Microfabric Analysis Research Articles

Palaeoenvironmental significance of lacustrine stromatolites of the Arnstadt Formation (“Steinmergelkeuper”, Upper Triassic, N-Germany)

Lacustrine stromatolites of the Norian Arnstadt Formation (“Steinmergelkeuper”) occur on top of asymmetric flooding–evaporation cycles of a closed lake basin. They have been investigated with regard to associated lithofacies and biota, microfabric and stable carbon and oxygen isotopes. The stromatolites of the “Middle Grey Series” are brecciated and reworked by a flooding event of a subsequent lake cycle. They comprise agglutinated stromatolites rich in fish scales as well as skeletal stromatolites composed of a rhythmically grown dendroid micropeloidal framework. The latter are characterized by a shift towards positive δ13C values relative to the associated lake carbonates. This points to an effective photosynthesis in biofilm calcification at low concentrations in dissolved inorganic carbon (DIC) in a perennial fresh to brackish water lake. The stromatolites of the “Upper Red Series” occur on top of perennial lake cycles intercalated between playa lake deposits. The fine-grained stromatolites are poor in microfabric characteristics but show a significant covariation of δ18O and δ13C. This points to evaporation/degassing acting as driving mechanism in biofilm calcification. The lack of biotic effects on carbon isotope fractionation may reflect high concentrations in dissolved inorganic carbon. Skeletal oncoids, which occur as allochthonous components within an intraformational lag deposit of the “Upper Red Series”, are composed of cyanobacterial tubes and probably represent lowest saline lakes with only poor DIC buffering. Stable isotope signatures in conjunction with stromatolite microfabric analyses may be used as a proxy of DIC concentrations in ancient closed lakes.

Microfabric analysis by manual and automated stereological procedures: a methodological approach to Antarctic tillite

AbstractFabric analysis is commonly used to infer former movement directions of diamictons; however, analysis techniques are typically time consuming and partly subjective. Stereological analysis provides an alternative for objective determination of the preferred orientation of particles. The ability of manual and automated stereological procedures to determine the preferred orientation of particles (120–4000 µm) in tillite is assessed, using core samples from Mount Feather, McMurdo Dry Valleys, Antarctica. Orthogonal sets of vertical thin sections were subjected to directed secant analysis involving the determination of the number of intersections between particle outlines and a rotating series of parallel lines. The manual analysis reveals a weakly developed three‐dimensional tilt angle (plunge) of 68°. This reflects the depth‐averaged subglacial deformational structure of the deposit. Because the preferred orientation signal is weak, in the automated set‐up this signal is obscured by the effect of digitization, hampering the reliable assessment of the orientation direction. The study shows microfabric variability within the studied cored section that is interpreted as a shear fabric. Reliable macrostructural information that might constrain ice flow directions requires a larger number of randomly or systematically drawn core samples from which averaged microstructural fabrics can be derived.

The effect of grain texture on the occurrence of microstructural properties in subglacial till

In this paper, we examine whether till grain size affects the range and occurrence of micromorphological features associated with subglacial shear. Our till samples were collected from two glaciers in Iceland, and varied in texture from a coarse, sandy clast-rich till (Fjallsjökull) to a fine-grained silty-sandy till (Vestari-Hagafellsjökull). We found a wide range of deformational microstructures that included skelsepic plasmic fabric, intraclasts of pre-existing eroded bedrock (basalt) and weathered clay and ‘mini-shear zones’ between clasts. We classified our micromorphological data into three classes; rotational, intermediate and linear. In addition to these observations, we performed extensive microfabric analysis at different scales on all of our samples. We found that the coarse-grained till contained a greater number and variety of microstructures than the fine-grained till. In addition, the fine-grained till showed a distinct lack of rotational structures that we attribute to the lack of significantly sized clasts in the matrix. We argue that the varied texture of the coarse-grained till provides a greater degree of perturbation within the shearing layer and so more distinct microstructures form. In a more fine-grained till, shearing is more homogeneous since there are less perturbations in the matrix and this leads to a more singular kind of microstructure. Our observations suggest that subglacial shear occurs within a multi-layered patchwork of different grain sizes, competence and pore water pressures. It is these factors that are so crucial in determining the occurrence and type of microstructural evidence we see in subglacial tills.

