Serpentine crack-seal veins: a unique record of fluid conditions during faulting

2021 ◽  
Author(s):  
Matthew S. Tarling ◽  
Steven A.F. Smith ◽  
Jeremy S. Rooney ◽  
Cecilia Viti ◽  
Keith C. Gordon
Keyword(s):  
Crack Opening ◽  
Shear Zones ◽  
Vein Formation ◽  
Crack Sealing ◽  
Transmission Electron ◽  
Individual Bands ◽  
Fault Kinematics ◽  
Repetitive Stress ◽  
Episodic Tremor ◽  
Stress Drops

<p>Serpentine veins are ubiquitous in hydrated and deformed ultramafic rocks, and have previously been used to track fault kinematics and understand the evolution of environmental conditions during vein formation. However, difficulties in unambiguously identifying and mapping serpentine types at sub-micron to mm scales has limited our understanding of vein precipitation kinetics and growth histories. Using recently developed techniques of Raman spectroscopy mapping, combined with scanning- and transmission-electron microscopy, we describe a new type of mineralogically banded serpentine crack-seal vein in six samples from different settings around the world. In all of the studied samples, individual bands comprise a thin layer (~0.4–2 µm) dominated by chrysotile and a much thicker layer (~0.5–30 µm) dominated by polygonal serpentine/lizardite. Existing field and experimental data suggest that disequilibrium conditions immediately following crack opening may favour rapid precipitation of chrysotile along one of the crack margins. Subsequently, diffusional transport of elements favours slower precipitation of polygonal serpentine/lizardite which leads to crack sealing. The similarities in layer thicknesses and mineralogy exhibited by samples collected from extension and shear veins, dilational jogs, foliation surfaces, and the margins of phacoids, suggest that a common set of processes involving crack opening and sealing are active in a range of different structural sites within serpentinite-dominated shear zones, potentially associated with frequent and repetitive stress drops such as those recorded during episodic tremor and slow slip.</p>

scholarly journals Direct Characterization of the Relation between the Mechanical Response and Microstructure Evolution in Aluminum by Transmission Electron Microscopy In Situ Straining

Materials ◽  
2021 ◽  
Vol 14 (6) ◽  
pp. 1431
Author(s):  
Seiichiro Ii ◽  
Takero Enami ◽  
Takahito Ohmura ◽  
Sadahiro Tsurekawa
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Ion Beam ◽  
Elastic Behavior ◽  
Annealed Specimen ◽  
Displacement Curve ◽  
Dislocation Loops ◽  
Transmission Electron ◽  
Stress Drops

Transmission electron microscopy in situ straining experiments of Al single crystals with different initial lattice defect densities have been performed. The as-focused ion beam (FIB)-processed pillar sample contained a high density of prismatic dislocation loops with the <111> Burgers vector, while the post-annealed specimen had an almost defect-free microstructure. In both specimens, plastic deformation occurred with repetitive stress drops (∆σ). The stress drops were accompanied by certain dislocation motions, suggesting the dislocation avalanche phenomenon. ∆σ for the as-FIB Al pillar sample was smaller than that for the post-annealed Al sample. This can be considered to be because of the interaction of gliding dislocations with immobile prismatic dislocation loops introduced by the FIB. The reloading process after stress reduction was dominated by elastic behavior because the slope of the load–displacement curve for reloading was close to the Young’s modulus of Al. Microplasticity was observed during the load-recovery process, suggesting that microyielding and a dislocation avalanche repeatedly occurred, leading to intermittent plasticity as an elementary step of macroplastic deformation.

scholarly journals From static alteration to mylonitization: a nano- to micrometric study of chloritization in granitoids with implications for equilibrium and fluid percolation length scales

2021 ◽  
Author(s):  
Laura Airaghi ◽  
Benoit Dubacq ◽  
Anne Verlaguet ◽  
Franck Bourdelle ◽  
Nicolas Bellahsen ◽  
...  
Keyword(s):  
Reaction Mechanisms ◽  
High Strain ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Low Grade ◽  
Zone Formation ◽  
Crustal Rocks ◽  
Transmission Electron ◽  
Variable Element ◽  
The Matrix

