Cathodoluminescence Microscopy Research Articles

RECOGNITION OF PORPHYRY QUARTZ IN STREAM SEDIMENTS BY FLUID INCLUSION PETROGRAPHY AND CATHODOLUMINESCENCE MICROSCOPY: RESULTS OF SYSTEMATIC DISPERSION STUDIES AND POTENTIAL APPLICATIONS IN PORPHYRY EXPLORATION

Abstract Regional stream sediment surveys are an important exploration tool used in the search for concealed or partially concealed porphyry deposits. It is shown here that quartz contained in the coarse fraction of stream sediments can be used as an indicator mineral to supplement geochemical analyses conducted on the fine fraction, such as the measurement of the bulk cyanide leach extractable gold content. A method is proposed that allows separation of quartz grains from the coarse rejects of stream sediment samples to prepare grain mounts for petrographic analysis. Based on optical cathodoluminescence microscopy and fluid inclusion petrography, the number of porphyry quartz grains in each grain mount is then identified. Case studies conducted at Vert de Gris in Haiti and Hides Creek in Papua New Guinea show that porphyry quartz grains could be confidently identified in sediments in the catchment areas of both porphyries. Because the cost of microscopic analysis of quartz is small compared to the expense of sampling and geochemical analysis, the developed technique could be routinely used in large greenfield exploration programs. It is envisaged here that petrographic analysis of quartz grains can contribute valuable information for prioritization of targets defined based on their geochemical signatures.

GaN/InGaN LED Sidewall Defects Analysis by Cathodoluminescence and Photosensitive Kelvin Probe Force Microscopy

GaN/InGaN LED Sidewall Defects Analysis by Cathodoluminescence and Photosensitive Kelvin Probe Force Microscopy

Diagenesis and pressure evolution in Ediacaran carbonate rocks in the North Slope (Penglai area) of the central Sichuan Basin (SW China)

The Penglai area is an emerging giant gas field in the Sichuan Basin supporting the carbon neutrality goal. To accelerate the exploration, this study combines petrographic observations using optical and cold cathodoluminescence microscopy, fluid inclusion microthermometry, Raman spectroscopy, 1D basin modeling and fluid inclusion PVTX modeling techniques to reveal the paleo-fluid P-T evolution in the second member of the Dengying Formation (Ediacaran age). The results firstly showed a diagenetic sequence which includes three phases of dolomite (1–3), bitumen, quartz (core and overgrowth) and fluorite. Then, ten paleo-fluid episodes were identified based on microthermometric histograms of different mineral phases. Finally, by integrating the Raman results, the hydrostatic and lithostatic gradients calculated by the constructed 1D basin model, and the existing equation of states, the homogenization pressure of fluid inclusions from each fluid episode was determined. By assuming the system was saturated by CH4 as suggested by Raman results, the homogenization pressure equals to the trapping pressure. The P-T trajectory indicates that the second member of the Dengying Formation experienced two sever pressure accumulation stages. One is between fluid episode #3 and #4 recorded by dolomite 3, due to oil generation, and the other is from fluid episode #5 in dolomite 3 to episode #7 in quartz overgrowth to episode #8 in fluorite due to oil cracking to gas. Also, two pressure releasing periods were recorded by fluid episode #5 in dolomite 3 and #9 and #10 in fluorite, caused by both basin uplifting and fluid pressure reaching the lithostatic pressure. This study provides insights into fluid pressure evolution in deeply buried carbonate reservoirs. The understandings can support further geoenergy utilization and reduce exploration risks.

Biogenic carbonates (queras) in loess-palaeosol sequences of the Ebro Basin and their potential use as a palaeoenvironmental proxy

Secondary carbonate accumulations are very common in arid and semi-arid soils but are often poorly described and classified. This is the case for queras that are observed in several loess-palaeosol sequences of the Ebro Basin. Micromorphologically, queras are complex pedogenic features basically consisting of infillings of calcified root cells in channels, surrounded by decarbonated hypocoatings. They apparently result from biologically mediated calcification/decalcification processes at a microscale. The aim of this study is to systematically characterize biocalcifications found in dated loess-palaeosol sequences, to determine the main factors of formation, and to discuss their possible use as a palaeoenvironmental proxy. Different queras show a similar number of biosparite layers (four to five) around a central channel, and a decarbonated hypocoating around the queras. This supports the hypothesis that queras originate from the calcification of root tips and are related to the acidification of surrounding soil for nutrient absorption. Under cathodoluminescence microscopy, biosparite has a different behaviour than non-biological calcite crystals: the latter appears reddish while biosparite remains black. The queras, with ages corresponding to Marine Isotope Stages 1–3, are younger than that of the loess where they are found in. The stable isotope values likely correspond to C3 plants, and the estimated temperatures of formation reflect a Mediterranean temperate climate. This suggests that the biocalcifications developed during short moist seasons amidst arid periods of the Quaternary.

