| 1. |
- Daly, L., et al.
(author)
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Boom boom pow : Shock-facilitated aqueous alteration and evidence for two shock events in the Martian nakhlite meteorites
- 2019
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In: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 5:9
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Journal article (peer-reviewed)abstract
- Nakhlite meteorites are similar to 1.4 to 1.3 Ga old igneous rocks, aqueously altered on Mars similar to 630 Ma ago. We test the theory that water-rock interaction was impact driven. Electron backscatter diffraction demonstrates that the meteorites Miller Range 03346 and Lafayette were heterogeneously deformed, leading to localized regions of brecciation, plastic deformation, and mechanical twinning of augite. Numerical modeling shows that the pattern of deformation is consistent with shock-generated compressive and tensile stresses. Mesostasis within shocked areas was aqueously altered to phyllosilicates, carbonates, and oxides, suggesting a genetic link between the two processes. We propose that an impact similar to 630 Ma ago simultaneously deformed the nakhlite parent rocks and generated liquid water by melting of permafrost. Ensuing water-rock interaction focused on shocked mesostasis with a high density of reactive sites. The nakhlite source location must have two spatially correlated craters, one similar to 630 Ma old and another, ejecting the meteorites, similar to 11 Ma ago.
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| 2. |
- Almqvist, Bjarne S.G., et al.
(author)
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Seismic anisotropy of mid crustal orogenic nappes and their bounding structures : An example from the Middle Allochthon (Seve Nappe) of the Central Scandinavian Caledonides
- 2021
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In: Tectonophysics. - : Elsevier. - 0040-1951 .- 1879-3266. ; 819
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Journal article (peer-reviewed)abstract
- We report compositional, microstructural and seismic properties from 24 samples collected from the Middle Allochthon (Seve Nappe) of the central Scandinavian Caledonides, and its bounding shear zones. The samples stem both from field outcrops and the continental drilling project COSC-1 and include quartzofeldspathic gneisses, hornblende gneisses, amphibolites, marbles, calc-silicates, quartzites and mica schists, of medium to high-strain. Seismic velocities and anisotropy of P (AVp) and S (AVs) waves of these samples were calculated using microstructural and crystal preferred orientation data obtained from Electron Backscatter Diffraction analysis (EBSD). Mica-schist exhibits the highest anisotropy (AVp similar to 31%; max AVs similar to 34%), followed by hornblende-dominated rocks (AVp similar to 5-13%; max AVs similar to 5-10%) and quartzites (AVp similar to 6.5-10.5%; max AVs similar to 7.5-12%). Lowest anisotropy is found in calc-silicate rocks (AVp similar to 4%; max AVs similar to 3-4%), where the symmetry of anisotropy is more complex due to the contribution to anisotropy from several phases. Anisotropy is attributed to: 1) modal mineral composition, in particular mica and amphibole content, 2) CPO intensity, 3) crystallization of anisotropic minerals from fluids circulating in the shear zone (calc-silicates and amphibolites), and to a lesser extent 4) compositional banding of minerals with contrasting elastic properties and density. Our results link observed anisotropy to the rock composition and strain in a representative section across the Central Scandinavian Caledonides and indicate that the entire Seve Nappe is seismically anisotropic. Strain has partitioned on the nappe scale, and likely on the microstructural scale. High- strain shear zones that develop at boundaries of the allochthon and internally within the allochthon show higher anisotropy than a more moderately strained interior of the nappe. The Seve Nappe may be considered as a template for deforming, ductile and flowing middle crust, which is in line with general observations of seismic anisotropy in mid-crustal settings.
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| 3. |
- Bazargan, Mohsen, et al.
(author)
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A Numerical Toolbox to Calculate the Seismic Properties of Micro Sized Isotropic and Anisotropic Minerals
- 2020
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In: Mineral Exploration Symposium. - : European Association of Geoscientists and Engineers. ; , s. 1-3
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Conference paper (other academic/artistic)abstract
- Seismology offers valuable information about the nature of lithosphere and asthenosphere. Seismic waves are used to investigation the Earth’s crust and upper mantel and to understand their placement and depth. In this respect, outcrop samples’ thin sections provide a wealth of information about the rocks seismic properties. A numerical toolbox is presented to investigate the seismic properties of rock samples. For this purpose, the toolbox makes use of image processing capabilities of MATLAB combined with computational power of FEM based COMSOL multiphysics. The toolbox provides variety of studies and analyses and it is specifically used to investigate the wave speed velocities in a Gabbro sample thin-section. There are several software packages in the technical community which can calculate the seismic velocities analytically by employing the effective medium theory. The toolbox is benchmarked against the existing software packages and additional features are discussed. An objective of the numerical measurements would be the investigate of the influence of grain sizes on elastic wave velocities and potential scattering due to the wavelength effects.
