Microstructure Analysis Using Electron Backscatter Diffraction 8:30 9:00 10:00 10:30 11:30 Noon 1:00 2:00 2:30 3:30

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1 Microstructure Analysis Using Electron Backscatter Diffraction Kibbe Science Symposium Saturday, April 16, 2005 Bowdoin College, Brunswick, Maine Organizer: Rachel Beane Schedule of Events 8:30 Registration, coffee/tea, and poster hanging (Druckenmiller atrium) 9:00 Donna Whitney, University of Minnesota Metamorphic Microstructures and Mountains (Druckenmiller 151) 10:00 Morning break and posters (Druckenmiller atrium) 10:30 David Prior, University of Liverpool Watching metamorphic processes happen: high temperature experiments inside an SEM (Druckenmiller 151) 11:30 Scott Sitzman, HKL Introduction to EBSD (Druckenmiller 151) Noon Lunch buffet (Druckenmiller atrium) 1:00 Greg Hirth, WHOI Lattice Preferred Orientations in Naturally Deformed Peridotites: A Link from the Lab to Mantle Dynamics (Druckenmiller 151) 2:00 Afternoon break and posters (Druckenmiller atrium) 2:30 Michael Cheadle, University of Wyoming Seismic Properties from EBSD Measurements: the Example of Mantle Peridotites (Druckenmiller 151) 3:30 Interest group discussions (Druckenmiller atrium)

2 Speaker Abstracts Metamorphic microstructures and mountains Donna L. Whitney, Geology & Geophysics, University of Minnesota, Minneapolis MN Textures of metamorphic rocks record the chemical and physical effects of changes in pressure and temperature - i.e., the path that a rock takes during mountain building and unroofing, the reactions that occur along that path, and the interaction of deformation and metamorphism. By integrating microstructural and petrologic data such as mineral chemistry with crystal shape and lattice orientation, we can better understand the driving forces, effects, and rates of chemical and mechanical processes that occur at depth in the Earth. With the general goal of linking the conditions and mechanisms of metamorphic crystallization to tectonic processes, the metamorphic geology group at the University of Minnesota is applying electron back-scattered diffraction (EBSD) and X-ray compositional mapping to metamorphic textures. Our investigations span the range from large crystals with low surface area to volume ratio (garnets) to numerous intergrown crystals with large surface area to volume ratio (symplectites). We are examining crystallographic controls on textures related to growth and reaction, deformation, and polymorphic transformations; for example, textures involving Al 2 SiO 5 minerals (kyanite, andalusite, sillimanite) in natural and experimentally deformed rocks. In the garnet study, we have documented morphologically single crystals of garnet that contain 2 to 3 distinct structural domains that meet at high angle boundaries (47-60 ). In garnet schists from Vermont, adjacent structural domains in garnet have no obvious spatial relationships to chemical zoning or internal (inclusion) fabrics. These boundaries did not form by late brittle deformation or twinning. If the boundaries formed when garnets coalesced during growth, garnets may grow by a different mechanism than other porphyroblasts in metamorphic rocks. Fine-scale observations of microstructures like these are important to understand because garnets are used in a wide variety of applications that provide information about large-scale tectonic processes such as the evolution of mountain systems.