Diagenesis of carbonate density-flow deposits controlled by differential uplift of platform segments: examples from the Cretaceous of the Gargano Promontory, Italy

ABSTRACT Thin-section investigation (polarized-light, cathodoluminescence, and ultraviolet microscopy) combined with isotopic (δ18O, δ13C,87Sr/86Sr) analyses of bulk carbonate samples form the basis for evaluating the diagenetic alteration of Albian–Cenomanian and Maastrichtian density-flow deposits off two segments of the Apulia Carbonate Platform in the Gargano Promontory, Italy. We propose that differential platform uplift south and north of the Mattinata Fault controlled the diagenesis of density-flow deposits during Albian–Cenomanian times. In both cases, the: i) abundant blocky cement and vuggy pores in clasts, and ii) remnant blocky cement on allochems in the corresponding matrix samples with interparticle pores, indicate disintegration of at least partially cemented deposits before failure and reworking into density flows. The abundant rudist fragments suggest that they were sourced from the margin and upper slope. However, the δ13C compositions of the density-flow deposits south and north of the Mattinata Fault are different, and geochemical modeling based on presumed marine and terrestrial δ13C compositions indicates: A) The marine δ13C values of deposits south of the Mattinata Fault suggest that the margin- and upper-slope deposits were subjected to predominantly marine-burial diagenesis before failure. Albian–Cenomanian Sr-isotope ages support the marine-burial diagenetic scenario where strontium was redistributed locally during calcitization of aragonitic allochems and during precipitation of calcite cements. However, post-uplift precipitation of vadose cement in pores formed during marine-burial diagenesis has lowered the δ13C and increased the 87Sr/86Sr ratio in many of the samples. B) Twenty-five km north of the Mattinata Fault, the negative δ13C values suggest that oxidation of terrestrial plants supplied 12C-enriched CO2 to the pore-water carbon pool during subaerial exposures, thus lowering the δ13C compositions of the margin- and upper-slope deposits from values obtained during marine-burial diagenesis. This diagenetic model requires that residual aragonitic and high-Mg calcitic allochems were available in the deposits during penetration of meteoric water. However, the Albian–Cenomanian Sr-isotope ages and the geochemical modeling support a predominantly marine-burial scenario, with intraformational redistribution of strontium during meteoric diagenesis. A similar diagenetic model is envisaged for the Maastrichtian density-flow deposits south of the Mattinata Fault, but the less negative δ13C and geochemical modeling suggest less influence of meteoric diagenesis before reworking. The majority of separate- and touching-vug pores likely also formed during marine burial before failure and reworking of the margin- and upper-slope deposits. The large range in porosity (4–31%) of density-flow deposits with predominantly interparticle pores are inferred to reflect varying degrees of compaction caused by variable overburden thickness as well as sedimentary processes. The results of the present study indicate that the geochemical imprint of meteoric diagenesis can be expected to vary in density-flows deposited along coeval segments of carbonate platforms in tectonically active regions.

The 450 nm (2.8 eV) cathodoluminescence emission in quartz and its relation to structural defects and Ti contents