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| 4. |
- Borthwick, Verity E., et al.
(author)
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Quantification of mineral behavior in four dimensions : grain boundary and substructure dynamics in salt
- 2012
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In: Geochemistry Geophysics Geosystems. - 1525-2027. ; 13, s. Q05005-
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Journal article (peer-reviewed)abstract
- Here we present the first four dimensional (time and three dimensional space resolved) experiment on a strongly deformed geological material. Results show that even complicated microstructures with large continuous and discontinuous changes in crystallographic orientation can be resolved quantitatively. The details that can be resolved are unprecedented and therefore the presented technique promises to become influential in a wide range of geoscientific investigations. Grain and subgrain scale processes are fundamental to mineral deformation and associated Earth Dynamics, and time resolved observation of these processes is vital for establishing an in-depth understanding of the latter. However, until recently, in situ experiments were restricted to observations of two dimensional surfaces. We compared experimental results from two dynamic, in situ annealing experiments on a single halite crystal; a 2D experiment conducted inside the scanning electron microscope and a 3D X-ray diffraction experiment. This allowed us to evaluate the possible effects of the free surface on grain and subgrain processes. The extent to which surface effects cause experimental artifacts in 2D studies has long been questioned. Our study shows that, although the nature of recovery processes are the same, the area swept by subgrain boundaries is up to 5 times larger in the volume than observed on the surface. We suggest this discrepancy is due to enhanced drag force on subgrain boundaries by thermal surface grooving. Our results show that while it is problematic to derive absolute mobilities from 2D experiments, derived relative mobilities between boundaries with different misorientation angles can be used.
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| 5. |
- Borthwick, V. E., et al.
(author)
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What happens to deformed rocks after deformation? : A refined model for recovery based on numerical simulations
- 2014
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In: Geological Society Special Publication. - 0305-8719 .- 2041-4927. ; 394, s. 215-234
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Journal article (peer-reviewed)abstract
- Deformation, in large parts of the middle crust, results in strained rocks consisting of grains with variable dislocation densities and microstructures which are characterized by gradual distortion and subgrain structures. Post-deformation residence of these rocks at elevated temperatures results in microstructural adjustments through static recovery and recrystallization. Here, we employ a numerical technique to simulate intragrain recovery at temperatures at or below the deformation temperature. The simulation is based on minimization of the stored energy, related to misorientation through local rotation of physical material points relative to their immediate environment. Three temperature-and/or deformation-geometry-dependent parameters were systematically varied: (1) deformation-induced dislocation types, (2) dislocation mobility and (3) size of dislocation interaction volume. Comparison with previously published in situ experiments shows consistency of numerical and experimental results. They show temperature- and dislocation-type-dependent small-scale fluctuations in subgrain-boundary misorientations and orientation variation within subgrains. These can be explained by the combined effect of increase in dislocation interaction volume and activation of climb. Our work shows microstructure can be significantly modified even if the post-deformational temperature is at or below the deformation temperature: a scenario relevant for most deformed rocks.
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| 6. |
- Rimsa, Andrius, et al.
(author)
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Brittle fracturing and fracture healing of zircon: An integrated cathodoluminescence, EBSD, U-Th-Pb, and REE study
- 2007
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In: American Mineralogist. - : Mineralogical Society of America. - 0003-004X. ; 92:7, s. 1213-1224
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Journal article (peer-reviewed)abstract
- The entire population of magmatic oscillatory zoned zircons in a migmatitic granite from the Tjarnesjo intrusion, southwest Sweden, reveal fine-scale brittle fracturing. The oscillatory zoned fragments are rotated but not dispersed. Fractures between individual fragments are sealed by newly formed CL-bright zircon. Hydraulic fracturing is the most probable mechanism. The internal structure of fractured zircons and the LREE-enriched, low Th character of CL-bright zircon both suggest that cracks between oscillatory zoned zircon fragments were rapidly sealed after fracturing by CL-bright zircon, precipitated from hydrothermal fluids. Zircon fracturing and crack-sealing has been dated by SIMS ion-probe and U-Th-Pb isotopes to 920 +/- 51 Ma (lower intercept age, 2 sigma, MSWD = 1.09) with a limit for the Youngest possible age of 960 +/- 16 Ma (Pb-207/(206) Pb, 2 sigma, MSWD = 0.23) dated by sector-zoned rims forming overgrowths on the fractured cores.
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