3 Watching metamorphic processes happen: high temperature experiments inside a SEM Dave Prior, Michel Bestmann, Sandra Piazolo, Nick Seaton and Gareth Seward Department of Earth and Ocean Sciences, Liverpool University, L693GP, UK We wish to understand the processes by which microstructures, preferred orientations and resultant physical properties of rocks evolve and to use this information to help interpret, quantitatively, the history of a rock. However, we are limited to looking at the final frozen in microstructures and and viable explanations of what has happened in the material are those that can explain the frozen in microstructures and textures. Being able to watch microstructures and textures as they evolve provides a much better set of constraints upon the processes that operate in materials. The CamScan X500 crystal probe was built with the aim of conducting high temperature experiments in-situ in an SEM and quantifying microstructural and textural changes during those experiments using EBSD and imaging. In this talk We will show some of the results of experiments in metals and in rock forming minerals. We will focus on the results of phase transformation experiments in titanium and recrystallization experiments in rock salt. We have observed phase transformations in Ti metal. The structure of Ti transforms at ~882 C from hexagonal close packed (HCP - α) to body centred cubic (BCC - β). The BCC phase nucleates as both intracrystalline plates and grain-boundary allotriomorphs. Electron backscatter diffraction analysis shows that intracrystalline plates have a Burgers orientation relationship with the parent HCP grain ({0001} // {110} and <11-20>//<111>). Boundary planes orientations and topography associated with plates suggest that plates nucleate and grow by a shear mechanism. Grain-boundary allotriomorphs have a Burgers orientation with one of their neighbouring HCP grains. The boundary plane orientations are arbitrary and have no associated topography suggesting that allotriomorphs grow by a diffusive process. Direct observations of boundary motion and textural analysis show that, during HCP to BCC transformation, BCC phase microstructure and texture are dominated by the growth of grain boundary allotriomorphs, rather than intracrystalline plates. We will explore the implications of these observations for the kinetics of phase transformations in the Earth. Recrystallization is an important process in metals and minerals. Dry deformed rock salt samples were heated to temperatures between 315 and 410ºC. Inter- and intracrystalline processes were observed using both secondary electron imaging and electron backscatter diffraction mapping techniques. Grain boundary migration was observed between substructured grains. During GBM some unexpected features were observed. Often the growing grain exhibits new low angle boundaries in the swept area and in some cases the existing substructure of the growing grain is continued in the swept area. More rarely substructures within the swept area develop into a low angle boundary within the growing grain. These data are not explicable with existing grain boundary migration models. The observations imply that general grain boundaries have a structure that allows structural features of the dying grain to be transferred to the growing grain. Grain boundary migration facilitating replacement of substructured grains by strain-free grains was also observed. With increasing temperature (T), not only the velocity but also the mode of grain boundary migration and resultant microstructure change continuously. While at low annealing temperature, impurities and subtle driving-force variations are most influential, at high temperature grain boundary anisotropy becomes more important. Transferring the knowledge from our experiments to geological samples, enables us to recognize similar microstructures in rocks. Such microstructural indicators can be used to distinguish microstructural characteristics that are due to either its pre-anneal history or post-deformational annealing.

4 Lattice Preferred Orientations in Naturally Deformed Peridotites: A Link from the Lab to Mantle Dynamics Greg Hirth, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA Our understanding of the rheological behavior of the Earth's mantle has been greatly enhanced by microstructural observations of both naturally and experimentally deformed rocks. The analysis of microstructures in naturally deformed rocks provides a critical link between theoretical and experimental studies and large-scale geologic processes. A current limitation in the application of both experimental observations on the development of lattice preferred orientations (LPO) in mantle rocks, and theoretical studies that are tested using the same data, is that the experiments are conducted at considerably higher stresses (or temperatures) than those experienced during deformation in the asthenosphere (or lithosphere). However, since specific deformation processes produce diagnostic microstructures, analyses of textures preserved in rocks can be used to constrain the applicability of experimental flow laws. In this talk I will present data obtained using EBSD (Electron Backscatter Diffraction) on LPOs in naturally deformed mantle rocks with well-constrained deformation histories. I will (1) discuss how the evolution of LPO with increasing strain in natural shear zones compares with that determined experimentally and theoretically. (2) Illustrate the possible effects of grain boundary sliding (GBS) on the development of LPO and describe microstructural evidence that a transition from dislocation creep to diffusion creep can result in randomization of a pre-existing LPO and promote strain localization in the mantle lithosphere. Throughout the talk I will compare the interpretations provided by the microstructural observations to predictions based on the extrapolation of experimentally determined flow laws for olivine aggregates and discuss their implications for the dynamics of the upper mantle.

