A Técnica de Difração de Elétrons Retro-Espalhados (EBSD) em Microscópio Eletrônico de Varredura (MEV) e sua Aplicação no Estudo de Rochas Deformadas

The electron backscattered diffraction technique in the scanning electron microscope (EBSD/SEM) is based on the diffraction of a high-energy electron beam by the crystalline structure of a given material, in all possible directions within the sample. Some of the diffracted electrons escape from the specimen with virtually the same initial energy, interact with a phosphorescent screen and the generated EBSP pattern can be picked up with a low-luminosity charge couple device (CCD) camera. These patterns can be indexed using pre-determined patterns for a large variety of minerals, which allows the determination of complete orientation of each single mineral within an aggregate. In this paper we briefly discuss the physical aspects related to the diffraction of an electron beam by crystalline matter and how the EBSP patterns are generated. We also present a short introduction of the necessary instruments to acquire EBSD data, as well as the calibration procedures, acquisition and indexing software of EBSPs. The pitfalls of the technique and possible error sources are also discussed with examples. Considering the scarce availability of literature on geological sample preparation, the polishing method of silicate-rich rocks for EBSP is described in detail in the last part of this paper.The electron backscattered diffraction technique in the scanning electron microscope (EBSD/SEM) is based on the diffraction of a high-energy electron beam by the crystalline structure of a given material, in all possible directions within the sample. Some of the diffracted electrons escape from the specimen with virtually the same initial energy, interact with a phosphorescent screen and the generated EBSP pattern can be picked up with a low-luminosity charge couple device (CCD) camera. These patterns can be indexed using pre-determined patterns for a large variety of minerals, which allows the determination of complete orientation of each single mineral within an aggregate. In this paper we briefly discuss the physical aspects related to the diffraction of an electron beam by crystalline matter and how the EBSP patterns are generated. We also present a short introduction of the necessary instruments to acquire EBSD data, as well as the calibration procedures, acquisition and indexing software of EBSPs. The pitfalls of the technique and possible error sources are also discussed with examples. Considering the scarce availability of literature on geological sample preparation, the polishing method of silicate-rich rocks for EBSP is described in detail in the last part of this paper

Estratigrafia do Batólito Florianópolis, Cinturão Dom Feliciano, na Região de Garopaba-Paulo Lopes, SC

