Zebra pattern in rocks as a function of grain growth affected by second-phase particles

Alternating fine grained dark and coarse grained light layers in rocks are often termed zebra patterns and are found worldwide. The crystals in the different bands have an almost identical chemical composition, however second-phase particles (e.g., fluid filled pores or a second mineral phase) are concentrated in the dark layers. Even though this pattern is very common and has been studied widely, the initial stage of the pattern formation remains controversial. In this communication we present a simple microdynamic model which can explain the beginning of the zebra pattern formation. The two dimensional model consists of two main processes, mineral replacement along a reaction front, and grain boundary migration affected by impurities. In the numerical model we assume that an initial distribution of second-phase particles is present due to sedimentary layering. The reaction front percolates the model and redistributes second-phase particles by shifting them until the front is saturated and drops the particles again. This produces and enhances initial layering. Grain growth is hindered in layers with high second-phase particle concentrations whereas layers with low concentrations coarsen. Due to the grain growth activity in layers with low second-phase particle concentrations these impurities are collected at grain boundaries and the crystals become very clean. Therefore, the white layers in the pattern contain large grains with low concentration of second-phase particles, whereas the dark layers contain small grains with a large second-phase particle concentration. The presence of the zebra pattern is characteristic for regions containing Pb-Zn mineralization. Therefore, the origin of the structure is presumably related to the mineralization process and might be used as a marker for ore exploration. A complete understanding of the formation of this pattern will contribute to a more accurate understanding of hydrothermal systems that build up economic mineralization

Reaction-induced porosity fingering: replacement dynamic and porosity evolution in the KBr-KCl system

In this contribution, we use X-ray computed micro-tomography (X-CT) to observe and quantify dynamic pattern and porosity formation in a fluid-mediated replacement reaction. The evolution of connected porosity distribution helps to understand how fluid can migrate through a transforming rock, for example during dolomitization, a phenomenon extensively reported in sedimentary basins. Two types of experiment were carried out, in both cases a single crystal of KBr was immersed in a static bath of saturated aqueous KCl at room temperature and atmospheric pressure, and in both cases the replacement process was monitored in 3D using X-CT. In the first type of experiment a crystal of KBr was taken out, scanned, and returned to the solution in cycles (discontinuous replacement). In the second type of experiment, 3 samples of KBr were continuously reacted for 15, 55 mins and 5.5 hours respectively, with the latter being replaced completely (continuous replacement). X-CT of KBr-KCl replacement offers new insights into dynamic porosity development and transport mechanisms during replacement. As the reaction progresses the sample composition changes from KBr to KCl via a K(Br,Cl) solid solution series which generates porosity in the form of fingers that account for a final molar volume reduction of 37% when pure KCl is formed. These fingers form during an initial and transient advection regime followed by a diffusion dominated system, which is reflected by the reaction propagation, front morphology, and mass evolution. The porosity develops as fingers perpendicular to the sample walls, which allow a faster transport of reactant than in the rest of the crystal, before fingers coarsen and connect laterally. In the continuous experiment, finger coarsening has a dynamic behaviour consistent with fingering processes observed in nature. In the discontinuous experiment, which can be compared to rock weathering or to replacement driven by intermittent fluid contact, the pore structure changes from well-organized parallel fingers to a complex 3D connected network, shedding light on the alteration of reservoir properties during weathering

Bio-oil yield potential of some tropical woody biomass

Six tropical biomass samples namely: Ogbono wood (Irvingia wombolu), Mango wood (Mangifera indica), Neem wood (Azadiracta indica), Ogbono shell (Irvingia wombolu), Ogirisi wood (Neubouldia laevis) and Tropical Almond wood (Terminalia catappa) were pyrolyzed in a bench scale screw reactor at 450oC. The physicochemical properties of the samples were determined prior to the pyrolysis experiments. The bio-oil and bio-char produced were similarly characterized using standard procedures established by American Standard and Test Methods (ASTM). The highest bio-oil yield of 66 wt% and least bio-oil yield of 53 wt% were obtained from Neem wood and Tropical Almond wood respectively. The characterization results of the products show that even though the moisture content of the bio-oil was quite higher than those of the original feedstock, their higher heating values were higher than those of the parent feedstock. Both characterization results show that the feedstock and their fast pyrolysis products are good materials for bioenergy production. The Gas Chromatography Mass Spectroscopy (GC-MS) analysis of the bio-oil shows the presence of useful chemicals such as phenols and levoglucosan, which could be harnessed for industrial applications