Abstract The origin of the common blue 450 nm (2.8 eV) cathodoluminescence (CL) emission in natural and synthetic quartz has been investigated using a combination of CL microscopy and spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and trace-element analysis by electron micro-probe analysis as well as inductively coupled plasma-mass spectrometry (ICP-MS). The study shows that the appearance of the ~450 nm emission band can be attributed to two different defects in quartz. First, a transient luminescence can be explained by structural defects in oxygen deficient quartz. The luminescence model implies self-trapped exciton (STE) emission related to oxygen vacancies. This type of CL emission is frequent in high-purity synthetic quartz and natural quartz of hydrothermal origin. Second, in Ti-rich quartz from natural samples (e.g., quartz phenocrysts in rhyolites) and synthetic quartz of Ti-diffusion experiments, an additional 450 nm (2.8 eV) emission was detected, which is stable under the electron beam. The intensity of this ~450 nm emission band correlates with the concentration of trace Ti in quartz, and substitutional Ti4+ at the Si4+ position was proved by EPR spectroscopy. In quartz crystals with elevated Ti concentrations both intrinsic and extrinsic blue CL emissions at ~450 nm can coexist, hindering a thorough characterization and quantification of the CL signal. A reliable distinction of the two different CL emission bands is possible by fitting the peaks of the CL spectra, and the peak width of the 450 nm emission can be used to differentiate the STE from the Ti4+ emission. However, the definitive technique is through the observation of CL peak shape change over time at a point by collecting a time series of CL spectra in conjunction with EPR spectroscopy and trace-element analysis of the Ti concentration.

New deposit of chocolate flint in Załęże gully (Kraków-Częstochowa Upland, Poland) – Raw material characterization and its availability for prehistoric communities

One of the valued chert raw materials used by prehistoric communities in Central and Eastern Europe was chocolate flint. Despite finding artefacts made of it over a wide area, for many years, it was thought to occur only in one location - in the Holy Cross Mountains in Poland. However, the discoveries of recent years have shown that deposits of this raw material also occur in another region of Poland, in the northern part of the Kraków-Częstochowa Upland (KCU). The result of intensive field surveys conducted in this area is the finding of a new outcrop of chocolate flint in Załęże gully (Lesser Poland voivodeship) described in this paper. Based on the results obtained from the radiocarbon and optically stimulated luminescence dating of sediments co-occurring with the flint deposit, a history of the paleogeographic development of this gully has been proposed. Access to the deposit in its present form is limited to modern times, but it was likely accessible elsewhere in this area. Macroscopic and microscopic (using optical and cathodoluminescence microscopy) examinations showed a high similarity with the material of other KCU provenance from the prehistoric mine in Poręba Dzierżna, site 24. In the vicinity of Załęże gully there are many archaeological sites where chocolate flint artefacts were found. Preliminary research suggests that they could have been made from the raw material from the newly discovered outcrop. The new location of chocolate flint occurrence in the KCU region described here is another premise to re-think the existing interpretations related to the extraction, use and distribution of this important chert raw material in prehistoric times in Central and Eastern Europe.

Imaging Strain-Localized Single-Photon Emitters in Layered GaSe below the Diffraction Limit.

Nanoscale strain control of exciton funneling is an increasingly critical tool for the scalable production of single photon emitters (SPEs) in two-dimensional materials. However, conventional far-field optical microscopies remain constrained in spatial resolution by the diffraction limit and thus can provide only a limited description of nanoscale strain localization of SPEs. Here, we quantify the effects of nanoscale heterogeneous strain on the energy and brightness of GaSe SPEs on nanopillars with correlative cathodoluminescence, photoluminescence, and atomic force microscopy, supported by density functional theory simulations. We report the strain-localized SPEs have a broad range of emission wavelengths from 620 to 900 nm. We reveal substantial strain-controlled SPE wavelength tunability over a ∼100 nm spectral range and 2 orders of magnitude enhancement in the SPE brightness at the pillar center due to Type-I exciton funneling. In addition, we show that radiative biexciton cascade processes contribute to observed CL photon superbunching. Also, the GaSe SPEs show excellent stability, where their properties remain unchanged after electron beam exposure. We anticipate that this comprehensive study on the nanoscale strain control of two-dimensional SPEs will provide key insights to guide the development of truly deterministic quantum photonics.

Characterization and Provenance of Carbonate Rocks for Quicklime and Dololime Production in Twin-Shaft Regenerative Kilns from the Arabian Peninsula and Neighboring Countries

This study analyzes high-grade carbonate rocks from several strategic deposits in the Arabian Peninsula and neighboring countries. The rocks are used locally for quicklime and dololime production in twin-shaft regenerative kilns. Stable C-O-Sr isotopes, along with chemical, mineralogical-petrographic analyses, micropaleontological investigations, cathodoluminescence microscopy, organic carbon speciation, and electron paramagnetic resonance spectroscopy, were used to trace the provenance of these rocks from economically significant non-metallic deposits. The resulting database can help identify and differentiate industrial raw materials that may appear similar chemically and/or macroscopically but have different textures/microstructures that can affect the properties of the derived burnt lime products. Various technological tests, including slaking reactivity, sticking tendency at high-temperature (i.e., 1300 °C), and physico-mechanical behavior of the lime, were performed to evaluate their suitability and predict lime performance in twin-shaft regenerative kilns. Comparison of laboratory and plant results validated the resulting database.