In southern Brazil, the Florianopolis Batholith results from prolonged, mainly granitic magmatism, as part of the Neoproterozoic Dom Feliciano Belt. Plutonic associations in this belt are related to transpressive tectonism (650-580 Ma) in post-collisional setting, where the translithospheric discontinuities of the Southern Brazilian Shear Belt have triggered magmatism, acting as channels for melts originated in deep crustal or mantle sources. In the region of Garopaba-Paulo Lopes, a fraction of this magmatism was studied, resulting in a formal proposition for its stratigraphic organization in igneous suites. A small volume of gneissic host rocks is found as roof pendants. The Paulo Lopes Suite comprises the foliated Paulo Lopes Granite, Garopaba Granitoids and Silveira Gabbro. It is characterized as porphyritic granitoids of high-K tholeiitic affinity, coeval with mafic, tholeiitic magmatism. It is followed by metaluminous, alkaline series granitoids of the Pedras Grandes Suite, namely the Vila da Penha Granite, comprising heterogranular and porphyritic facies, and the Serra do Tabuleiro Granite, comprising heterogranular, equigranular and porphyritic facies. The Cambirela Suite, last magmatic episode in the study area, encompasses alkaline plutonic, volcanic and subvolcanic rock types, as the Ilha Granite, the Cambirela Rhyolite and the Itacorumbi Granite. Hypabissal rocks are also part of this suite, and result from recurrent acid and basic magma pulses emplaced either as idividual dykes or composite ones. The magmatic associations described in this region attest to magma emplacement under a moderate to low stress field, possibly extinguished by the time the Cambirela Suite crystallized. Their age values indicate that they are mostly contemporaneous to the post-collisional, syntectonic magmatism of the Southern Brazilian Shear Belt, and the preservation of magmatic structures such as modal layering, relatively uncomon in granitoids, may also be attributed to their emplacement far from the main deformation sites. Features indicative of coeval mafic and felsic magmas are described in all three magmatic associations, and are especially significant in the Paulo Lopes and Cambirela suites. They attest to continuous mantle participation in the batholith construction.In southern Brazil, the Florianopolis Batholith results from prolonged, mainly granitic magmatism, as part of the Neoproterozoic Dom Feliciano Belt. Plutonic associations in this belt are related to transpressive tectonism (650-580 Ma) in post-collisional setting, where the translithospheric discontinuities of the Southern Brazilian Shear Belt have triggered magmatism, acting as channels for melts originated in deep crustal or mantle sources. In the region of Garopaba-Paulo Lopes, a fraction of this magmatism was studied, resulting in a formal proposition for its stratigraphic organization in igneous suites. A small volume of gneissic host rocks is found as roof pendants. The Paulo Lopes Suite comprises the foliated Paulo Lopes Granite, Garopaba Granitoids and Silveira Gabbro. It is characterized as porphyritic granitoids of high-K tholeiitic affinity, coeval with mafic, tholeiitic magmatism. It is followed by metaluminous, alkaline series granitoids of the Pedras Grandes Suite, namely the Vila da Penha Granite, comprising heterogranular and porphyritic facies, and the Serra do Tabuleiro Granite, comprising heterogranular, equigranular and porphyritic facies. The Cambirela Suite, last magmatic episode in the study area, encompasses alkaline plutonic, volcanic and subvolcanic rock types, as the Ilha Granite, the Cambirela Rhyolite and the Itacorumbi Granite. Hypabissal rocks are also part of this suite, and result from recurrent acid and basic magma pulses emplaced either as idividual dykes or composite ones. The magmatic associations described in this region attest to magma emplacement under a moderate to low stress field, possibly extinguished by the time the Cambirela Suite crystallized. Their age values indicate that they are mostly contemporaneous to the post-collisional, syntectonic magmatism of the Southern Brazilian Shear Belt, and the preservation of magmatic structures such as modal layering, relatively uncomon in granitoids, may also be attributed to their emplacement far from the main deformation sites. Features indicative of coeval mafic and felsic magmas are described in all three magmatic associations, and are especially significant in the Paulo Lopes and Cambirela suites. They attest to continuous mantle participation in the batholith construction

Estudo da orientação cristalográfica em dobras, limites de grãos e anisotropia sísmica em moscovita-quartza milonitos