&lt;p&gt;Strain accommodation in upper crustal rocks is often accompanied by fluid-mediated crystallization of phyllosilicates, which influence rock strength and shear zone formation. The composition of these phyllosilicates is commonly used for pressure-temperature-time constraints of deformation events, although it is often highly heterogeneous. This study investigates the reactions producing a phyllosilicate, chlorite, in and below greenschist-facies conditions and the variations in chlorite composition, along a strain gradient in the Bielsa granitoid (Axial Zone, Pyrenees). Compositional maps of chlorite (including iron speciation) are compared to nanostructures observed by transmission electron microscopy in increasingly-strained samples and related to mechanisms of fluid percolation and scales of compositional homogenisation. In the Bielsa granitoid, altered at the late Variscan, Alpine-age shear zones are found with high strain gradients. The undeformed granitoid exhibits local equilibria, pseudomorphic replacement and high compositional heterogeneities in chlorite. This is attributed to: (i) variable element supply and reaction mechanisms observed at nanoscale and (ii) little interconnected intra- and inter-grain nanoporosity causing isolation of fluid evolving in local reservoirs. In samples with discrete and mm-sized fractures, channelized fluid triggered the precipitation of homogeneous Alpine chlorite in fractures, preserving late-Variscan chlorite within the matrix. In low-grade mylonites, where brittle-ductile deformation is observed, micro-, nano-cracks and defects allows the fluid percolating into the matrix at the scale of hundreds of &amp;#181;m. This results in a more pervasive replacement of late-Variscan chlorite by Alpine chlorite. Local equilibria and high compositional heterogeneities in phyllosilicates as chlorite are therefore preserved according (i) matrix-fracture porosity contrasts at nanoscale and (ii) the location and interconnection of nanoporosity between crystallites of phyllosilicates that control reaction mechanisms and element mobility. In low grade mylonites, mineral and compositional replacement remains incomplete despite the high strain.&lt;/p&gt;

scholarly journals Magnetic properties of pseudotachylytes, Jämtland, central Sweden

2019 ◽  
Author(s):  
Hagen Bender ◽  
Bjarne S. G. Almqvist ◽  
Amanda Bergman ◽  
Uwe Ring
Keyword(s):  
Magnetic Susceptibility ◽  
Shear Zones ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Fault Zones ◽  
Small Sample ◽  
Magnetic Fabric ◽  
Normal Faults ◽  
Magnetic Fabrics ◽  
Nappe Complex ◽  
Fault Kinematics

Abstract. Nappe assembly in the Köli Nappe Complex, Jämtland, Sweden, has been associated with in- and out-of-sequence thrusting. Kinematic data from shear zones bounding the Köli Nappe Complex are compatible with this model, but direct evidence from fault zones internally subdividing the nappe complex does not exist. We studied a series of pseudotachylyte exposures in these fault zones for deciphering the role seismic faulting played in the assembly of the Caledonian nappe pile. To constrain the fault kinematics, microstructural and magnetic fabrics of pseudotachylyte in foliation-parallel fault veins have been investigated. Because the pseudotachylyte veins are thin, we focused on small (c. 0.2 cm3) samples for measuring the anisotropy of magnetic susceptibility. The results show inverse proportionality between specimen size and anisotropy of magnetic susceptibility degree, which is most likely an analytical artifact related to instrument sensitivity and small sample dimensions. This finding implies magnetic anisotropy results acquired from small specimens demand cautious interpretation. However, analysis of structural and magnetic fabric data indicates that seismic faulting occurred during exhumation into the upper crust but yield no kinematic in-formation. Structural field data suggest that seismic faulting was postdated by brittle E–W extensional deformation along steep normal faults. Therefore, it is likely that the pseudotachylytes formed late during out-of-sequence thrusting of the Köli Nappe Complex over the Seve Nappe Complex.