Inhomogeneous Beam Brightness Intensity Converter (IBBIC) as support software for cathodoluminescence microscopy studies and images pre-processing

Inhomogeneous Beam Brightness Intensity Converter (IBBIC) as support software for cathodoluminescence microscopy studies and images pre-processing

Calcium aluminate phosphor-containing mortar achieved via aqueous metal-ethylenediaminetetraacetic acid complex solution and high-heat treatment

Calcium aluminate phosphors were synthesized on mortar substrates using an aqueous solution comprising metal (Al,Eu,Nd)-ethylenediaminetetraacetic acid (EDTA) complexes, followed by treatment with a high-heat hydrogen/oxygen (H2-O2) coupled flame with a variable gas flow rate. The infrastructure functionalizationperformance of the cement system was evaluated through crystal structure analysis and emission/excitation properties investigation. The emission phase of the calcium aluminates was identified using cathodoluminescence microscopy. The afterglow properties were also investigated. Furthermore, a model for the formation of calcium-aluminate phosphors using a metal-EDTA aqueous solution with H2-O2 flame treatment was developed.

Nanoscale imaging of He-ion irradiation effects on amorphous TaOx toward electroforming-free neuromorphic functions

Resistive switching in thin films has been widely studied in a broad range of materials. Yet, the mechanisms behind electroresistive switching have been persistently difficult to decipher and control, in part due to their non-equilibrium nature. Here, we demonstrate new experimental approaches that can probe resistive switching phenomena, utilizing amorphous TaOx as a model material system. Specifically, we applied scanning microwave impedance microscopy and cathodoluminescence (CL) microscopy as direct probes of conductance and electronic structure, respectively. These methods provide direct evidence of the electronic state of TaOx despite its amorphous nature. For example, CL identifies characteristic impurity levels in TaOx, in agreement with first principles calculations. We applied these methods to investigate He-ion-beam irradiation as a path to activate conductivity of materials and enable electroforming-free control over resistive switching. However, we find that even though He-ions begin to modify the nature of bonds even at the lowest doses, the films' conductive properties exhibit remarkable stability with large displacement damage and they are driven to metallic states only at the limit of structural decomposition. Finally, we show that electroforming in a nanoscale junction can be carried out with a dissipated power of <20 nW, a much smaller value compared to earlier studies and one that minimizes irreversible structural modifications of the films. The multimodal approach described here provides a new framework toward the theory/experiment guided design and optimization of electroresistive materials.

Asian dust-deposition flux to the subarctic Pacific estimated using single quartz particles

Iron availability limits marine ecosystem activities in large areas of the ocean. However, the sources and seasonal supply of iron, critically important for controlling surface ocean biogeochemistry and carbon cycling, are poorly understood. The western subarctic Pacific is a high-nutrient and low-chlorophyll region, and despite high concentrations of macronutrients, iron limits phytoplankton production in summer. Here, we determine the seasonal deposition flux of Asian dust using scanning electron microscope–cathodoluminescence analysis of single quartz particles derived from the western subarctic Pacific during 2003–2022 to trace provenance. We found a high (up to 6.9 mg m−2 day−1) deposition flux of Asian dust in May, June, and early July, with an annual average of 1.0 ± 0.2 mg m−2 day−1. The supply of dissolved-iron flux calculated from Asian dust was 0.9 ± 0.3 µg m−2 day−1 during the high productivity season (April–July), which is approximately half that from the deeper part of the ocean, calculated from vertical profiles of dissolved iron. Our study provides a reliable approach for estimating iron supply from dust to the surface ocean that may be critical for sustaining biological productivity under future ocean stratification, which suppresses nutrient supply from the subsurface ocean.