Strain investigations on calcite marbles using neutron time-of-flight diffraction

In order to describe and explain the effect of bowing of marble facade panels, neutron time-of-flight diffraction was applied to determine residual macro- and microstrain on the calcite mineral phase. The measurements were combined with investigations of the crystallographic preferred orientation (texture) measurements by neutron diffraction, macroscopic measuring of the bowing on marble building stones, as well as microfabric analyses. Three samples were investigated to explain the bowing effect: a fresh broken sample, a good conditioned facade panel and a strongly deformed facade panel. Residual intracrystalline strain was detected in all investigated samples, which differed in the degree of bowing. For the first time, the preferred orientation and the residual strain were found to be related. The results show that different strain magnitudes are reflected by residual strains, which differ significantly in magnitude and direction. Furthermore, different Bragg peak widths have been detected as an indication of microscopic strain. The observed residual strain values in the samples are related with the grain shape and texture properties.

Microfabric of fractured Boom Clay at depth: a case study of brittle–ductile transitional clay behaviour

Recent Underground Research Facility (URF) gallery excavation works in the Boom Clay at 224 m below the surface revealed intense fracturing of this weak claystone. Large fault planes (meter-scale) and joints were observed during tunnelling and found to belong to the excavation-disturbed zone (EDZ). Samples taken from the new gallery have been investigated using scanning electron microscope (SEM) imagery and microstructural analysis. This revealed a brittle–ductile transitional behaviour of the mudstone sediment regarding deformation, implying dilatant shear and hybrid fractures in high strain zones and ductile, compacted plastic shear bands in the lower strain zones, following an elasto-plastic constitutive model. Microfabric analysis contributed in this way to a better understanding of clay behaviour and effective fracturing. This information is primordial for understanding the rheology of the clay and assessing the influence of fracturing on permeability, critical issues in radioactive waste management.

Steepland resources: characteristics, stability and micromorphology

Steeplands of Central America are a major land resource, but due to population pressures they are rapidly being deforested leading to landscape instability. Microwatersheds of Southern Honduras were examined to correlate soil type, landform, slope gradient and slump potential. Soils most susceptible to slumping were moderately deep (0.5–1 m) Haplustalfs and Haplustepts. These soils occupy about 25% of the landscape, and have slope gradients of 45–90%. Soils are developed on plagioclase-rich andesitic parent material, are loam to clay loam in texture and have high base status. However, due to weathering of parent material, sand and silt fractions are dominated by quartz with smaller amounts of plagioclase, vermiculite and kaolinite. The A and B horizons have an open porphyric related distribution with plagioclase as the coarse fraction in a fine-grained groundmass. The andesitic parent rock contains abundant hydrothermally altered plagioclase phenocrysts in a single-spaced porphyric-related (c/f 5μm ratio of 4:1) distribution. Some feldspars are partially to completely altered to clay pseudomorphs, probably vermiculite. Microfabric analysis of soil and pararock did not show any striated b-fabric indicative of shear failure. However, microfabrics do confirm translocated clay into the subsoil as typic pore coatings indicating long-term landscape stability prior to deforestation.

Microfabric analysis of laminated diatom ooze (Holocene) from the eastern Bransfield Strait, Antarctic Peninsula

Selected intervals of laminated diatom ooze (Holocene) from the eastern Bransfield Strait, Antarctic Peninsula were analyzed using high-resolution section images of impregnated samples to reveal potential annual-to-seasonal scale signatures of the lamination. The results show a probable seasonality within individual diatom ooze laminae which mainly reflects spring blooms ofChaetoceros followed by summer production ofCorethron orRhizosolenia. Alternating monospecific diatom ooze and terrigenous laminae are suggestive of annual cyclicities, representing alternation of diatom mass flux during the spring or autumn with terrigenous flux during the rest of year.