scholarly journals Dynamic Recrystallization in a Naturally Deformed Albite

1983 ◽  
Vol 5 (4) ◽  
pp. 219-237 ◽  
Author(s):  
J. D. Fitz Gerald ◽  
M. A. Etheridge ◽  
R. H. Vernon
Keyword(s):  
Dynamic Recrystallization ◽  
Grain Boundaries ◽  
Boundary Migration ◽  
Experimental Studies ◽  
Shear Zones ◽  
Optical Microscope ◽  
Coarse Grained ◽  
Transmission Electron ◽  
Subgrain Growth ◽  
Diffraction Patterns

Coarse-grained, deformed albite occurs in veins within a blueschist from the Cazadero region, California. In some grains, deformation and recrystallization are concentrated in narrow shear zones less than 50 μm wide. We have examined the substructural progression across these zones by transmission electron microscopy (TEM), in an attempt to determine the details of the dynamic recrystallization mechanism. The misorientation across subgrain and recrystallized grain boundaries has been determined by analysis of electron diffraction patterns.Dynamic recrystallization apparently proceeded by the following stages: 1) the formation of a well-ordered substructure from a more tangled, cell-like array, 2) increasing misorientation between subgrains, 3) rapid growth of subgrains at a misorientation between 3° and 5° to produce new “grains” with straighter grain boundaries and lower internal dislocation densities and 4) continued deformation and rotation of the recrystallized grains with local grain-boundary migration to maintain relatively equiaxed shapes. The ultimate recrystallized structure in the narrow deformation zones consists of grains misoriented by between 5° and at least 30°, most of them containing a well-developed substructure.The combination of subgrain growth and rotation explains a number of features common to dynamically recrystallized minerals. The smaller subgrains present prior to growth and also within recrystallized grains form a population distinct from the larger subgrains and recrystallized grains of approximately equal size, which are those observed in an optical microscope. The smaller subgrains are visible only in TEM. Individual recrystallized grains may remain through substantial straining, rotating in response to dislocation and sub-boundary motion within them, thus preserving and even enhancing the crystallographic fabric (texture). The retention of an initial recrystallized grain population throughout significant continuing deformation may explain the absence of strain softening in some recent experimental studies.

scholarly journals Orogenic Gold in Transpression and Transtension Zones: Field and Remote Sensing Studies of the Barramiya–Mueilha Sector, Egypt

2019 ◽  
Vol 11 (18) ◽  
pp. 2122 ◽  
Author(s):  
Basem Zoheir ◽  
Mohamed Abd El-Wahed ◽  
Amin Beiranvand Pour ◽  
Amr Abdelnasser
Keyword(s):  
Remote Sensing ◽  
Hydrothermal Alteration ◽  
Gold Mineralization ◽  
Shear Zones ◽  
Island Arc ◽  
Quartz Veins ◽  
Vein Formation ◽  
Time Space ◽  
Band Ratioing ◽  
Sentinel 2

Multi-sensor satellite imagery data promote fast, cost-efficient regional geological mapping that constantly forms a criterion for successful gold exploration programs in harsh and inaccessible regions. The Barramiya–Mueilha sector in the Central Eastern Desert of Egypt contains several occurrences of shear/fault-associated gold-bearing quartz veins with consistently simple mineralogy and narrow hydrothermal alteration haloes. Gold-quartz veins and zones of carbonate alteration and listvenitization are widespread along the ENE–WSW Barramiya–Um Salatit and Dungash–Mueilha shear belts. These belts are characterized by heterogeneous shear fabrics and asymmetrical or overturned folds. Sentinel-1, Phased Array type L-band Synthetic Aperture Radar (PALSAR), Advanced Space borne Thermal Emission and Reflection Radiometer (ASTER), and Sentinel-2 are used herein to explicate the regional structural control of gold mineralization in the Barramiya–Mueilha sector. Feature-oriented Principal Components Selection (FPCS) applied to polarized backscatter ratio images of Sentinel-1 and PALSAR datasets show appreciable capability in tracing along the strike of regional structures and identification of potential dilation loci. The principal component analysis (PCA), band combination and band ratioing techniques are applied to the multispectral ASTER and Sentinel-2 datasets for lithological and hydrothermal alteration mapping. Ophiolites, island arc rocks, and Fe-oxides/hydroxides (ferrugination) and carbonate alteration zones are discriminated by using the PCA technique. Results of the band ratioing technique showed gossan, carbonate, and hydroxyl mineral assemblages in ductile shear zones, whereas irregular ferrugination zones are locally identified in the brittle shear zones. Gold occurrences are confined to major zones of fold superimposition and transpression along flexural planes in the foliated ophiolite-island arc belts. In the granitoid-gabbroid terranes, gold-quartz veins are rather controlled by fault and brittle shear zones. The uneven distribution of gold occurrences coupled with the variable recrystallization of the auriferous quartz veins suggests multistage gold mineralization in the area. Analysis of the host structures assessed by the remote sensing results denotes vein formation spanning the time–space from early transpression to late orogen collapse during the protracted tectonic evolution of the belt.

