Host Rock Material Research Articles

Bioconcentration and translocation of rare earth elements in plants collected from three legacy mine sites in Portugal

Rare earth elements (REE), a group of emerging contaminants with commercial and technological applications, share many physical and chemical characteristics and have thus been used as accurate tracers of various environmental samples. They have been shown to increase in receiving waters following the dissolution of host-rock material during mining activities. In this study, spontaneous vegetation and related media were collected from three Portuguese legacy mine sites in November 2020 to evaluate the phytoavailability and fate of REE. Water, soil and plant data were analyzed in the context of the 1) prevailing geochemical context, 2) the mining context, and 3) plant effects. This study presents the REE signatures for different plant species and links the signatures to a potential source of bioavailable REE. The REE accumulated in plant tissue seems to reflect the REE signature of surface waters in the mining areas, showing enrichment in middle REE. Although the soils, sediments, and waters in this study had similar features, certain plants seemed better adapted to translocating Light REE and Eu over others. Given that REE are readily available within the field conditions of a mining site, this study shows how plant physiology and biologic preference towards particular REE contribute to the fractionation of REE and create a unique signature dependent on plant type.

Numerical modeling of caldera formation using Smoothed Particle Hydrodynamics (SPH)

SUMMARYCalderas are kilometer-scale basins formed when magma is rapidly removed from shallow magma storage zones. Despite extensive previous research, many questions remain about how host rock material properties influence the development of caldera structures. We employ a mesh-free, continuum numerical method, Smoothed Particle Hydrodynamics (SPH) to study caldera formation, with a focus on the role of host rock material properties. SPH provides several advantages over previous numerical approaches (finite element or discrete element methods), naturally accommodating strain localization and large deformations while employing well-known constitutive models. A continuum elastoplastic constitutive model with a simple Drucker–Prager yield condition can explain many observations from analogue sandbox models of caldera development. For this loading configuration, shear band orientation is primarily controlled by the angle of dilation. Evolving shear band orientation, as commonly observed in analogue experiments, requires a constitutive model where frictional strength and dilatancy decrease with strain, approaching a state of zero volumetric strain rate. This constitutive model also explains recorded loads on the down-going trapdoor in analogue experiments. Our results, combined with theoretical scaling arguments, raise questions about the use of analogue models to study caldera formation. Finally, we apply the model to the 2018 caldera collapse at Kīlauea volcano and conclude that the host rock at Kīlauea must exhibit relatively low dilatancy to explain the inferred near-vertical ring faults.

A numerical study on THM coupled behavior in the high-level radioactive waste disposal system

It is essential to securely isolate high-level radioactive waste from the biosphere, and geologic disposal of it at a deep underground repository is considered the most effective method. Therefore, it is crucial to research the complex thermo-hydro-mechanical-chemical (THMC) coupled behavior in geological disposal systems parallel with numerical simulation. DECOVALEX is an international cooperating project to efficiently develop numerical methods and models and validate through test results for predicting the THMC interactions in the disposal systems. In Task C of DECOVALEX-2023, the modeling teams focus on understanding pore pressure development and THM interactions in the host rock and buffer material during the FE experiment, a 1:1 scale in-situ heater test based on Nagra’s Reference repository design. We used OGS-FLAC3D for the numerical simulation, combining OpenGeoSys for TH simulation and FLAC3D for M simulation. At the first phase of the task, a simple two-dimensional benchmark problem was defined to set up the numerical model. THM coupled processes in the bentonite were simulated with a two-phase flow system, and we investigated the temperature and pressure variations on the given monitoring position. Vaporization induced by a temperature increase in the bentonite was observed at the heater’s vicinity, and flow occurred by capillarity, and pressure difference was well simulated. Additionally, the flow process was dominant in the near field of engineering barrier, while we observed thermal pressurization in the far-field area. We plan to apply the developed model to a full-scale three-dimensional numerical simulation for the next phase.

How geomechanical attributes control the morphology of stylolites in carbonate sequences? A case from Permian–Triassic gas reservoirs in the Persian Gulf