Shark Bay stromatolites: Microfabrics and reinterpretation of origins

Detailed analysis of microfabrics in Hamelin Pool stromatolites leads to reinterpretation of the origins of these structures. Previous studies have concluded that Shark Bay stromatolites form primarily as a result of sediment trapping and binding by microorganisms. Our results suggest that microbial precipitation of microcrystalline carbonate (micrite), as both framework and cement in these stromatolites, is also a fundamental, heretofore unrecognized, process in their formation. Microbial trapping and binding is the primary mechanism of stromatolite accretion in the intertidal zone, forming grainy, calcarenite structures. Microbial precipitation is the primary accretionary mechanism in the subtidal zone, forming muddy, micritic stromatolites. Microbial precipitation also lithifies trapped and bound sediment in the calcarenite stromatolites. Recognition of microbially precipitated micrite in Shark Bay stromatolites is important, as many ancient stromatolites are micritic.

Microfabric analyses applied to the Witwatersrand gold- and uranium-bearing conglomerates: constraints on the provenance and post-depositional modification of rock and ore components

Microfabrics of detrital quartz grains and quartz cement of four gold- and uranium-bearing conglomerates of various goldfields of the Witwatersrand Basin, South Africa, were investigated by optical and cathodoluminescence microscopy. The study revealed that the vast majority of quartz grains (<5 mm) originate from felsic magmatic source rocks. Cherts and polymineralic mineral grains, pointing to greenschist to amphibolite facies metasedimentary rocks, follow in abundance, whereas diagenetic to weakly metamorphosed quartzites are subordinate. Material from hydrothermal vein quartz is almost completely lacking, both in the sand and in the pebble fraction. No obvious relationships are discernible between the varying proportions of detrital siliciclastic components of the different reefs and their ore grades. Assuming a sedimentological control of gold distribution, this finding supports multiple sources for the detrital components, which were thoroughly mixed during transport. The post-depositional history of the sediments is characterized by a complex polyphase succession of deformation, cementation and hydrothermal alteration events. Both fragmentation and pressure solution features within detrital quartz, quartz cement and round grains of pyrite, zircon, chromite and uraninite demonstrate that these materials were present in the conglomerates during diagenesis, and, thus, are true detrital grains with abraded, rounded grain morphologies. By analogy, it is assumed that gold is also a detrital component, although most of the gold grains display characteristics of hydrothermal overprinting. During subsequent metamorphism, micro-shear zones are developed, and brittle–ductile crystal–plastic deformation and limited quartz recrystallization occur. Maximum temperatures of about 350 °C were reached on the prograde metamorphic path. Recrystallization and redistribution of detrital siliciclastic and ore minerals took place, and various hydrothermal/metamorphic minerals including chlorite, sericite, pyrophyllite and chloritoid were formed. These redistribution processes involved existing detrital minerals only and were generally isochemical because little evidence exists for the development of a secondary porosity and permeability that would allow major external inputs into the Witwatersrand conglomerates. Most of the gold grains have hydrothermal characteristics, as evidenced by their authigenic, crystalline shapes and their chemical compositions. However, these features are regarded to result from overprinting. Most likely, the gold grains experienced more drastic modifications relative to other ore components because of the ductile and mobile nature of gold. The retrograde metamorphic path is characterized by percolating radioactive fluids at T<300 °C, recorded by radiation damage indicated by cathodoluminescence alteration rims along quartz grain boundaries and microcracks. The degrees of radiation damage observed are proportional to the uranium contents of the conglomerate ores. The collective evidence of our study supports the modified placer model for the genesis of the Witwatersrand ores. This model explains most of the observations on the detrital mineral assemblage and its post-depositional modification elegantly and in a satisfactory manner.

