Cathodoluminescence as a tracing technique for quartz precipitation in low velocity shear experiments

Two simulated gouges (a pure quartz and a quartz-muscovite mixture) were experimentally deformed in a ring shear apparatus at a constant low velocity under hydrothermal conditions favourable for dissolution–precipitation processes. Microstructural analysis using scanning electron microscope cathodoluminescence imaging and cathodoluminescence spectroscopy combined with chemical analysis showed that quartz dissolution and precipitation occurred in both experiments. The starting materials and deformation conditions were chosen so that dissolution–precipitation microstructures could be unambiguously identified from their cathodoluminescence signal. Precipitated quartz was observed as blue luminescent fracture fills and overgrowths with increased Al content relative to the original quartz. In the pure quartz gouge, most of the shear deformation was localized on a boundary-parallel slip surface. Sealing of fractures in a pulverized zone directly adjacent to the slip surface may have helped keeping the deformation localized. In the quartz-muscovite mixture, some evidence was observed of shear-accommodating precipitation of quartz in strain shadows, but predominantly in fractures, elongating the original grains. Precipitation of quartz in fractures implies that the length scale of diffusive mass transfer in frictional-viscous flow is shorter than the length of the quartz domains. Additionally, fracturing might play a more important role than generally assumed. Our results show that cathodoluminescence, especially combined with chemical analysis, is a powerful tool in microstructural analyses of experimentally deformed quartz-bearing material and visualizing quartz precipitation

Sedimentary geology of the Middle Carboniferous of the Donbas region (Dniepr-Donets basin, Ukraine)

Peer reviewedPublisher PD

Recrystallisation and Trace-Element Mobility in Zircons: Implications for U-Pb Dating

Complex microstructures of zircon recrystallisation have been discovered in igneous and metamorphic zircons from the Jack Hills Metasedimentary Belt in Western Australia, and the granitic injection complex of Harris in north-west Scotland. Cathodoluminescence (CL), electron backscatter diffraction (EBSD) and wavelength dispersive spectroscopy (WDS) analysis reveal that recrystallisation in the studied zircons is often characterised by (1) translation, bending and fading of the oscillatory zoning; (2) structural recovery of the zircon; (3) an enrichment in Hf and depletion in Y in recrystallised zircon; and (4) the formation of a recrystallisation interface with a minor (<2°) misorientation. A new, composite recrystallisation model in which trace element and dislocation migration occurs shortly after crystallisation during magmatic cooling and pipe diffusion along dislocation arrays formed by the accumulation of dislocations allows enhanced diffusion to enrich Hf and leach U, Pb and Y. After the recrystallisation event, subsequent metamictization of primary zircon (typically with oscillatory zoning) creates zones that can structurally recover through a diffusion-reaction re-equilibration mechanism, resulting in further trace-element mobility. These mechanisms can create complex microstructures in zircons, suggesting that a detailed understanding of the crystallisation and recrystallisation history of a zircon may be required to accurately interpret its U-Pb ages

Перебіг анемії вагітних у хворих на туберкульоз легень

В статье представлены данные течения анемии беременных и нарушения фетоплацентарного комплекса у 86 женщин, больных туберкулезом легких. Выявлено, что анемия на фоне туберкулезного процесса приводит к существенным функциональным и морфологическим нарушениям в плаценте. показано диагностическое значение оценки плацентарной дисфункции для профилактики и лечения анемии у данного контингента женщин.The article presents the data of course of pregnancy anemia and failures of fetoplacental complex in 86 women with pulmonary tuberculosis. It is found that anemia under tuberculous process leads to significant functional and morphological disturbances in placenta. Diagnostic significance of assessment of placental dysfunction for the prevention and treatment of anemia in this contingent of women is shown

Observational and theoretical evidence for frictional-viscous flow at shallow crustal levels