Outcrop scale mixing enhanced by permeability variations: the role of stationary and travelling waves of high saturation indices

To study the ore mineralization at the outcrop scale we merge an advection–diffusion simulation with the geochemical software iphreeqc to model the mixing of two realistic fluids. We simulate the infiltration of a metal-rich fluid into a rock that is saturated with pore fluid. We test the feedback effects with a number of scenarios based on an outcrop-scale 5 × 5 m model consisting of two high-permeable vertical faults within a low-permeable host rock that lead into a permeable layer. The hot metal-rich fluid enters the model through the faults from below. We solve the advection–diffusion equation for 12 chemical species and temperature, and use iphreeqc to determine the resulting properties of local fluid domains as well as related saturation indices for minerals. The faults in the model act as pathways for the metal-rich fluid, with the infiltrating fluid displacing the pore fluid. Mixing in the model takes place as a function of advection along permeable faults coupled with diffusion of chemical species at the interface between two fluids, while heat diffusion is fast enough (103 times faster) to equilibrate temperature. Simulations show a high saturation index of mixing-derived minerals such as barite at the interface between the two fluids as a result of fluid mixing. Fast fluid pathways (i.e. faults) show travelling waves of high saturation indices of barite, while low-permeability zones such as fault walls and areas below less permeable layers experience stationary waves of high saturation indices. Our results show that, depending on the dominant transport process (advection or diffusion), mineralization will localize next to permeability contrasts in zones where local diffusion dominates

Relative rates of fluid advection, elemental diffusion and replacement govern reaction front patterns

Replacement reactions during fluid infiltration into porous media, rocks and buildings are known to have important implications for reservoir development, ore formation as well as weathering. Natural observations and experiments have shown that in such systems the shape of reaction fronts can vary significantly ranging from smooth, rough to highly irregular. It remains unclear what process-related knowledge can be derived from these reaction front patterns. In this contribution we show a numerical approach to test the effect of relative rates of advection, diffusion, and reaction on the development of reaction fronts patterns in granular aggregates with permeable grain boundaries. The numerical model takes (i) fluid infiltration along permeable grain boundaries, (ii) reactions and (iii) elemental diffusion into account. We monitor the change in element concentration within the fluid, while reactions occur at a pre-defined rate as a function of the local fluid concentration. In non-dimensional phase space using Péclet and Damköhler numbers, results show that there are no rough fronts without advection (Péclet10−3). As advection becomes more dominant and reaction slower, roughness develops across several grains with a full microstructure mimicking replacement in the most extreme cases. The reaction front patterns show an increase in roughness with increasing Péclet number from Péclet 10 to 100 but then a decrease in roughness towards higher Péclet numbers controlled by the Damköhler number. Our results indicate that reaction rates are crucial for pattern formation and that the shape of reaction fronts is only partly due to the underlying transport mechanism

Zebra dolomites of the Spessart, Germany: implications for hydrothermal systems of the European Zechstein Basin

Zebra dolomites have a distinctive texture and are a peculiar structural variety of dolostones often encountered in the vicinity of base metal deposits commonly in the Mississippi Valley-Type (MVT). We investigate origin and evolution of the zebra dolomites found in the region of the Spessart, northwestern Bavaria, Germany, through diagenetic and petrogenetic analysis using SEM, CL microscopy, O–C isotopes, and fluid inclusion micro-thermometry. Here, we aim to shed light on the nature of the fluids that altered the zebra dolomite of the Zechstein formation. We distinguish the geochemical signatures of two different fluid flow regimes post-dating texture formation, each characterized by specific homogenization temperatures and oxygen–carbon isotope ratios (Event 1: Th = 120 °C; δ18Ofluids = [0 to 2‰]; Event 2; Th = 300 °C; δ18Ofluids = 18‰). Comparison of these fluids and the associated mineralization with published regional fluid flow data support that the zebra dolomites in the Spessart most likely coincided with the Permian large-scale fluid flow event that occurred throughout the European Zechstein Basin

