New microstructural data on experimentally deformed “wet” and “dry” natural olivine rocks (Anita Bay and Åheim dunite), together with the other reliable experimental data, indicate that the experimental stress‐recrystallized grain size relationship in olivine‐rocks is largely independent of water content and temperature, and is only slightly dependent on the flow properties of the material. The experimental data cover a stress range of 30–300 MPa, water contents from <30 ppm to 300 ppm, and temperatures in the range 1100–1650°C. Local melt contents of up to 10 volume% cannot be demonstrated to have a significant effect on the stress—grain size relationship.
Abstract We present a selective overview of current issues and outstanding problems in the field of deformation mechanisms, rheology and tectonics. A large part of present-day research activities can be grouped into four broad themes. First, the effect of fluids on deformation is the subject of many field and laboratory studies. Fundamental aspects of grain boundary structure and the diffusive properties of fluid-filled grain contacts are currently being investigated, applying modern techniques of light photomicrography, electrical conductivity measurement and Fourier Transform Infrared (FTIR) microanalysis. Second, the interpretation of microstructures and textures is a topic of continuous attention. An improved understanding of the evolution of recrystallization microstructures, boundary misorientations and crystallographic preferred orientations has resulted from the systematic application of new, quantitative analysis and modelling techniques. Third, investigation of the rheology of crust and mantle minerals remains an essential scientific goal. There is a focus on improving the accuracy of flow laws, in order to extrapolate these to nature. Aspects of strain and phase changes are now being taken into account. Fourth, crust and lithosphere tectonics form a subject of research focused on large-scale problems, where the use of analogue models has been particularly successful. However, there still exists a major lack of understanding regarding the microphysical basis of crust- and lithosphere-scale localization of deformation.
Deformation of ice in continental sized ice sheets determines the flow behavior of ice towards the sea. Basal dislocation glide is assumed to be the dominant deformation mechanism in the creep deformation of natural ice, but non-basal glide is active as well. Knowledge of what types of deformation mechanisms are active in polar ice is critical in predicting the response of ice sheets in future warmer climates and its contribution to sea level rise, because the activity of deformation mechanisms depends critically on deformation conditions (such as temperature) as well as on the material properties (such as grain size).
One of the methods to study the deformation mechanisms in natural materials is Electron Backscattered Diffraction (EBSD). We obtained ca. 50 EBSD maps of five different depths from a Greenlandic ice core (NEEM). The step size varied between 8 and 25 micron depending on the size of the deformation features. The size of the maps varied from 2000 to 10000 grid point. Indexing rates were up to 95%, partially by saving and reanalyzing the EBSP patterns.
With this method we can characterize subgrain boundaries and determine the lattice rotation configurations of each individual subgrain. Combining these observations with arrangement/geometry of subgrain boundaries the dislocation types can be determined, which form these boundaries. Three main types of subgrain boundaries have been recognized in Antarctic (EDML) ice core (Weikusat et al. 2010, 2011).
Here, we present the first results obtained from EBSD measurements performed on the NEEM ice core samples from the last glacial period, focusing on the relevance of dislocation activity of the possible slip systems. Preliminary results show that all three subgrain types, recognized in the EDML core, occur in the NEEM samples. In addition to the classical boundaries made up of basal dislocations, subgrain boundaries made of non-basal dislocations are also common.
The probable slip systems responsible for subgrain boundaries in ice can be determined using EBSD, providing the subgrain boundaries can be localized in the coarse grained natural ice and small scanning electron microscope (SEM) samples, and providing the subgrain boundary geometry can be characterized precisely. This can be realized by observation of etch features imaged with light microscopy (LM microstructure mapping). The surface then has to be retained during EBSD data acquisition in an SEM.