Along the Hikurangi Subduction Margin, accretionary prism uplift has exposed the Hungaroa fault zone, an inactive thrust developed within the Middle to Late Eocene Wanstead Formation. Within the ~33 m-wide fault core, deformation of the smectitic, calcareous mudstone matrix produced a penetrative foliation that locally wraps around clasts. Deformation occurred at temperatures constrained by syntectonic calcite vein clumped isotope thermometry, which yielded a narrow range of Δ47 values between 0.445 ± 0.024‰ and 0.482 ± 0.013‰, corresponding to a mean calcite precipitation temperature of 82−12+13 °C. Optical and scanning electron microscopy analyses reveal that calcite underwent: dissolution along stylolites and clast, vein, and microlithon margins; precipitation in foliation-parallel and foliation-perpendicular extension veins; and precipitation in hybrid veins and strain fringes. Maximum differential stress estimates obtained from calcite twin densities (44.1 ± 13.9 to 96.6 ± 20.8 MPa) are consistent with those sustainable by a cohesionless fault at ~3 km depth with a friction coefficient in the range measured for two calcareous mudstones (μ = 0.38 to 0.50) and a micrite clast (μ = 0.61 and 0.64). Marlstone clasts within the foliated calcareous mudstone matrix contain mutually cross-cutting shear fractures and extension veins with crack-seal textures, providing evidence for temporal fluctuations in shear strength resulting from pore fluid overpressure transients. At strain rates imposed during laboratory experiments, frictional sliding involves granular flow processes. Yet, calcite microstructures indicate that diffusive mass transfer played an important role in accommodating deformation. We model the fault zone rheology assuming diffusion-controlled frictional-viscous flow, with deformation at strain rates γ˙≤ 10−9 s−1 able to have taken place at very low shear stresses (τ < 10 MPa) given sufficiently short diffusion distances (d < 0.1 mm), even in the absence of pore fluid overpressures. However, if grain-scale and fracture-scale processes change the diffusion distance, fault zones deforming via frictional-viscous flow can exhibit temporally variable strain rates. Thus, our results suggest that the shallow (up-dip) limit of the seismogenic zone is not a simple function of temperature in fault zones governed by a frictional-viscous flow rheology

The fossil bivalve Angulus benedeni benedeni: A potential seasonally resolved stable-isotope-based climate archive to investigate Pliocene temperatures in the southern North Sea basin

Bivalves record seasonal environmental changes in their shells, making them excellent climate archives. However, not every bivalve can be used for this end. The shells have to grow fast enough so that micrometre- to millimetre-sampling can resolve sub-annual changes. Here, we investigate whether the bivalve Angulus benedeni benedeni is suitable as a climate archive. For this, we use ca. 3-million-year-old specimens from the Piacenzian collected from a temporary outcrop in the Port of Antwerp area (Belgium). The subspecies is common in Pliocene North Sea basin deposits, but its lineage dates back to the late Oligocene and has therefore great potential as a high-resolution archive. A detailed assessment of the preservation of the shell material by micro-X-ray fluorescence, X-ray diffraction, and electron backscatter diffraction reveals that it is pristine and not affected by diagenetic processes. Oxygen isotope analysis and microscopy indicate that the species had a longevity of up to a decade or more and, importantly, that it grew fast and large enough so that seasonally resolved records across multiple years were obtainable from it. Clumped isotope analysis revealed a mean annual temperature of 13.5±3.8°C. The subspecies likely experienced slower growth during winter and thus may not have recorded temperatures year-round. This reconstructed mean annual temperature is 3.5°C warmer than the pre-industrial North Sea and in line with proxy and modelling data for this stratigraphic interval, further solidifying A. benedeni benedeni's use as a climate recorder. Our exploratory study thus reveals that Angulus benedeni benedeni fossils are indeed excellent climate archives, holding the potential to provide insight into the seasonality of several major climate events of the past ∼25 million years in northwestern Europe

Cathodoluminescence as a tracing technique for quartz precipitation in low velocity shear experiments

Abstract Two simulated gouges (a pure quartz and a quartz-muscovite mixture) were experimentally deformed in a ring shear apparatus at a constant low velocity under hydrothermal conditions favourable for dissolution–precipitation processes. Microstructural analysis using scanning electron microscope cathodoluminescence imaging and cathodoluminescence spectroscopy combined with chemical analysis showed that quartz dissolution and precipitation occurred in both experiments. The starting materials and deformation conditions were chosen so that dissolution–precipitation microstructures could be unambiguously identified from their cathodoluminescence signal. Precipitated quartz was observed as blue luminescent fracture fills and overgrowths with increased Al content relative to the original quartz. In the pure quartz gouge, most of the shear deformation was localized on a boundary-parallel slip surface. Sealing of fractures in a pulverized zone directly adjacent to the slip surface may have helped keeping the deformation localized. In the quartz-muscovite mixture, some evidence was observed of shear-accommodating precipitation of quartz in strain shadows, but predominantly in fractures, elongating the original grains. Precipitation of quartz in fractures implies that the length scale of diffusive mass transfer in frictional-viscous flow is shorter than the length of the quartz domains. Additionally, fracturing might play a more important role than generally assumed. Our results show that cathodoluminescence, especially combined with chemical analysis, is a powerful tool in microstructural analyses of experimentally deformed quartz-bearing material and visualizing quartz precipitation