The tectonometamorphic evolution of high-pressure low-temperature metamorphic rocks of eastern Crete, Greece: constraints from microfabrics, strain, illite crystallinity and paleodifferential stress

Analysis of strain, paleodifferential stress, illite crystallinity, and microfabrics of quartz and calcite suggest that multiply alternating late Oligocene/early Miocene N–S convergence and N–S extension caused significant vertical discontinuities in the grade of high-pressure/low-temperature metamorphism of the Phyllite–Quartzite unit (PQU) of eastern Crete. Constrictional fabrics and parallelism of D2 stretching lineations and fold axes are attributed to enhanced slab pull forces at greater depth of subduction. A major D3 extensional detachment is present in the upper part of the PQU where slates with lowest metamorphic grade (diagenesis/anchizone transition) are resting on top of anchimetamorphic rocks. The highest metamorphic grade (anchi-/epizone transition) has been determined for Skythian rocks on top of pre-Alpine basement. Revived N–S convergence (D4) led to E–W-trending map-scale folds and top-to-the-S directed brittle thrusts. There is a clear correlation between D2-related metamorphic grade, finite strain, differential stress, and deformation mechanisms. At the anchi-/epizone transition, solution–precipitation and dislocation creep of quartz and calcite caused maximum finite strains (RXZ=up to 7) at high strain rates and moderate differential stresses (80–200MPa). At the diagenesis/anchizone transition, deformation was accommodated by cataclasis, dislocation glide and solution–precipitation creep resulting in relatively low finite strains (RXZ<2) at high differential stresses of up to 300MPa.

Quantitative orientation and micro-porosity analysis of recent marine sediment microfabric

Impregnated samples taken from three different post depositional regimes within the Mariana Mounds hydrothermal area in the Mariana Trough were observed in the back-scattered mode in the SEM. Intensity gradient orientation analysis methods have been adapted to study the orientation patterns within the sediments representing one of the first applications of such quantitative microfabric methods to the study of natural materials. Orientation was specified by two separate parameters, an index of anisotropy and a direction of orientation. The index is generally low for any single image, but by plotting the direction and anisotropy as a vector centred on the relevant specimen co-ordinates, consistent patterns of orientation were observed in the samples. Differences between the orientation patterns from the three environmental regimes were noted. A new dispersion ratio parameter was used to assess within sample variations. In a complementary study, objective thresholding methods based on Kohler's relative histogram method were used to segment the images. Assessments of porosity showed differences which were generally consistent with the trends noted from orientation analysis. While this paper uses image analysis techniques to examine deep sea sediments, the methods of analysis described are equally applicable to sediments on the continental shelf.

Crystallographic fabrics of omphacite, rutile and quartz in Vendée eclogites (Armorican Massif, France). Consequences for deformation mechanisms and regimes

This study aims at further understanding of the mechanisms how lattice-preferred orientations (LPO) develop during deformation in the main eclogite minerals. Microstructures and textures of deformed eclogites from the Les Essarts complex (Western France) were investigated using optical microscopy and electron backscatter diffraction (EBSD) in the scanning electron microscope. Microfabric analyses of eclogite-facies minerals are used to identify their deformation mechanisms, which define the rheology at high-pressure metamorphic conditions. Mechanisms of intracrystalline deformation by dislocation movement (dislocation creep) result usually in a non-linear flow law (typically power law), while diffusive processes (diffusion creep) correspond to linear flow laws. General microstructural observations may suggest intracrystalline deformation (dislocation creep) of omphacite. The omphacite LPO vary between S- and L-type and correlate with oblate or prolate grain shape fabrics, respectively. Until now, these LPO types have not been understood by plasticity models based on dislocation glide on the known slip systems in clinopyroxene. An alternative interpretation is given in terms of anisotropic growth and dissolution, with grain boundary diffusion as the rate controlling process. There are further indications suggesting diffusion creep with concomitant anisotropic growth and dissolution as a main deformation mechanism in omphacite. In omphacite around a hollow garnet, crystallographic and shape fabrics align with the c[001] axes parallel to the grain elongations defining the mineral lineation, which rotates locally with the inferred flow direction. In this part, the grain sizes of omphacite and rutile are larger than in the surrounding matrix. The geometry of both the shape and crystallographic fabrics is interpreted to represent the local stress regime (directions and ratios of the principal stresses). The LPO of rutile duplicate the LPO of omphacite and a similar distinction between S- and L-type was used. Rutile deformation mechanisms probably involve dislocation creep as well as diffusion creep. Quartz mainly occurs as an interstitial phase with weak LPO patterns interpreted as random. No representative obliquity of the LPO in omphacite nor rutile with respect to foliation and lineation was observed to be used as potential shear sense criteria. However, the rutile LPO was slightly rotated relative to the omphacite LPO consistently in most samples. The results suggest that diffusion processes are strongly involved in the deformation of eclogites. A linear flow law should be taken into account in tectonic models where eclogites are incorporated.