A deformação em zonas de cisalhamento dúcteis geralmente é responsável pelo desenvolvimento de orientações cristalográficas preferenciais dos minerais. Além de fornecer informações das condições de fluxo ao longo dessas zonas, o desenvolvimento de orientações cristalográficas preferenciais possui forte influência nas propriedades sísmicas das rochas. Neste trabalho são apresentados os resultados de um estudo detalhado sobre as relações entre mecanismos de deformação, orientações cristalográficas preferenciais e mecanismos de dobramento, formação de limites de grão e propriedades sísmicas anisotrópicas de moscovitaquartzo milonitos. Obteve-se dados de orientação preferencial através da técnica de difração de elétrons retroespalhados (EBSD) em um microscópio eletrônico de varredura, de moscovitaquartzo milonitos provenientes da região de Saas Fee (Alpes Internos do Oeste). As amostras foram coletadas ao longo de uma dobra cuja linha de charneira é paralela à lineação de estiramento regional da área. Esse tipo de feição é comum em zonas de alta deformação, mas contradizem as teorias de mecanismos de dobramento simples. Dados de orientações preferenciais foram utilizados para acessar as relações temporais relativas e cinemáticas entre as dobras e a deformação por cisalhamento que gerou as lineações de estiramento. No presente caso, as orientações preferenciais de quartzo estão relacionadas com a atuação de processos de dobramento ativo, concomitantemente à deformação por cisalhamento, em dobras parasitas de estruturas de maior escala, cujas linhas de charneira são perpendiculares à lineação de estiramento local. Os moscovita-quartzo milonitos estão intensamente recristalizados dinamicamente. Através do tratamento dos dados de orientação preferencial, foi possível calcular os pares eixo/ângulo de desorientação para cada par de cristais desses agregados, e determinar os tipos e as orientações desses limites de grão. Nessas rochas, os limites de grãos são do tipo inclinado, paralelos às formas prismáticas {m} e basal (c). Os limites rotacionados resultam da atuação conjunta de deslizamentos romboédricos e prismáticos em , basais e prismáticos em ou mesmo da atuação de dois planos romboédricos {r e H} em . Em tectonitos ricos em quartzo, as maclas Dauphiné parecem ser um tipo especial de limite, que auxiliam na cominuição do tamanho de grão. As medidas de orientação preferencial de quartzo e moscovita permitiram o cálculo das propriedades sísmicas dos moscovita-quartzo milonitos. Estas rochas possuem anisotropia relativamente alta (AVp entre 7 e 9 % e AVs entre 6 e 10 %) e variam em magnitude com a proporção modal de moscovita. O desenvolvimento de orientação preferencial tem o efeito de dispersar as propriedades sísmicas dos monocristais e atenuar suas magnitudes. A trama de moscovita possui um efeito importante no controle das direções máximas e mínimas de propagação em relação às estruturas do sistema de referência. Além disso, aparentemente as dobras podem causar um efeito nas direções de propagação das ondas P e S lentas. A partir de simulações das propriedades sísmicas com diferentes proporções modais de quartzo e moscovita, observou-se que o incremento das proporções desse último mineral e a redução do primeiro provocam um aumento das velocidades de ondas P e das anisotropias das ondas P e S, mas causam diminuição das velocidades das ondas S. O aumento da quantidade de moscovita nos agregados ocasiona aumento das densidades e dos coeficientes de Poisson dos mesmos. O coeficiente de refletividade na interface entre uma rocha com 100 % de quartzo e uma com 100% de moscovita é relativamente alto, da ordem de 0,06 para ondas P e 0,05 para ondas S, indicando bons refletores sísmicos para esse tipo de limite. Os resultados desse trabalho demonstram a utilidade da análise microestrutural e de orientações cristalográficas preferenciais na investigação de rochas de alta deformação e suas implicações para as determinações das propriedades sísmicas anisotrópicas das mesmas.Rock deformation within ductile shear zones generally produces lattice preferred orientation of minerals. In addition the information about flow conditions of these zones, the development of lattice preferred orientations has a strong influence on the petrophysical properties of deformed rocks. This work presents a detailed study on the relationships between deformation mechanisms, lattice preferred orientation and folding mechanisms, crystallographic textures, grain boundary development and seismic properties of muscovite-quartz mylonites. The lattice preferred orientation data of the muscovite-quartz mylonites from the Saas Fee region (Western Internal Alps) was obtained using the technique of electron backscattered diffraction (EBSD) in a scanning electron microscope. The samples have been collected around a fold whose hinge line is parallel to the local stretching lineation. Such situation is very common within high strain zones, but the presence of these structures can refute simple folding mechanisms theories. The crystallographic fabric data were used to understand the temporal and kinematic relationships between folding development and shear deformation responsible for the development of the associated stretching lineation. In the present case, the quartz lattice preferred orientation seems to be related to the active folding mechanisms that took place in parasitic folds of a larger structure with hinge line at high angle with to the main stretching lineation, simultaneously to the shear deformation event. The muscovite-quartz mylonites are intense dynamically recrystallized. Using the mathematical treatment of lattice preferred orientation data it was possible to calculate the pair angle/misorientation axis on each pair of crystals of these rocks, which allowed the determination of types and orientation of grain boundaries. In these muscovite-quartz mylonites, the grain boundaries included tilted ones, parallel to the prismatic {m} and basal (c) planes, as well as twisted boundaries, which can be related to the concurrent slip in both rhomb and prismatic forms in slip direction, basal and prismatic in or even in two rhombs {r e H} in direction. Dauphiné twinning seems to be a special type of boundary which is related to the initial stages of grain boundary comminuition. The lattice preferred orientation of quartz and muscovite allowed calculations of the seismic properties of the muscovite-quartz mylonites. These rocks shows a relatively high anisotropy for both compressional and shear waves (AVp between 7 and 9 % and AVs between 6 and 10 %) and their magnitude vary with the modal proportion of muscovite. Cristallographic fabric development has the effect of scattering and attenuating single crystal seismic properties. The muscovite crystallographic fabric has an important effect on the directional properties of maximum and minimum propagation velocities in relation to the structures of the reference frame. Besides, it seems that the presence of folds can affect in the propagation directions of P and slow S waves. From simulations of the seismic properties with different modal quartz and muscovite content, it was observed that the increase of the later and decrease of the former causes an increase of P wave velocities and both P and S anisotropies, but also produces a decrease of both shear waves velocities. Increasing mica contents provoke an increment of densities and of Poisson’s ratio. The reflectivity coefficient along an interface between quartz and muscovite bearing rocks is relatively high, of the order of 0,06 for P waves and 0,05 for S waves, indicating that these types of surfaces are good seismic reflectors. The results of the work demonstrate the usefulness of microstructural and lattice preferred orientation analysis in the study of high strain rocks, as well as their important control on the anisotropic seismic properties of these rocks