Hydration- and dehydration-induced rheological heterogeneities on the deep subduction interface, and possible relationships to episodic tremor and slow slip

2020 ◽  
Author(s):  
Whitney Behr ◽  
Carolyn Tewksbury-Christle ◽  
Alissa Kotowski ◽  
Claudio Cannizzaro ◽  
Robert Blass ◽  
...  
Keyword(s):  
Subduction Zones ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Coarse Grained ◽  
Slow Slip ◽  
Brittle Deformation ◽  
Interface Shear ◽  
Dehydration Reactions ◽  
Viscous Shear ◽  
Episodic Tremor

&lt;p&gt;Episodic tremor and slow slip (ETS) is observed in several subduction zones down-dip of the locked megathrust, and may provide clues for preparatory processes before megathrust rupture. Exhumed rocks provide a unique opportunity to evaluate the sources of rheological heterogeneity on the subduction interface and their potential role in generating ETS-like behavior. We present data from two subduction interface shear zones representative of the down-dip extent of the megathrust: the Condrey Mountain Schist (CMS) in northern CA (greenschist to blueschist facies conditions) and the Cycladic Blueschist Unit (CBU) on Syros Island, Greece (blueschist to eclogite facies). Both complexes highlight the propensity for fluid-mediated metamorphic reactions to produce strong rheological heterogeneities:&lt;/p&gt;&lt;p&gt;In the CMW, hydration reactions led to progressive serpentinization of peridotite bodies that were entrained from the overriding plate and underplated along with oceanic-affinity sediments. The margins of each peridotite-serpentinite lens show extreme strain localization accommodated by dislocation glide and minor pressure solution in antigorite, whereas lens interiors show evidence for more distributed, alternating, frictional-viscous deformation, with abundant crack-seal veins occupied by antigorite, brucite and oxides that are in some places also ductilely sheared. Deformation in the surrounding metasedimentary matrix was purely viscous.&lt;/p&gt;&lt;p&gt;In the CBU on Syros Island, dehydration reactions in MORB-affinity basalts, subducted and underplated with oceanic and continental-affinity sediments, led to progressive development of strong eclogitic lenses within a weaker blueschist and metasedimentary matrix. The eclogite lenses are commonly coarse-grained and massive and show brittle deformation in the form of dilational and shear fractures/veins filled with quartz, white mica, glaucophane and/or chlorite. Brittle deformation in the eclogites is coeval with ductile deformation in the surrounding blueschist and metasedimentary matrix, indicating concurrent frictional-viscous flow.&lt;/p&gt;&lt;p&gt;Although we cannot easily distinguish transient deformation processes in exhumed rocks, we can use the following three approaches to assess whether these heterogeneities could have generated deformation behaviors similar to deep ETS: 1) We measure displacements within, and dimensions of the heterogeneities in outcrop/map-scale to estimate the maximum possible seismic moment that would be released when the frictional heterogeneities slip;&amp;#160; 2) We compare deformation mechanisms inferred from field and microstructural observations to their expected mechanical behavior from rock deformation experiments; and 3) We use seismo-thermo-mechanical modeling to examine expected slip velocities and moment-duration ratios for frictional-viscous shear zones that are scaled to observations from nature and the lab.&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;All three approaches suggest that frictional-viscous heterogeneities of the types and length-scales we observe in the exhumed rock record are compatible with ETS as documented in modern subduction zones.&lt;/p&gt;

