a sample than does a treatment in terms of boundary number.The strain in the quartzite sample is heterogeneous. A 100×150 μm low-strain partially recrystallised subarea C was compared with a high-strain completely recrystallised subarea E. The density of high-angle (>10°) boundaries in E is roughly double that in C, reflecting the greater degree of recrystallisation. Low-angle boundaries in C and E are produced by subgrain rotation. In the low-angle range 0–10° boundary densities in both C and E show an exponential decrease with increasing misorientation. The densities scale with exp(−θ/λ) where λ is approximately 2° in C and 1° in E; in other words, E has a comparative dearth of boundaries in the 8–10° range. We explain this dearth in terms of mobile high-angle boundaries sweeping through and consuming low-angle boundaries as the latter increase misorientation through time. In E, the density of high-angle boundaries is larger than in C, so this sweeping would have been more efficient and could explain the relative paucity of 8–10° boundaries.The boundary density can be generalised to a directional property that gives the degree of anisotropy of the boundary network and its preferred orientation. Despite the imposed strain, the analysed samples show that boundaries are not, on average, strongly aligned. This is a function of the strong sinuosity of high-angle boundaries, caused by grain boundary migration. Low-angle boundaries might be expected, on average, to be aligned in relation to imposed strain but this is not found.Boundary densities and their generalisation in terms of directional properties provide objective measures of microstructure. In this study the patterns they show are interpreted in terms of combined subgrain rotation and migration recrystallisation, but it may be that other microstructural processes give distinctive patterns when analysed in this fashion.  相似文献   

11.
Abstract: Biotite and muscovite inclusions inside mica host minerals from the Sutlej section of the Higher Himalayan Crystalline were studied under an optical microscope. These inclusions formed possibly by local recrystallization of mica grains during regional prograde metamorphism, with some affected by top-to-SW shear leading to parallelogram shapes. Recrystallization may have been assisted by solution transfer along the cleavage planes of the host grains. The relative competency of deformed phyllosilicate inclusions with the same or different composition to the host depends on the size and orientation of (001) cleavage planes of the inclusions relative to the host. Shearing of mica inclusions led to their parallelogram geometries within the contained mica inclusions. Some of the sheared inclusions deflect cleavage planes in the host minerals and define flanking microstructures. Trapezoid-shaped inclusions are a new finding that deserves more attention for their genesis. These structurally anisotropic inclusions did not originate from sub-grains, secondary infillings or retrogression. These inclusions are also not related to pseudomorphism, isomorphism, folding of the bulk rock etc. Some of the inclusions formed by recrystallization of the host mineral during top-to-SW ductile shear.  相似文献   

12.
The deformation and recrystallization microstructures in biotite from the Woodroffe Thrust mylonites are described and interpreted. The degree of strain causing recrystallization and the nucleation mechanisms differ across the mylonite zone. These differences are associated with the contrast in water content between the granulite and amphibolite facies felsic gneisses on either side of the zone. p]In moderately mylonitized granulite facies felsic gneisses (0.1–0.6% H2O) subgrains form in intensely deformed host biotite and recrystallization mechanisms involve subgrain rotation both on host grain boundaries and associated with kink band bulge. In the amphibolite facies felsic gneisses (0.9–1.2% H2O) the biotite recrystallizes by a mechanism involving localized internal kinking of the host and subsequent migration of high angle boundaries generated on the kink limbs. This combined with rotation due to the concurrent deformation generates high angle grain boundaries around the entire original kink limb and thus a new grain.  相似文献   

13.
Octachloropropane (C3C18) was sheared using a press mounted on an optical microscope and then allowed to adjust its microstructure statically, at the deformation temperature. Depending on strain rate and deformation temperature, the post-deformational changes in microstructure are strikingly different. After low temperature-high strain-rate deformation, fast growth of new strain-free grains on the boundaries of deformed grains results in the obliteration of grain-shape foliation and intracrystalline deformation features, and the development of a foam texture. After high temperature—low strain-rate deformation, on the other hand, grain-shape foliation and grains with subgrain boundaries tend to survive the adjustment. Lattice preferred orientation is maintained after the post-deformational adjustment at both deformation conditions and thus remains a good indicator of deformation.  相似文献   