Carbonate sequences are strongly affected by chemical compaction during their burial diagenesis. In the Persian Gulf, mechanically and chemically-compacted Permian–Triassic reservoirs host the World’s largest gas reservoir. Statistical analysis of sedimentological, geomechanical and petrophysical parameters of stylolite-bearing rock samples from drilled cores has been used to unravel the controls of geo-mechanical properties on the formation and distribution of stylolites and solution seams in these rocks. The textural and mineralogical composition of samples has been defined based on petrographic studies. Geomechanical tests (i.e., Schmidt hammer, Brazilian, Uniaxial compressional and Triaxial tests) of drill core and plug samples have been integrated with petrophysical logs (i.e., sonic, density, and gamma-ray) and, accordingly, seven geomechanical units (GMUs) have been defined. For the morphological analysis of stylolites, their amplitude, wavelength, insoluble residue thickness, length of insoluble materials along the stylolite, density, spacing, intersection type and connectivity have been measured from cores and plugs. All these data collectively allow defining the statistical relationships of stylolite morphology with the textural–mineralogical classes of their host rocks and their GMUs. The results of this study revealed that there is a generally negative correlation between rock strength, stylolite frequency and amplitude. The GMUs with lower rock strength and stylolite number (and higher porosity) show higher stylolite amplitude. Stylolites have larger wavelengths in limestone and limy dolomite lithologies and, textually, in mudstone/ wackestone and packstone facies. The thickness of insoluble materials shows a power-law distribution, with a weak correlation with stylolite amplitudes. There is a negative relationship between stylolite intensity and matrix porosity, which is related to the dissolution of host rock materials along the stylolite surface and then precipitation in adjacent pore spaces and cause differences in cementation in vicinity of the stylolites. Consequently, deciphering these relationships will help to model the stylolite distribution at well- to field scales, and to make more reliable predictions of Permian–Triassic reservoirs in the Persian Gulf and in other carbonate sequences all around the World.

Analysis of coupled thermal-hydro-mechanical processes in Callovo-Oxfordian clay rock: From full-scale experiments to the repository scale

Management of energy waste such as high-level waste (HLW) disposal in deep geological repositories is one of most pressing challenges in the context of nuclear energy. The general research question addressed in the present work is how to represent coupled physical processes after the emplacement of heat-emitting waste in such a repository in claystone at multiple scales by numerical models and how can they validated by experimental results in underground research laboratories. Numerical modelling of the coupled thermal hydraulic mechanical (THM) processes to assess the long-term safety of deep geological repositories rests on adequate constitutive laws and quantification of in-situ host rock material properties during long-term heating as well as a reliable numerical approach for solving the underlying governing equations. For this purpose, various in-situ experiments have been conducted in the Callovo-Oxfordian claystone (COx) at the Meuse/Haute-Marne (M/HM) Underground Research Laboratory (URL) by the French National Radioactive Waste Management Agency (ANDRA) since 2003. A subset of these is investigated here using a numerical THM model employed at different spatial scales. Two main results were achieved. First, the previously developed numerical THM model (Wang et al., 2021) has been compared agains in-situ experimental data from the full-scale heating (ALC) experiment at the M/HM URL in an attempt to validate the model. The numerically simulated temperature and pore pressure data of the ALC experiment are in good agreement with in-situ measured data. This model was then employed to simulate fully coupled THM processes of an entire repository section for High-Level Waste (HLW) in a COx clay formation according to the Cigeo project by ANDRA. This upscaling was enabled by running the numerical code on high-performance-computing platforms. Three-dimensional, fully coupled and efficient THM models for the numerical simulation of deep geological repositories such as the one presented in this study will help in the design of nuclear waste repositories under realistic geological conditions. Moreover, the present work is a contribution to Task E of the DECOVALEX-2019 project for model validation and comparison against experimental results (Plúa et al., 2021; Seyedi et al., 2020).

A two-scale time dependent damage model for preferential gas flow in clayey rock materials

Understanding of the gas migration within the host rock is critical for the safety assessment of a deep geological repository for radioactive wastes. However, localized gas flow in clayey host rock materials is a complex behavior associated with dynamic and unstable network of dilatant pathways, which are accompanied by micro-cracking that indicates macroscopic tensile fractures. The subcritical crack propagation at the microscale may represent the mechanism of time dependent damage observed at the macroscale. A two-scale damage model is developed in this study to explicitly simulate the preferential gas flow in clayey rock materials. A homogenization method based on asymptotic developments is employed to deduce the macroscopic damage behavior coupled with the poroelastic system, which initiates from the periodically distributed microstructures with micro-cracks. A time dependent damage evolution law is constructed based on the microscopic phenomena and the corresponding intrinsic permeability model is proposed which implicitly accounts for the fracture opening induced permeability change. The local macroscopic response of the model is analyzed and validated against the experimentally measured direct tensile strength, which illustrates the homogenized elastic and permeability coefficients, and highlights the influence of several parameters, i.e., the initial damage, the microstructural size and the strain rate. Numerical examples are presented in order to illustrate the global macroscopic response, in which a pure mechanical test, i.e., uniaxial tension test is simulated and verified against the laboratory results, then the simulation of preferential gas flow is illustrated together with the comparison of experimental explanation. The numerical results showed that the proposed two-scale model can explicitly simulate the gas induced fracturing, in which the damage propagation and the dilatant gas pathways are well captured.