5 Seismic Properties from EBSD measurements: the example of Mantle Peridotites M. J. Cheadle, Department of Geology & Geophysics, University of Wyoming, Dept. 3006, 1000 E. University Ave., Laramie, Wyoming, USA Harry Hess in 1964 was the first to suggest that seismic anisotropy in the uppermost mantle is fundamentally controlled by the non-random orientation of its primary anisotropic phases: olivine and orthopyroxene and the statistical alignment of their crystal axes or lattice-preferred orientation (LPO). Since then, many authors have confirmed the relationship between seismic anisotropy and the LPO of minerals in both crust and mantle rocks, and have used this observation to explain various phenomena reported from seismic experiments including shear wave-spitting and velocity anomalies. It has recently been realized that seismic anisotropy is a tool that has the potential to map patterns of flow in the uppermost mantle beneath mid-oceanic ridges, hotspots and at subduction zones. However, successful interpretation of seismic anisotropy requires a knowledge of the constituent crystal LPO s that control the elasticity and therefore seismic wave propagation through mantle rocks. Electron Backscatter Diffraction (EBSD) is a technique that can provide this information efficiently and accurately. Further, EBSD can also provide grain size information, which intrinsically controls both the shear modulus and seismic attenuation of mantle rocks. Recent studies suggest that a change in grain size from 1mm to 1cm causes a seismic velocity anomaly that is equivalent to that caused by a 100 o C mantle temperature anomaly. Seismic anisotropy of a polycrystalline rock can be calculated if the volume fraction, density, elastic constants and LPO of each mineral are known. This method has two great advantages over the physical measurement of seismic properties of rock samples: i) the effect of cracks and limited alteration on seismic velocity are easily removed, and ii) the method is relatively simple and cost efficient. I will illustrate this approach using the example of partially serpentinised peridotite from slow spreading mid-ocean ridges. In these localities mantle can be exposed on the sea floor and arguably the most fresh (up to 60%) and longest section of in-situ abyssal peridotite ever recovered was collected during ODP Leg 209, at the 15 20' N Fracture Zone on the Mid Atlantic Ridge. ODP Hole 1274 drilled into 154m of relatively fresh harzburgite and dunite. EBSD analysis reveals that a weak lattice preferred orientation of olivine exists throughout the harzburgite in the core from Site 1274, where at least 20% fresh olivine is present. The LPO exhibits patterns that are similar to those produced in deformation experiments and observed in ophiolites with distinct [010] maximum subperpendicular to foliation, with less strongly developed [100] maximum parallel to the foliation plane. Orthopyroxene crystals show a strong [001] maximum parallel to the foliation plane, with a weaker [100] maximum subperpendicular to the foliation plane. This fabric is consistent with a relatively low strain rate operating on the olivine (010)[100] and the orthopyroxene (100)[001] slip systems. When orientation data from different depth are rotated to a common paleo-north direction, the resulting orientation maxima are similar indicating that the LPO is consistent over the 154m section drilled. The LPO-calculated seismic anisotropy for these rocks compares well with mantle anisotropy results of a recent wide-angle seismic experiment in the Atlantic, and raises the possibility that the strength of anisotropy in the uppermost oceanic mantle is a function of plate spreading rate.

6 Poster Abstracts Development and Application of a New Method for Calculating the Strength of Lattice- Preferred Orientation (LPO) Philip Skemer, Ikuo Katayama, Zhenting Jiang, Shun-ichiro Karato, Yale University, Department of Geology and Geophysics, 210 Whitney Ave., New Haven, CT Using orientation data from experimentally deformed olivine, we explore some practical problems with the J-index, a commonly applied measure of fabric strength. We show that the J- index is highly dependent on several factors, including the number of discrete data in the orientation distribution function (ODF), and arbitrary numerical parameters specified for its calculation. Because of this non-uniqueness, we conclude that the J-index is difficult to interpret and should only be applied with caution. As an alternative to the J-index, we propose a new measure of fabric strength that is based on the distribution of uncorrelated misorientation angles. This M-index is shown to be insensitive to the parameters specified for its calculation. For typical deformed olivine samples, we show that ~150 discrete data are adequate to quantify fabric strength using the M-index technique. The M-index correlates well with seismic anisotropy, particularly for materials of the same fabric type. Therefore, we conclude that the M- index technique is well-suited for the quantification of fabric strength and the comparison of like materials. We have applied the M-index to the study of recrystallized orthopyroxene bands in sheared lherzolite xenoliths from the Jagersfontein kimberlite, South Africa. Using the M-index, we can observe and quantify the progressive randomization of LPO inherited during dynamic recrystallization. This fabric randomization is used to infer a transition to grain-size sensitive deformation, indicating that strain-weakening of orthopyroxene may play an important role in the rheology of the upper mantle. Olivine LPO Variation in a Shear Zone of the Josephine Peridotite Jessica M. Warren, MIT/WHOI Joint Program, Woods Hole MA; jmwarren@whoi.edu Greg Hirth, Woods Hole Oceanographic Institution, Woods Hole MA Constraining the rheology of peridotites is important for models of melt extraction at mid-ocean ridges. Knowledge of the behavior of olivine during shear deformation is also necessary when using seismic anisotropy to infer mantle kinematics. The Josephine Peridotite is ideal for analysis of the interaction of deformation and melts, due to the presence of shear zones with associated melt migration structures. We use these shear zones to understand olivine rheology by measuring olivine lattice preferred orientations (LPOs) as a function of strain. Results on harzburgites from the largest Josephine shear zone, which is 100m wide and contains syn-deformational dunite, indicate that the olivine LPO rotates in the shear zone so that the [100] maxima lies parallel to the shear direction. Outside of the shear zone, the harzburgite has a preexisting LPO which may be coincident with the existing stress regime during deformation or may represent a previous mantle flow regime. At the center of the shear zone, the olivine LPO indicates that the [100] axis is aligned with the flow direction. This result agrees with the experimental results of Zhang and Karato (1995), providing a link between experimental data and LPO evolution models, which can be applied to the interpretation of mantle seismic anisotropy.