14.
In situ observations of polycrystalline ice deformed in simple shear between −10 and −1°C are presented. This study illustrates the processes responsible for the deformation, the development of a preferred crystallographic orientation and the formation of a preferred dimensional orientation. Intracrystalline glide on the basal plane, accompanying grain rotations and dynamic recrystallization, helps to accommodate the large intragranular strains. These are the most important mechanisms for crystallographic reorientation and produce a stable fabric that favours glide on the basal plane. Localized kinks, developed in grains unfavourably oriented for easy glide, are unstable and are overprinted by dynamic recrystallization. Dynamic recrystallization is a strain softening process with nucleation occurring in the form of equiaxed grains that grow subparallel to pre-existing grain anisotropies and become elongate during deformation. Plots of grain axial ratio against orientation ( ) indicate a weak shape fabric which does not correspond to the theoretical foliation and elongation for the appropriate increment of shear strain. We argue that estimates of the strain magnitude made from orientation of elongate grains are unreliable in high temperature shear zones. These results are applicable to both geological and glacial shear environments.  相似文献   

15.
This work contributes to the experimental investigations of the origin and 3-D orientation of micropores in low porosity crystalline rocks. The origin and spatial orientation of microporosity in two eclogites with different microstructures were studied by 1) quantitative and qualitative microstructural analysis of grains and grain boundaries, 2) measurement of lattice preferred orientation using the SEM-EBSD method and 3) experimental measurement of velocity of elastic P-waves in spherical samples in 132 directions under confining pressures up to 400 MPa. Results show good correlation between the elastic properties and the orientation of grain boundaries and cleavage planes in clinopyroxene. The magnitude and anisotropy of velocity change with pressure shows that microporosity in the fine-grained sample is relatively large and strongly preferentially oriented, whereas it is significantly lower and less preferentially oriented in the coarse-grained sample. Seeing that the lattice preferred orientation of clinopyroxene is similar in both samples we can deduce from velocity changes that the grain size of the rock forming minerals controls the amount of microporosity. Also, the orientation of microporosity depends mostly on preferred orientation of grain boundaries and somewhat less on the orientation of cleavage planes. Grain boundaries are therefore the most important contributors to the bulk microporosity in the studied rocks.  相似文献   

16.
The development of microstructures and textures (i.e. crystallographic preferred orientations) during recrystallisation of naturally deformed quartz polycrystals has been studied via electron diffraction techniques in the scanning electron microscope. In the investigated sample series of quartz-rich rocks originating from different deformation regimes, the microstructural and textural changes in quartz have been significantly influenced by dynamic recrystallisation. Based on microstructural observations paired with orientation and misorientation analyses down to the scale of grains and subgrains, criteria could be established which characterise the dominant recrystallisation process and its influence on texture development. It is shown that the texture development during dynamic recrystallisation is controlled by a differential activation of slip systems in grains of ‘soft’ and ‘hard’ orientations. The analyses provide further evidence that specific grain orientations are preferred during crystal plastic deformation, recrystallisation and grain growth. The influence of twinning after the Dauphiné law was also investigated. Observations of a progressive reduction in the population of Dauphiné-twin boundaries during recrystallisation and a penetrative deformation in both hosts and twins indicate a generation prior to deformation and recrystallisation. A mechanical origin for twinning and possible influence on texture development was therefore discarded.  相似文献   

17.
We use quantitative microstructural analysis including misorientation analysis based on electron backscatter diffraction (EBSD) data to investigate deformation mechanisms of naturally deformed plagioclase in an amphibolite gabbro mylonite. The sample is from lower oceanic crust exposed near the Southwest Indian Ridge, and it has a high ratio of recrystallized matrix grains to porphyroclasts. Microstructures preserved in porphyroclasts suggest that early deformation was achieved principally by dislocation creep with subgrain rotation recrystallization; recrystallized grain (average diameter ∼8 μm) microstructures indicate that subsequent grain boundary sliding (GBS) was active in the continued deformation of the recrystallized matrix. The recrystallized matrix shows four-grain junctions, randomized misorientation axes, and a shift towards higher angles for neighbor-pair misorientations, all indicative of GBS. The matrix grains also exhibit a shape preferred orientation, a weak lattice preferred orientation consistent with slip on multiple slip systems, and intragrain microstructures indicative of dislocation movement. The combination of these microstructures suggest deformation by dislocation-accommodated GBS (DisGBS). Strain localization within the recrystallized matrix was promoted by a transition from grain size insensitive dislocation creep to grain size sensitive GBS, and sustained by the maintenance of a small grain size during superplasticity.  相似文献   