Biaxial, Hybrid Plastic-Elastic Relaxation in Micropatterned Semipolar InGaN Buffer Layers Improves Quality of Templated Quantum Well Emitters

Despite significant progress and multiple creative approaches taken toward the realization of long-wavelength III-nitride based light emitters over the past decade, high double-digit external quantum efficiencies remain elusive for GaN/InGaN based devices for wavelengths longer than 525 nm. This stems from the large mismatch between the GaN and InGaN lattice that manifests in deleterious crystal strain gradients, degrades surface growth morphology, and introduces diverse and potent nonradiative defects.A promising route to mitigation of strain-based effects is templating epitaxial growth on relaxed InGaN epilayers; in particular, a known and controllable platform for uniaxial strain relaxation is the creation of 1-D misfit dislocation arrays at semipolar GaN/InGaN heterointerfaces. Leveraging work by Hardy et al. on achieving >200 nm thick, partially relaxed semipolar InGaN layers [1] and precise determination of InGaN epilayer strain state and composition by Young et al. [2], we explore the potential for 2-D buffer layer relaxation in (11-22) InGaN layers via micron- and sub-micron patterning. Cathodoluminescence microscopy and x-ray diffraction indicated significant increases in biaxial epilayer relaxation with increased pillar aspect ratio; quantum wells deposited atop these relaxed InGaN buffers showed dramatic threading dislocation reduction, luminescence and morphology improvements, and evidence of improved indium incorporation relative to planar samples at nominally identical growth conditions.[1] Hardy, M.T., Young, E.C., Hsu, P.S., Haeger, D.A., Koslow, I.L., Nakamura, S., DenBaars, S.P., Speck, J.S. Suppression of m-plane and c-plane slip through Si and Mg doping in partially relaxed (20-21) InGaN/GaN heterostructures. Appl. Phys. Lett. 101, 132102 (2012).[2] Young, E. C., Romanov, A.E., Speck, J.S., Determination of composition and lattice relaxation in semipolar ternary (In,Al,Ga)N strained layers from symmetric x-ray diffraction measurements. Appl. Phys. Express. 4, 061001 (2011).

Examining the impact of cement on porosity evolution in the upper triassic Kurra Chine Formation of Sirwan valley, Kurdistan region, Iraq: A study integrating cathodoluminescence and scanning electron microscopy

The study of carbonate petrography is essential for understanding reservoir properties and the quality of rock formations. High-resolution cathodoluminescence spectroscopy and optical microscopy were used to determine the impact of cement on the porosity evolution of carbonate rocks of the Kurra Chine Formation at Sirwan Valley, Northern Iraq. This study represents the first-ever attempt to investigate sedimentology at this location. A total, of 183 thin-section slides, 18 X-Ray Diffraction (XRD) samples, 14 Scanning Electron Microscope (SEM) samples, and 6 Cathodoluminescence (CL) samples were analyzed from the outcrop section of the Kurra Chine Formation. Four types of dolomite textures are recognized and classified based on the crystal size distribution and crystal-boundary shape. The porosity within Kurra Chine Formation includes primary and secondary porosity, with the majority of primary porosity being occluded by cementation. Cathodoluminescence analysis revealed two phases of calcite cement in the Kurra Chine Formation. The first phase (C1), is characterized by weak-zoned blocky calcite, displaying a dull luminescence with typical homogenous red color, which fills most of the interparticle and moldic pores. The second type of calcite cement (C2) is ghost-zoned calcite exhibiting red to slight orange luminesces interfingering with dolomite cement. XRD analysis indicated a predominant of calcite and common dolomite. SEM analysis revealed the predominance of secondary porosity occurring within fractures of the dolomite.

The Origin of the Upper Cambrian Basin-Scale Massive Dolostones of the Xixiangchi Formation, Sichuan Basin, China

The thick Upper Cambrian Xixiangchi dolostones, developed in the Sichuan Basin, are an important deep exploration target, but their genesis is still controversial, which hinders predicting the porous dolomite distribution and related potential hydrocarbon play. Herein, based on the observation and sampling of field outcrops, combined with a microscopic thin section analysis, cathodoluminescence analysis, and geochemical study, their characteristics and genesis were investigated. The results showed that there are mainly three types of dolomite that can be distinguished: (1) fine crystalline dolomite with a low crystallinity (Type 1); (2) granular dolomite with coarse grains, maintaining the original particle structure (Type 2); and (3) grain-texture relict dolomite with a higher degree of crystal form and obvious recrystallization (Type 3). The Type 1 dolomite with a common lamina structure and the highest 87Sr/86Sr ratio implies the most continental-influenced seawater in a supratidal environment around paleouplift, where there is an evaporation pump effect in its formation. The Type 2 dolomite recorded a slightly higher diagenesis temperature and slightly lower brine salinity, which would be formed in a seepage-reflux model in the beach environment of the platform. The lowest REY content and higher dolomite temperature with structural residuals indicate that the Type 3 dolomite is the result of further burial dolomitization during the diagenetic process.