Propriedades sísmicas anisotrópicas derivadas da orientação cristalográfica preferencial de muscovita-quartzo milonitos

Seismic wave propagation in organized matter usually results in azimuthal variations of longitudinal waves (Pwaves), as well as the effect of birefringence in transversal waves (S-waves), which results in two orthogonal shear waves with contrasting velocities. In this paper we present the results of the anisotropic seismic properties of five samples of muscovitequartz mylonites collected in different parts of a fold in the Saas Fee region, Western Internal Alps. The P-wave velocities in these rocks varies from 5.73 to 6.32 km/s, whereas the high-velocity shear wave (S1) varies from 3.82 to 4.22 km/s and the low velocity (S2) from 3.73 to 4.09 km/s. The anisotropy in these rocks is relatively high and reaches values from 9.5% for P-waves, and almost 11% for shear wave splitting. Both anisotropy and propagation directions seem to be related to from the strong preferred orientation of quartz and muscovite but also depend of muscovite modal content within the different specimens. Development of preferred orientation of minerals destroys and disperses the single crystal seismic properties, which causes a decrease of wave velocities and a dispersion of propagation directions, of both compressional and shear waves. Since the preferred orientation of quartz and muscovite can be directly related to the main macroscopic structures in these rocks (foliation, lineation, and pole of foliation) and the anisotropic seismic properties are related to the preferred orientation, it is possible to determine the propagation directions in terms of these structures. Due to the relatively high muscovite content, many of the maximum propagation velocities are parallel/subparallel to the foliation and some parallel to the lineation of the reference frame. On the other hand, directions of minimum propagation cannot be directly related to the foliation pole. The presence of folds in the mid-to lower crust can exert changes in the propagation directions due to the foliation variation around such structures, mainly in the P-waves

Crystallographic fabric development along a folded polycrystalline hematite.

Detailed analyses of microstructures and crystallographic preferred orientations (CPOs) of hematite rocks were conducted in samples of polycrystalline hematite around a tight fold in the Quadrilátero Ferrífero region, southeastern Brazil. Grain size is dominantly very fine in all samples (z15–30 mm), with a slight increase toward the hinge. Grain aspect ratio increases substantially toward the hinge, from an average of 2 in the limb to 9 in the hinge. Distribution patterns of [0001] axes and poles of planes {1010}, {1120} and {1014} suggest that intracrystalline slip operate on the basal plane along the <a> direction. The distribution of the poles of prismatic planes parallel to the foliation of reference frame indicate that the all the symmetric <a>-slip directions of hematite crystals were equally efficient during activation of basal intracrystalline slip. Increasing aspect ratio is accompanied by CPO intensification toward the hinge, where intensification was aided during deformation by bulk rotation of the hematitic plates and possibly by some grain boundary sliding. Such CPO intensification with the increase of aspect ratio toward the hinge is used to infer that the fold developed by flexural flow

Propriedades sísmicas anisotrópicas derivadas da orientação cristalográfica preferencial de muscovita-quartzo milonitos