Application of cathodoluminescence to fine-grained pelitic schists of the Imjingang Belt, Korea

Cathodoluminescence (CL) was used to investigate the mode and compositional zoning of plagioclase in fine-grained metapelites of the Imjingang Belt, Korea. Minute, untwinned grains of plagioclase are indistinguishable from quartz under the polarized light microscope. However, distinct colour and intensity of the CL of plagioclase permit not only rapid identification but also characterisation of its compositional zoning on a thin-section scale without extensive compositional mapping. The abundance of plagioclase in the analysed samples suggests that this mineral could be more common in low- to medium-grade metapelites than previously recognized. CL colours of plagioclase vary from dull green through green to bright greenish yellow as the An content increases. Furthermore, CL petrography suggests the syn-kinematic growth of outer part of plagioclase (An 51-69 ). Hence, CL microscopy can provide a wealth of information not only for deciphering P-T-t paths but also for microfabric analysis of fine-grained schists.

Erste Ergebnisse von Makro- und Mikrostrukturuntersuchungen im metamorphen Beloretsk-Komplex (Süd-Ural)

Erste Ergebnisse von Makro- und Mikrostrukturuntersuchungen im metamorphen Beloretsk-Komplex (Süd-Ural)

Abstract: Microfabric Analysis of Cyclic Rhythmites: A Comparison of Modern and Ancient Samples

A cyclic rhythmite is represented by alternating silt-rich and clay-rich laminae, which can be used to interpret original depositional environments. This research on cyclic rhythmites compares fabric features of sediment from modern tidal environments with possible ancient analogs in order to determine the depositional conditions of the ancient samples. Samples from both modem and ancient cyclic rhythmites were collected and prepared for study at the macroscopic and microscopic level. Layer thickness, components of layers, and fabric features were studied using hand samples, thin-section analyses, and the scanning electron microscope. The study of the samples helped in the determination of depositional processes and the variations that operated within the environments. A number of factors, other than tides, such as climate, season, or varying influx of material through time from river systems, have been considered to be influential in the formation of these rhythmites. This research has helped determine how these may have played roles in the rhythmite formation. Examining the rhythmite laminae at different scales allows the layering observed megascopically in hand samples to be related to microscopic clay fabric that was sensitive to environmental depositional controls. This study characterizes the features of fabric representative of rhythmite sediments and rocks and uses these features in identifying sedimentary processes operative in the original depositional environment.

A quick and inexpensive sample preparation method for AMS microfabric analysis in fine-grained sediment

Unless disturbed by postdepositional processes, sedimentary grain fabric reflects the physical conditions at the time of sediment deposition. Anisotropy of magnetic susceptibility, or AMS, is a sophisticated yet simple method for measuring grain fabric. In most cases this technique requires a self-supporting, right-cylindrical sample, and as a result unlithified sediment needs first to be lithified. Generally this is done with expensive, time-consuming resin impregnation procedures. In consolidated, mud-rich, clast-poor sediment, however, a new technique of cyclical steaming and (steam) soaking proved to be highly effective in softening the sediment sample enough so that it could be cored for AMS-sample preparation. Using this new method, samples were prepared in about 90% less time and for only 10% of the cost of standard impregnation techniques.

Research Methods Paper A Quick and Inexpensive Sample Preparation Method for AMS Microfabric Analysis in Fine-Grained Sediment

ABSTRACT Unless disturbed by postdepositional processes, sedimentary grain fabric reflects the physical conditions at the time of sediment deposition. Anisotropy of magnetic susceptibility, or AMS, is a sophisticated yet simple method for measuring grain fabric. In most cases this technique requires a self-supporting, right-cylindrical sample, and as a result unlithified sediment needs first to be lithified. Generally this is done with expensive, time-consuming resin impregnation procedures. In consolidated, mud-rich, clast-poor sediment, however, a new technique of cyclical steaming and (steam) soaking proved to be highly effective in softening the sediment sample enough so that it could be cored for AMS-sample preparation. Using this new method, samples were prepared in about 90% less time and for only 10% of the cost of standard impregnation techniques.