<p>Induced subsidence and seismicity caused by the production of hydrocarbons in the Groningen gas field (the Netherlands) is a widely known issue facing this naturally aseismic region (Smith et al., 2019). Extraction reduces pore-fluid pressure leading to accumulation of small elastic and inelastic strains and an increase in effective vertical stress driving compaction of reservoir sandstones.</p><p>Recent studies (Pijnenburg et al., 2019a, b and Verberne et al., 2021) identify grain-scale deformation of intergranular and grain-coating clays as largely responsible for accommodating (permanent) inelastic deformation at small strains relevant to production (&#8804;1.0%). However, their distribution, microstructure, abundance, and contribution to inelastic deformation remains unconstrained, presenting challenges when evaluating grain-scale deformation mechanisms within a natural system. Traditional methods of mineral identification are costly, labor-intensive, and time-consuming. Digital imaging coupled with machine-learning-driven segmentation is necessary to accelerate the identification of clay microstructures and distributions within reservoir sandstones for later large-scale analysis and geomechanical modeling.</p><p>We performed digital imaging on thin-sections taken from core recovered from the highly-depleted Zeerijp ZRP-3a well located at the most seismogenic part of the field. The core was kindly made available by the field operator, NAM. Optical digital images were acquired using the Zeiss AxioScan optical light microscope at 10x magnification with a resolution of 0.44&#181;m and compared to backscattered electron (BSE) digital images from the Zeiss EVO 15 Scanning Electron Microscope (SEM) at varying magnifications with resolutions ranging from 0.09&#181;m - 2.24 &#181;m. Digital images were processed in ilastik, an interactive machine-learning-based toolkit for image segmentation that uses a Random Forest classifier to separate clays from a digital image (Berg et al., 2019).</p><p>Comparisons between segmented optical and BSE digital images indicate that image resolution is the main limiting factor for successful mineral identification and image segmentation, especially for clay minerals. Lower resolution digital images obtained using optical light microscopy may be sufficient to segment larger intergranular/pore-filling clays, but higher resolution BSE images are necessary to segment smaller micron to submicron-sized grain-coating clays. Comparing the same segmented optical image (~11.5% clay) versus BSE image (~16.3% clay) reveals an error of ~30%, illustrating the potential of underestimating the clay content necessary for geomechanical modeling.</p><p>Our analysis shows that coupled automated electron microscopy with machine-learning-driven image segmentation has the potential to provide statistically relevant and robust information to further constrain the role of clay films on the compaction behavior of reservoir rocks.</p><p>&#160;</p><p>References:</p><p>Berg, S. et al., Nat Methods 16, 1226&#8211;1232 (2019).</p><p>(NAM) Nederlandse Aardolie Maatschappij BV (2015).</p><p>Pijnenburg, R. P. J. et al., Journal of Geophysical Research: Solid Earth, 124 (2019a).</p><p>Pijnenburg, R. P. J. et al., Journal of Geophysical Research: Solid Earth, 124, 5254&#8211;5282. (2019b)</p><p>Smith, J. D. et al., Journal of Geophysical Research: Solid Earth, 124, 6165&#8211;6178. (2019)</p><p>Verberne, B. A. et al., Geology, 49 (5): 483&#8211;487. (2020)</p>
We present the results of a structural and petrological study of mantle rocks from the strongly dismembered Othris Ophiolite. Part of the mantle section was impregnated with melt, crystallizing plagioclase and clinopyroxene as cumulate phases and refertilizing previously depleted peridotites. Melt impregnation occurred late in the deformation history of the host peridotites. The deformation took place at stresses of 13–26 MPa and at temperatures around 1000–1200°C, at the base of the thermal lithosphere. The melt therefore impregnated relatively cold mantle rocks, implying that the thermal lithosphere reached into the mantle during magmatic activity. We conclude that the Othris Ophiolite represents a spreading environment with a relatively thick lithosphere, such as that near an axial discontinuity or transform fault of a slow-spreading ridge. The proposed magmatic and deformation history of the peridotites is in agreement with episodic magmatism at slow-spreading ridges. We thus conclude that the heterogeneous character of the mantle section of the Othris Ophiolite results from melt impregnation processes. We suggest that the presence of lherzolitic ophiolite types among harzburgitic ophiolite types in the Hellenic–Dinaric chain reflects variable degrees of melt impregnation and refertilization rather than partial melting and melt extraction.
An unexpected microstructure, with important implications for melt distribution in the upper mantle, has been found in a 70% olivine +30% orthopyroxene rock experimentally deformed at 1500K, 300 MPa, where incipient partial melting occurred. As found in previous studies of comparable systems, most melt in the olivine‐orthopyroxene sample resided in a network of grain‐edge tubes and occasional thick (50–500 nm) layers. We infer, using electron microscopy at its highest resolution, that melt also existed in another form as glass films 1.0–1.5 nm thick, along all grain boundaries, with total film fraction (F) of 0.0002. All of these melts are unusually SiO 2 ‐rich. Further work is needed to confirm that the thin films are not transient, although their coexistence with smoothly curved solid‐melt interfaces and flat crystal faces suggests they are stable. If thin high‐silica melt films are stable they might influence physical properties and melt extraction processes in regions of incipient melting and metasomatism in the upper mantle.
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