7 Crystallographic orientation evidence for the formation of atoll garnet in eclogite Rachel Beane 1 and David Prior 2 1 Department of Geology, 6800 College Station, Bowdoin College, Brunswick, Maine Department of Earth Sciences, University of Liverpool, Liverpool L69 3GP, UK, Atoll garnets from eclogite in the Ural Mountains have faceted rings of garnet that surround a lagoon of other phases and subhedral islands of garnet. Electron backscatter diffraction shows that the islands of garnet have exactly the same orientation as the atoll ring that surrounds them. We propose a two-stage model for the formation of these atoll garnets. First, a poikiloblast, with inclusions concentrated in the core, grows. Then, the garnet and inclusions are redistributed by diffusion to form the atoll shape, resulting in decreased surface to volume ratio, and lowered interfacial energy. This model has important implications for interpretations of P-Tt traverses of garnet, and for solid-state diffusion in metamorphic rocks. Using Simultaneously Collected EBSD and EDS Data for Improved Phase Differentiation and Orientation Imaging Matthew M Nowell and Stuart I. Wright, TSL/EDAX, 392 East South, Suite H, Draper, UT The automated analysis of electron backscatter diffraction (EBSD) patterns and subsequent orientation imaging has become a well-accepted microstructural analysis technique. However when applied to multi-phase samples, the EBSD technique can have significant limitations if the phases of interest are crystallographically similar. For example, consider the minerals Magnetite (Fe3O4) and Chromite (FeCr2O4). Both have a cubic crystal structure (Fd3m) with similar lattice parameters (a=8.40ǻ for Magnetite and a= 8.36Ǻ for Chromite) and similar diffracting intensities. These similarities make differentiation by EBSD difficult. However these are easy to differentiate chemically using EDS. After simultaneously collecting both EBSD and EDS data, it is possible to manually or automatically determine the chemical limits which define each phase of interest, and then use these limits to differentiate phases while using EBSD to determine crystallographic orientation. When polymorphous minerals are considered only EBSD is used to determine both phase and orientation. In addition to more accurate phase differentiation, overall data acquisition times are reduced using this combined method.

8 Examination of an Unusual Grain Boundary in CaF 2 M.E. Msall 1, W. Dietsche 2, R. Beane 1, R. Wichard 3, and J. Carpenter 1 1 Bowdoin College, Brunswick, ME, USA 2 Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, Stuttgart, Germany 3 Carl Zeiss SMT AG, Oberkochen, Germany We have examined a grain boundary in CaF 2 using Phonon Imaging and Electron Backscatter Diffractometry (EBSD). Unlike the <111> twin boundary typically found in CaF 2, the crystal grains on either side of the boundary are not simply related to any principle symmetry directions, and are not related to one another by symmetry operations of the cubic group. In spite of the high degree of misalignment of the crystalline lattices, phonons can pass this grain boundary without excessive energy loss. Phonon images of samples taken from different sections of the grain boundary show that the structural properties of the grain boundary are constant over a large area. Computer simulations of phonon scattering at the interface based on acoustic mismatch models demonstrate that the caustic positions are sensitive to small changes in the relative orientation of the two pieces and to the projection of the grain boundary on the image plane. EBSD gives the needed high precision measurement of the relative orientation, resulting in a superior model of phonon transmission through this very asymmetric interface.