Bulk sediment properties interpreted in light of qualitative and quantitative microfabric analysis

Sediments from Eckernforde Bay, Germany, are characterized by an aggregate and channel microstructure, with channel dimensions about two orders of magnitude larger than interparticle distances within aggregates. Porosity within aggregates as determined by image analysis of transmission electron micrographs was about 12% less than bulk porosity. Illite and smectite formed the bulk of most aggregates, while numerous biogenic particles generally occurred outside or on the periphery of aggregates. Microfabric analysis provides insights into permeability and consolidometer behavior of this sediment, reveals characteristics not apparent from bulk analyses, and may have implications for geochemistry and physical behavior of the sediment.

The evolution of high-temperature mylonitic microfabrics: evidence from simple shearing of a quartz analogue (norcamphor)

Plane strain simple shearing of norcamphor (C 7H 10O) in a see-through deformation rig to a shear strain of γ = 10.5 at a homologous temperature of T h = 0.81 yields a microfabric similar to that of quartz in amphibolite facies mylonite. Synkinematic analysis of the norcamphor microfabric reveals that the development of a steady-state texture is linked to changes in the relative activities of several grain-scale mechanisms. Three stages of textural and microstructural evolution are distinguished: (1) rotation and shearing of the intracrystalline glide planes are accommodated by localized deformation along three sets of anastomozing microshears. A symmetrical c-axis girdle reflects localized pure shear extension along the main microshear set ( S a) oblique to the bulk shear zone boundary (abbreviated as SZB); (2) progressive rotation of the microshears into parallelism with the SZB increases the component of simple shear on the S a microshears. Grain-boundary migration recrystallization favours the survival of grains with slip systems oriented for easy glide. This is associated with a textural transition towards two stable c-axis point maxima whose skeletal outline is oblique with respect to the S a microshears and the SZB; and (3) at high shear strains (γ > 8), the microstructure, texture and mechanism assemblage are strain invariant, but strain continues to partition into rotating sets of microshears. Steady state is therefore a dynamic, heterogeneous condition involving the cyclic nucleation, growth and consumption of grains.

The cause of frequency-dependent seismic absorption in crustal rock

SUMMARY The absorption of 5 to 200 Hz seismic wave energy propagating at depth in fracture-bearing crystalline basement in southern California is observed to increase as a power law in wave frequency as ?f0.43±0.1, so that energy absorption per unit frequency Q−1 decreases with frequency as ?f−0.57, and the seismic quality factor Q increases with frequency as f0.57; the value of Q at 10 Hz is ?1000. Crustal fracture density, which elastic scattering and borehole logs show to vary with fracture dimension L as a power law L0.4, cannot easily explain the observed frequency dependence of absorption. A likely hypothesis identifies the f0.43±0.1 frequency dependence of seismic absorption with the f1/12 frequency dependence of a thermal diffusion process. Seismic absorption computed for thermal gradients induced by localized strain concentrations depends on a single free parameter, the thermal absorption domain characteristic size D. If D? 0.2 mm the computed magnitude of absorption agrees with the observed Q? 1000 at 10 Hz. Thin sections from deep borehole rock samples show that ?0.2 mm is the characteristic dimension of microfracture shear-fabric and crystalline grains. Where seismic elastic scattering occurs over a wide range of fracture sizes, seismic absorption occurs predominantly at the dimension of microfractures and granularity. Systematic wide-band borehole seismic observation of seismic absorption magnitude and particularly frequency dependence, combined with thin-section analysis of the rock microfabric, can test the hypothesis for a range of crustal rocks. The evidence of thermal activity at the scale of microfractures suggests that static stress concentrations possess a free energy that can act to align microfractures, thus explaining the shear-wave birefringence widely observed in the crust.