18.
Mylonitized biotites from below the Woodroffe Thrust recrystallized by a mechanism which involved intense internal deformation of host grains. At first localized kinks formed within the host lattice with increasing strain. The limbs of these kinks migrated into the intensely deformed, adjacent host generating high angle grain boundaries and eventually new grains.  相似文献   

19.
A number of microstructural features indicate a difference in the dominant deformation mechanism between the higher temperature Ryoke and the lower temperature Sambagawa and Shimanto metamorphic belts of Japan. The microstructures of metacherts containing deformed radiolaria are divided into two types: in both the Sambagawa and Shimanto belts the quartz grains are tabular while in the Ryoke belt they are equiaxed. TEM studies of these metacherts revealed that the tabular grains contain abundant subboundaries consisting of large numbers of network dislocations and bowe-out dislocations, while the equiaxed grains contain no subboundaries and have low densities of dislocations which are not bowed-out. There is a corresponding difference in the textures (lattice preferred orientation of quartz): the Ryoke metacherts display randomly distributed c-axes of quartz, while the Sambagawa and Shimanto metacherts show conspicuous crossed girdle patterns with some asymmetry. There is a third difference between these regions: in the metacherts of the Ryoke metamorphic belt, the strain magnitudes determined from deformed radiolaria increase with increasing volume fraction of mica in the same metamorphic P and T conditions, while in the Sambagawa and the Shimanto metamorphic cherts the strain magnitudes decrease with increasing the mica fraction.These microstructures, textures, and rheological behaviours of quartz-mica rocks suggest a change of deformation mechanism between the lower temperature Sambagawa and Shimanto, and the higher temperature Ryoke metamorphic belts. Since random fabrics of c-axes of quartz are inconsistent with lattice rotation due to dislocation glide, the Ryoke metacherts may have deformed by pressure-solution.  相似文献   

20.
A detailed field study reveals a gradual transition from high‐grade solid‐state banded orthogneiss via stromatic migmatite and schlieren migmatite to irregular, foliation‐parallel bodies of nebulitic migmatite within the eastern part of the Gföhl Unit (Moldanubian domain, Bohemian Massif). The orthogneiss to nebulitic migmatite sequence is characterized by progressive destruction of well‐equilibrated banded microstructure by crystallization of new interstitial phases (Kfs, Pl and Qtz) along feldspar boundaries and by resorption of relict feldspar and biotite. The grain size of all felsic phases decreases continuously, whereas the population density of new phases increases. The new phases preferentially nucleate along high‐energy like–like boundaries causing the development of a regular distribution of individual phases. This evolutionary trend is accompanied by a decrease in grain shape preferred orientation of all felsic phases. To explain these data, a new petrogenetic model is proposed for the origin of felsic migmatites by melt infiltration from an external source into banded orthogneiss during deformation. In this model, infiltrating melt passes pervasively along grain boundaries through the whole‐rock volume and changes completely its macro‐ and microscopic appearance. It is suggested that the individual migmatite types represent different degrees of equilibration between the host rock and migrating melt during exhumation. The melt topology mimicked by feldspar in banded orthogneiss forms elongate pockets oriented at a high angle to the compositional banding, indicating that the melt distribution was controlled by the deformation of the solid framework. The microstructure exhibits features compatible with a combination of dislocation creep and grain boundary sliding deformation mechanisms. The migmatite microstructures developed by granular flow accompanied by melt‐enhanced diffusion and/or melt flow. However, an AMS study and quartz microfabrics suggest that the amount of melt present did not exceed a critical threshold during the deformation to allow free movements of grains.  相似文献   

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