Pattern formation in Mississippi valley-type deposits - identifying one of nature's fundamental processes in geologic systems

Nature has a range of distinct mechanisms that cause initially heterogeneous systems to break their symmetry and form patterns. The study of pattern formation and the behaviour of non-linear systems have interested scientists across many disciplines from physics, chemistry, biology, and economics to geosciences. In study, a new mechano-chemical process that leads to the formation of complex periodic wave- or stripe-like zebra patterns in rocks will be presented. The genesis of periodically banded dolostones, which host lead-zinc mineralization, has been studied for several years, because an evolutionary relationship between the banded dolomites and mineralized areas is highly likely. To date, a complete generic model has not been formulated for the formation of these zebra rocks and there is an ongoing debate on the exact processes leading to the genesis of the pattern. In the first part of this work, new analytical findings obtained from zebra dolomites from Peru and Germany will be presented. The zebra dolomites from Germany have never been described before and represent the first known zebra dolomite deposit in Germany. Based on the analytical finding, a numerical and an analytical model were developed in the second part of this thesis. The combination of the numerical and the analytical model yields a new approach to the zebra pattern formation based on one of nature’s fundamental processes for wave-like pattern formation in geological systems. This approach also includes a new inversion routine based on the spacing of the respective pattern

Messung geogener und anthropogener elektromagnetischer Strahlung zur Erkundung von tektonischen Störungen

Verschiedene Studien haben ergeben, dass sowohl vor als auch während eines Erdbebens natürliche elektromagnetische Strahlung ausgesandt werden kann. Mittels Laborexperimenten erfolgte zudem der Nachweis, dass die Emission der elektromagnetischen Wellen mit Bruchvorgängen im Gestein korreliert. Da aktive tektonische Störungen durch permanent auftretende Mikrobrüche gekennzeichnet sind, sollte in deren Umfeld eine erhöhte elektromagnetische Strahlung im Frequenzbereich von MHz bis ggf. kHz messbar sein. Für die hier vorgestellten Kartierungen von Störungen im Südschwarzwald, an der Hunsrück-Südrand-Störung sowie im nordöstlichen Mainzer Becken wurde ein Gerät verwendet, welches die Magnetfeldkomponente im Frequenzbereich von 5 bis 50 kHz erfasst. Allerdings überlagern elektromagnetische Wellen militärischer VLF-Sender mögliche geogene Signale aktiver Störungen. Bei der VLF-Methode werden gerade diese anthropogenen Signale zur Erkundung des Untergrundes genutzt, da sie in leitfähigen Strukturen Ströme induzieren, die ihrerseits ein sekundäres phasenverschobenes Magnetfeld hervorrufen. Somit stellt sich die Frage, ob sich die kartierten Störungen infolge ihrer Aktivität oder infolge ihrer elektrischen Leitfähigkeit in den Messkurven abzeichnen. Zukünftige Arbeiten sollen daher Aufschluss über die mögliche Ursache geben

Promoting effects of metal iodides in Ni(isoq)<SUB>4</SUB>Cl<SUB>2</SUB>-catalyzed carbonylation of ethanol and n-propanol

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Zebra rocks: Compaction waves create ore deposits

© 2017 The Author(s). Nature has a range of distinct mechanisms that cause initially heterogeneous systems to break their symmetry and form patterns. One of these patterns is zebra dolomite that is frequently hosting economically important base metal mineralization. A consistent generic model for the genesis of these periodically banded rocks is still lacking. In this contribution, we present for the first time a fully consistent mathematical model for the genesis of the pattern by coupling the reactive fluid-solid system with hydromechanics. We show that visual banding develops at a given stress and host-rock permeability indicating that the wavelength and occurrence of the pattern may be predictable for natural settings. This finding offers the exciting possibility of estimating conditions of formation of known deposits as well as forecasting potential exploration targets