Structure and kinematics of an extensional growth fold, Hadahid Fault System, Suez Rift, Egypt

Abstract. Normal faulting drives extensional growth folding of the Earth's upper crust during continental extension, yet we know little of how fold geometry relates to the structural segmentation of the underlying fault. We use field data from the Hadahid Fault System, Suez Rift, Egypt, to investigate the geometry and kinematics of a large (30 km long, up to 2.5 km displacement), exceptionally well-exposed normal fault system and to test and develop models for extensional growth folding. The Hadahid Fault System comprises eight up to 5 km long segments that are defined by unbreached or breached monoclines. These segments are soft-linked, hard-linked, or defined by a more subtle along-strike transition in overall structural style. High overlap : separation (O:S) ratios between its segments suggest the Hadahid Fault System comprises a single, now hard-linked structure at depth. We demonstrate that a progressive loss of at-surface displacement along-strike of the Hadahid Fault System results in surface-breaking faults and breached monoclines being replaced by unbreached monoclines developed above blind faults. However, shorter along-strike length-scale variations in structural style also occur, with unbreached monoclines developed between breached monoclines. The origin of this variability is unclear, but it might reflect local variations in host rock material properties that drive short length-scale variations in fault propagation-to-slip ratio, and thus the timing and location of fold breaching. We show that folding is a key expression of the strain that accumulates in areas of continental extension, arguing that tectono-sedimentary models for rift development should capture the related structural complexity.

Three-dimensional lattice simulation of hydraulic fracture interaction with natural fractures

Determination of the hydraulic fracture (HF) propagation patterns in fractured unconventional reservoirs typically assumes that natural fractures (NFs) persist through the full height of the reservoir/HF with uniform properties. Here a fully-coupled lattice simulator is employed to simulate the 3D interaction between HFs and NFs with spatially-varied mechanical and geometrical properties, thus capturing the dependence of HF propagation patterns on the NF cemented proportions and/or height observed in analogue laboratory experiments, and matching a 3D analytical criterion for simulation cases with different NF cohesion, friction coefficient, tensile strength, location of cemented regions, cemented proportions, and/or host rock materials.

Experimental and DEM investigations of temperature effect on pure and interbedded rock salt

Rock salt is considered as a favorable host rock material for natural gas storage due to its low permeability, fast self-healing ability and unique creep behavior. Characterizing engineering properties of rock salt especially interbedded salt under complicated geological storage conditions associated with temperature variations remains challenging. This research integrates physical and numerical experiments to investigate the temperature effect on the engineering behavior of pure and interbedded salt. Thermo-mechanical coupled triaxial compression tests are carried out on both pure (Ash-grey Rock Salt and Charcoal-grey Rock Salt) and impure (Rock Salt with Mudstone interbedded) rock salt specimens. To reveal the underlying mechanism of thermal-induced deformation and cracking of salt at grain scale, we configure a discrete element model and incorporate smooth-joint contacts to capture the interfacial behavior in the bedded salt. Experimental results indicate that the compressive strength and failure mode of rock salt is highly susceptible to temperature. The increasing temperature enhances the ductility of salt whereas lowers its peak compressive strength. Numerical results provide a micromechanical explanation that the attenuated compressive strength of salt under high temperature is attributed to the evolution of micro-cracks. The presence of the mudstone layer increases the overall material strength, confines the crack propagation orientations, and therefore limits the transverse deformation of the bedded salt. This study is expected to bring new insights into the micro-mechanical study of bedded salt and improve the long-term assessment of geological storage facilities.

Variable sulfur isotope composition of sulfides provide evidence for multiple sources of contamination in the Rustenburg Layered Suite, Bushveld Complex

The Rustenburg Layered Suite (RLS) of the Bushveld Complex (BC) is famous for its platinum group element (PGE) ore, which is hosted in sulfides. The source of sulfur necessary to generate this type of mineralization is inferred to be the host rock of the intrusion. The RLS has a sulfur isotopic signature that indicates the presence of Archean surface-derived material (Δ33S≠0) in the magma. This signature, with an average value of Δ33S=0.112±0.024‰, deviates from the expected Δ33S value of the mantle of 0±0.008‰. Previous work suggested that this signature is uniform throughout the RLS, which contrasts with radiogenic isotopes which vary throughout the igneous stratigraphy of the RLS.In this study, samples from key intervals within the igneous stratigraphy were analyzed, showing that Δ33S values vary in the same stratigraphic levels as Sr and Nd isotopes. However, the variation is not consistent; in some levels there is a positive correlation and in others a negative correlation. This observation suggests that in some cases distinct magma pulses contained assimilated sulfur from different sources. Textural analysis shows no evidence for late addition of sulfur. These results also suggest that it is unlikely that large-scale assimilation and/or efficient mixing of host rock material in a single magma chamber occurred during emplacement.The data do not uniquely identify the source of sulfur in the different layers of the RLS, but the variation in sulfur isotope composition and its relationship to radiogenic isotope data calls for a reevaluation of the models for the formation and evolution of the RLS, which has the potential to impact the knowledge of how PGE deposits form.