Seismic wave propagation in organized matter usually results in azimuthal variations of longitudinal waves (Pwaves), as well as the effect of birefringence in transversal waves (S-waves), which results in two orthogonal shear waves with contrasting velocities. In this paper we present the results of the anisotropic seismic properties of five samples of muscovitequartz mylonites collected in different parts of a fold in the Saas Fee region, Western Internal Alps. The P-wave velocities in these rocks varies from 5.73 to 6.32 km/s, whereas the high-velocity shear wave (S1) varies from 3.82 to 4.22 km/s and the low velocity (S2) from 3.73 to 4.09 km/s. The anisotropy in these rocks is relatively high and reaches values from 9.5% for P-waves, and almost 11% for shear wave splitting. Both anisotropy and propagation directions seem to be related to from the strong preferred orientation of quartz and muscovite but also depend of muscovite modal content within the different specimens. Development of preferred orientation of minerals destroys and disperses the single crystal seismic properties, which causes a decrease of wave velocities and a dispersion of propagation directions, of both compressional and shear waves. Since the preferred orientation of quartz and muscovite can be directly related to the main macroscopic structures in these rocks (foliation, lineation, and pole of foliation) and the anisotropic seismic properties are related to the preferred orientation, it is possible to determine the propagation directions in terms of these structures. Due to the relatively high muscovite content, many of the maximum propagation velocities are parallel/subparallel to the foliation and some parallel to the lineation of the reference frame. On the other hand, directions of minimum propagation cannot be directly related to the foliation pole. The presence of folds in the mid-to lower crust can exert changes in the propagation directions due to the foliation variation around such structures, mainly in the P-waves

Microestruturas das rochas miloníticas da porção sul do terreno Nico Pérez, Uruguai : indicadores cinemáticos e mecanismos de deformação

03

First results on the LPO-derived seismic properties of iron ores from the Quadrilátero Ferrífero region, southeastern Brazil.

Determinations of the LPO-derived seismic properties of iron ores were carried out in five samples with contrasting mineralogy (hematite, magnetite and quartz) and deformed in different conditions. All the samples are seismically quasi-isotropic or weakly anisotropic, which reflect (i) an absent or weak crystallographic preferred orientation (CPO) in some samples; (ii) the high modal content of magnetite, (iii) the relatively weak anisotropy of elastic stiffness of hematite single crystal. Such variables induce a low anisotropic seismic behavior even in high strained iron ores with strong preferred orientation of hematite. A plane of seismic transversal isotropy parallel to the foliation of the aggregates is developed in hematite ± magnetite aggregates and in itabirites with strong CPO of hematites. In these high strained aggregates, some relationships between the crystallographic axes of hematite and the propagation velocities can be observed. The magnitudes of P and S-wave velocities derived of hematite CPO are lower than the values experimentally determined in iron ores. Such a difference probably reflects other microstructural variable which were not taken into account in the present contribution (e.g. shape preferred orientation)

A técnica de difração de elétrons retro-espalhados (EBSD) em microscópio eletrônico de varredura (MEV) e sua aplicação no estudo de rochas deformadas

The electron backscattered diffraction technique in the scanning electron microscope (EBSD/SEM) is based on the diffraction of a high-energy electron beam by the crystalline structure of a given material, in all possible directions within the sample. Some of the diffracted electrons escape from the specimen with virtually the same initial energy, interact with a phosphorescent screen and the generated EBSP pattern can be picked up with a low-luminosity charge couple device (CCD) camera. These patterns can be indexed using pre-determined patterns for a large variety of minerals, which allows the determination of complete orientation of each single mineral within an aggregate. In this paper we briefly discuss the physical aspects related to the diffraction of an electron beam by crystalline matter and how the EBSP patterns are generated. We also present a short introduction of the necessary instruments to acquire EBSD data, as well as the calibration procedures, acquisition and indexing software of EBSPs. The pitfalls of the technique and possible error sources are also discussed with examples. Considering the scarce availability of literature on geological sample preparation, the polishing method of silicate-rich rocks for EBSP is described in detail in the last part of this paper