Mineralogy and petrology of the Highland Border Suite serpentinites

1983 ◽  
Vol 47 (344) ◽  
pp. 301-310 ◽  
Author(s):  
N. P. Ikin ◽  
R. S. Harmon
Keyword(s):  
Low Temperatures ◽  
Minor Element ◽  
Shear Zones ◽  
Greenschist Facies ◽  
Ultramafic Rocks ◽  
Brittle Deformation ◽  
Basaltic Rocks ◽  
Vein Formation ◽  
Ductile Shearing ◽  
High Level

AbstractPrior to serpentinization, the ultramafic rocks of the ophiolitic Highland Border Suite were depleted harzburgites, dunites, and orthopyroxenites. They may have been the residual mantle after melting of primary lherzolite that produced the basaltic rocks of the Suite. Serpentinization of these peridotites occurred at low temperatures (75±50°C) and produced pseudomorphic-textured lizardite serpentinites in which the minor-element compositions of the primary phases were preserved in the lizardite grains. Olivine was almost completely serpentinized before orthopyroxene was hydrated. Concomitant peripheral alteration of chromespinels, during serpentinization of orthopyroxene, caused release of Cr and Al cations which were incorporated into the lizardite. Under greenschist-facies conditions (c. 350°C at 3–4 kbar), the lizardite serpentinites were subsequently metamorphosed and recrystallized to nonpseudomorphic-textured antigorite serpentinites. Postmetamorphic processes included high-level brittle deformation that resulted in fracturing and cross-fibre chrysotile vein-formation, and ductile shearing (T ∼ 200 °C, P < 2 kbar) which produced talc-chrysotile shear zones.

The Watershed Tungsten Deposit, Northeast Queensland, Australia: Permian Metamorphic Tungsten Mineralization Overprinting Carboniferous Magmatic Tungsten

2020 ◽  
Author(s):  
Jaime A. Poblete ◽  
Paul H.G.M. Dirks ◽  
Zhaoshan Chang ◽  
Jan Marten Huizenga ◽  
Martin Griessmann ◽  
...  
Keyword(s):  
Depositional Environment ◽  
Weighted Average ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Tungsten Deposit ◽  
Metasedimentary Rocks ◽  
Vein Formation ◽  
Ductile Shear Zones ◽  
Stage 4 ◽  
Tungsten Mineralization

Abstract The Watershed tungsten deposit (49.2 Mt avg 0.14% WO3) lies within the Mossman orogen, which comprises deformed Silurian-Ordovician metasedimentary rocks of the Hodgkinson Formation intruded by Carboniferous-Permian granites of the Kennedy Igneous Association. The Hodgkinson Formation in the Watershed area comprises skarn-altered conglomerate, psammite, and slate units that record four deformation events evolving from ductile, isoclinal, colinear folding with transposition (D1–D3) to brittle ductile shear zones (D4). Multiple felsic to intermediate dikes cut across the metasedimentary rocks at Watershed including: (1) Carboniferous, monzonite dikes (zircon U/Pb age of 350 ± 7 Ma) emplaced during D1–2; and (2) Permian granite plutons and dikes (zircon U/Pb ages of 291 ± 6, 277 ± 6, and 274 ± 6 Ma) and diorite (zircon U/Pb age of 281 ± 5 Ma) emplaced during D4. Tungsten mineralization is largely restricted to skarn-altered conglomerate, which preserves a peak metamorphic mineralogy formed during ductile deformation and comprises garnet (Grt40–87 Alm0–35Sps1–25Adr0–16), actinolite, quartz, clinopyroxene (Di36–59Hd39–61Jhn1–5), and titanite. A first mineralization event corresponds to the crystallization of disseminated scheelite in monzonite dikes (pre-D3) and adjacent units, with scheelite grains aligned in the S1–2 fabric and affected by D3 folding. This event enriched the Hodgkinson Formation in tungsten. The bulk of the scheelite mineralization formed during a second event and is concentrated in multistaged, shear-related, quartz-oligoclase-bearing veins and vein halos (muscovite 40Ar-39Ar weighted average age of 276 ± 6 Ma), which were emplaced during D4. The multistage veins developed preferentially in competent, skarn-altered conglomerate units and formed synchronous with four retrograde alteration stages. The retrograde skarn minerals include clinozoisite after garnet, quartz, plagioclase, scheelite, and phlogopite with minor sodium-rich amphibole, which formed during retrograde stages 1 and 2, accompanied by later muscovite, calcite, and chlorite formed during retrograde stage 3. Retrograde stage 4 was a late-tectonic, noneconomic sulfide stage. The principal controls on scheelite mineralization at Watershed were the following: (1) early monzonite dikes enriched in scheelite; (2) D4 shear zones that acted as fluid conduits transporting tungsten from source areas to traps; (3) skarn-altered conglomerate lenses that provide a competent host to facilitate vein formation and a source for calcium to form scheelite; and (4) an extensional depositional environment characterized by vein formation and normal faulting, which provide trapping structures for tungsten-bearing fluids, with decompression being a likely control on scheelite deposition. The coexistence of scheelite with oligoclase in monzonite dikes and veins suggests that tungsten was transported as NaHWO40. Exploration in the area should target Carboniferous monzonite, associated with later syn-D4 shear zones cutting skarn-altered conglomerate.