Distinction between amorphous and healed planar deformation features in shocked quartz using composite color scanning electron microscope cathodoluminescence (SEM-CL) imaging

Abstract–Planar deformation features (PDFs) in quartz are one of the most reliable and most widely used forms of evidence for hypervelocity impact. PDFs can be identified in scanning electron microscope cathodoluminescence (SEM-CL) images, but not all PDFs show the same CL behavior: there are nonluminescent and red luminescent PDFs. This study aims to explain the origin of the different CL emissions in PDFs. Focused ion beam (FIB) thin foils were prepared of specific sample locations selected in composite color SEMCL images and were analyzed in a transmission electron microscope (TEM). The FIB preparation technique allowed a direct, often one-to-one correlation between the CL images and the defect structure observed in TEM. This correlation shows that composite color SEM-CL imaging allows distinction between amorphous PDFs on one hand and healed PDFs and basal Brazil twins on the other: nonluminescent PDFs are amorphous, while healed PDFs and basal Brazil twins are red luminescent, with a dominant emission peak at 650 nm. We suggest that the red luminescence is the result of preferential beam damage along dislocations, fluid inclusions, and twin boundaries. Furthermore, a high-pressure phase (possibly stishovite) in PDFs can be detected in color SEM-CL images by its blue luminescence

Heterogeneous stresses and deformation mechanisms at shallow crustal conditions, Hungaroa Fault Zone, Hikurangi Subduction Margin, New Zealand

The Hungaroa Fault Zone (HFZ), an inactive thrust fault along the Hikurangi Subduction Margin, accommodated large displacements (~4-10 km) at the onset of subduction in the early Miocene. Within a 40 m-wide high-strain fault core, calcareous mudstones and marls display evidence for mixed-mode viscous flow and brittle fracture, including: discrete faults; extensional veins containing stretched calcite fibers; shear veins with calcite slickenfibers; calcite foliation-boudinage structures; calcite pressure fringes; dark dissolution seams; stylolites; embayed calcite grains; and an anastomosing phyllosilicate foliation.Multiple observations indicate a heterogeneous stress state within the fault core. Detailed optical and electron backscatter diffraction-based texture analysis of syntectonic calcite veins and isoclinally folded limestone layers within the fault core reveal that calcite grains have experienced intracrystalline plasticity and interface mobility, and local subgrain development and dynamic recrystallisation. The recrystallized grain size in two calcite veins of 6.0±3.9 µm (n=1339; 1SD; HFZ-H4-5.2m_A;) and 7.2±4.2µm (n=406; 1SD; HFZ-H4-19.9m) indicate high differential stresses (~76-134 MPa). Hydrothermal friction experiments on a foliated, calcareous mudstone yield a friction coefficient of μ≍0.35. Using this friction coefficient in the Mohr-Coulomb failure criterion yields a maximum differential stress of 55 MPa at 4 km depth, assuming a minimum principal stress equal to the vertical stress, an average sediment density of 2350 kg/m3, and hydrostatic pore fluid pressure. Interestingly, calcareous microfossils within the foliated mudstone matrix are undeformed. Moreover, calcite veins are oriented both parallel to and highly oblique to the foliation, indicating spatial and/or temporal variations in the maximum principle stress azimuth.To further constrain HFZ deformation conditions, clumped isotope geothermometry was performed on six syntectonic calcite veins, yielding formation temperatures of 79.3±19.9°C (95% confidence interval). These temperatures are well below those at which dynamic recrystallisation of calcite is anticipated and exclude shear heating and the migration of hotter fluids as an explanation for dynamic recrystallisation of calcite at shallow crustal levels

Drill core from seismically active sandstone gas reservoir yields clues to internal deformation mechanisms

Europe’s largest gas field, the Groningen field (the Netherlands), is widely known for induced subsidence and seismicity caused by gas pressure depletion and associated compaction of the sandstone reservoir. Whether compaction is elastic or partly inelastic, as implied by recent experiments, is a key factor in forecasting system behavior and seismic hazard. We sought evidence for inelastic deformation through comparative microstructural analysis of unique drill core recovered from the seismogenic center of the field in 2015, 50 yr after gas production started, versus core recovered before production (1965). Quartz grain fracturing, crack healing, and stress-induced Dauphiné twinning are equally developed in the 2015 and 1965 cores, with the only measurable effect of gas production being enhanced microcracking of sparse K-feldspar grains in the 2015 core. Interpreting these grains as strain markers, we suggest that reservoir compaction involves elastic strain plus inelastic compression of weak clay films within grain contacts