Correlating cathodoluminescence and scanning transmission electron microscopy for InGaN platelet nano-LEDs

Structural defects are detrimental to the efficiency and quality of optoelectronic semiconductor devices. In this work, we study InGaN platelets with a quantum well structure intended for nano-LEDs emitting red light and how their optical properties, measured with cathodoluminescence, relate to the corresponding atomic structure. Through a method of spectroscopy–thinning–imaging, we demonstrate in plan-view how stacking mismatch boundaries intersect the quantum well in a pattern correlated with the observed diminished cathodoluminescence intensity. The results highlight the importance of avoiding stacking mismatch in small LED structures due to the relatively large region of non-radiative recombination caused by the mismatch boundaries.

Super‐Resolved Bulk States of Photonic Crystals With Cathodoluminescence Microscopy

AbstractProgress in photonic crystals (PhCs) enables the advanced manipulation of light propagation, leading to strong light–matter interactions and unconventional applications. To explore the physical nature and realize the precise and efficient control of position‐dependent light–matter interactions, the direct deep‐subwavelength characterization of PhC states is pivotal. Here, the super‐resolved visualization of bulk states in the z‐direction asymmetric dielectric PhC is realized by characterizing the radiative local density of optical states, including the etched hole area. An accelerating voltage scanning approach is proposed to selectively probe bulk states and substrates, elucidating the electron–matter interactions affected by the electron‐beam energy. The bulk state energy is demonstrated to be confined inside structure holes as shown by the cathodoluminescence intensity mappings of PhCs with different geometrical parameters, enabling the design of a silicon‐based high‐quality cavity in the visible range. This work provides a deep investigation of the depth‐dependent interaction between the electron beam and dielectric PhCs, promoting the characterization of PhC delocalized modes. The deep‐subwavelength detail of bulk state benefits the future nanoscale manipulation of light–matter interactions and the optimization of quantum emission devices.

Palaeo-environmental significance of fibrous carbonate cement in Marinoan cap carbonates

Marinoan cap carbonates are the first sedimentary rocks deposited after the Marinoan Snowball Earth glaciation and likely record the marine geochemical signals across the Cryogenian-Ediacaran boundary. The nature of the parent fluids, the possible precursor mineralogies and the dolomitisation models of these carbonates are debated to the present day. Cap carbonates and their fibrous dolomite cement samples from the Jiulongwan and Huajipo sections in South China were analysed for their petrography and geochemistry. Data obtained shed light on the deposition and alteration history of cap carbonates. Two types of fibrous dolomite cement precipitated penecontemporaneously with platform carbonate sediment deposition: (i) isopachous fascicular-optic dolomite fabrics with a characteristic botryoidal habit and mottled cathodoluminescence colour; they likely formed via an aragonitic precursor; and (ii) radial-slow dolomite cement, representing a direct marine precipitate. Growth zones are recognised under the SEM and visible in alternating dull- and bright-red zones under the cathodoluminescence microscope. Basal cap carbonates (Ce anomalies: 0.85 ± 0.11; Mn: 3324.3 ± 558.2 ppm) and the base (Ce anomalies: 0.83 ± 0.22; Mn: 2349.2 ± 1276.4 ppm) of fascicular-optic dolomite cement from platform facies indicate anoxic marine seawater with high Mn concentrations. Based on the geochemical data of fibrous dolomite and blocky calcite cement, the marine porewater evolved from the Mn reduction zone (Ce anomalies: 1.04 ± 0.26) to the sulphate reduction zone (Ce anomalies: 1.00 ± 0.48) and the Fe reduction zone (Ce anomaly: 1.40). In situ trace and rare earth elemental contents and the transition from a dull to bright fluorescence colour along the fibrous dolomite transects point to the former activity of microbial sulphate reduction and the degradation of organic matter. These two factors likely facilitated the formation of cap dolostone and multiphase dolomite cement crusts in a marine seawater (porewater) environment. Data shown here are significant for those concerned with the palaeoenvironmental archive and the dolomitisation process of Marinoan cap dolostones.

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.