Influence of Cell Wall Composition on the Fossilisation of Bacteria and the Implications for the Search for Early Life Forms

1997 ◽  
Vol 161 ◽  
pp. 491-504 ◽  
Author(s):  
Frances Westall
Keyword(s):  
Cell Wall ◽  
Life Forms ◽  
Cation Binding ◽  
Positive Bacterium ◽  
Gram Positive ◽  
Gram Positive Bacteria ◽  
Transmission Electron ◽  
Hydrothermal Sediments ◽  
Identification Of Bacteria ◽  
The Difference

AbstractThe oldest cell-like structures on Earth are preserved in silicified lagoonal, shallow sea or hydrothermal sediments, such as some Archean formations in Western Australia and South Africa. Previous studies concentrated on the search for organic fossils in Archean rocks. Observations of silicified bacteria (as silica minerals) are scarce for both the Precambrian and the Phanerozoic, but reports of mineral bacteria finds, in general, are increasing. The problems associated with the identification of authentic fossil bacteria and, if possible, closer identification of bacteria type can, in part, be overcome by experimental fossilisation studies. These have shown that not all bacteria fossilise in the same way and, indeed, some seem to be very resistent to fossilisation. This paper deals with a transmission electron microscope investigation of the silicification of four species of bacteria commonly found in the environment. The Gram positiveBacillus laterosporusand its spore produced a robust, durable crust upon silicification, whereas the Gram negativePseudomonas fluorescens, Ps. vesicularis, andPs. acidovoranspresented delicately preserved walls. The greater amount of peptidoglycan, containing abundant metal cation binding sites, in the cell wall of the Gram positive bacterium, probably accounts for the difference in the mode of fossilisation. The Gram positive bacteria are, therefore, probably most likely to be preserved in the terrestrial and extraterrestrial rock record.

A Reproducible Selective Stain for Cardiac Sarcoplasmic Reticulum

1974 ◽  
Vol 32 ◽  
pp. 214-215
Author(s):  
R. A. Waugh ◽  
J. R. Sommer
Keyword(s):  
Complex System ◽  
Electron Microscope ◽  
Sarcoplasmic Reticulum ◽  
Transmission Electron Microscope ◽  
Electron Microscopic ◽  
Cardiac Sarcoplasmic Reticulum ◽  
Transmission Electron

Cardiac sarcoplasmic reticulum (SR) is a complex system of intracellular tubules that, due to their small size and juxtaposition to such electron-dense structures as mitochondria and myofibrils, are often inconspicuous in conventionally prepared electron microscopic material. This study reports a method with which the SR is selectively “stained” which facilitates visualizationwith the transmission electron microscope.

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