Constraining the timing of shale detachment faulting: A geochemical approach

K-Ar dating of illite in fault gouges is a useful tool for constraining the timing of brittle fault movement; however, this can be problematic in fault gouges hosted in clay-rich rocks due to the influence of host-rock material. Therefore, this study employs a multianalytical geochemical approach to unravel the influence of host-rock mineralogy, as well as fault zone development, on ages from fault-gouge samples in a shale detachment zone. K-Ar dating of the ≥2 μm fraction of 6 samples from the Sap Bon Formation detachment zone and associated fault zones in the Khao Khwang fold-thrust belt of central Thailand yielded an age range of 262 ± 5.4 to 208 ± 4.6 Ma. Carbon and oxygen stable isotope analysis along with X-ray diffraction mineralogy indicate that the samples with the youngest K-Ar ages are characterized by higher grade clay mineralogy, and hotter, orogenic fluid temperatures. Using these proxies and comparison to existing geochronology of the study area, we correlated K-Ar illite ages to one of three stages of fault zone evolution: detrital, diagenetic (burial), and authigenic (fault movement). The youngest K-Ar dates in the Sap Bon Formation are contemporaneous with recently published zircon province data indicating that faulting and detachment zone formation in the Sap Bon Formation were occurring by the mid-Late Triassic, with deformation continuing as late as the Rhaetian.

Experimental study and stress analysis of rock bolt anchorage performance

A new method was developed to apply pull-and-shear loads to the bolt specimen in order to evaluate the anchorage performance of the rebar bolt and the D-Bolt. In the tests, five displacing angles (0°, 20°, 40°, 60°, and 90°), two joint gaps (0 mm and 30 mm), and three kinds of host rock materials (weak concrete, strong concrete, and concrete-granite) were considered, and stress–strain measurements were conducted. Results show that the ultimate loads of both the D-Bolt and the rebar bolt remained constant with any displacing angles. The ultimate displacement of the D-Bolt changed from 140 mm at the 0° displacing angle (pure pull) to approximately 70 mm at a displacing angle greater than 40°. The displacement capacity of the D-Bolt is approximately 3.5 times that of the rebar bolt under pure pull and 50% higher than that of the rebar bolt under pure shear. The compressive stress exists at 50 mm from the bolt head, and the maximum bending moment value rises with the increasing displacing angle. The rebar bolt mobilises greater applied load than the D-Bolt when subjected to the maximum bending. The yielding length (at 0°) of the D-Bolt is longer than that of the rebar bolt. The displacement capacity of the bolts increased with the joint gap. The bolt subjected to joint gap effect yields more quickly with greater bending moment and smaller applied load. The displacement capacities of the D-Bolt and the rebar bolt are greater in the weak host rock than that in the hard host rock. In pure shear condition, the ultimate load of the bolts slightly decreases in the hard rock. The yielding speed in the hard rock is higher than that in the weak rock.

Extension fracture propagation in rocks with veins: Insight into the crack‐seal process using Discrete Element Method modeling

AbstractWe investigate how existing veins interact with extension fractures in rocks using 3‐D Discrete Element Method models with a geometry inspired by tension tests with notched samples. In a sensitivity study, we varied (a) the angle between the vein and the bulk extension direction and (b) the strength ratio between host rock and vein material. Results show a range of vein‐fracture interactions, which fall into different, robust, “structural styles.” Veins, which are weaker than the host rock, tend to localize fracturing into the vein, even at high‐misorientation angles. Veins, which are stronger than the host rock, cause deflection of the fracture tip along the vein‐host rock interface. Fractures are arrested at the interface from weak to stronger material. When propagating from a stronger to a weaker material, macroscopic bifurcation of the fracture is common. Complex interactions are favored by a low angle between the vein and the fracture and by high‐strength contrast. The structural styles in the models show good agreement with microstructures and mesostructures of crack‐seal veins found in natural systems. We propose that these structural styles form the basis for criteria to recognize strength contrasts and stress of crack‐seal systems in nature.

Comparison of microbiological influences on the transport properties of intact mudstone and sandstone and its relevance to the geological disposal of radioactive waste

AbstractThe role of the microbial activity on the transport properties of host rocks for geological repositories, particularly in the far-field, is an area of active research. This paper compares results from experiments investigating changes in transport properties caused by microbial activity in sedimentary rocks in Japan (mudstones) and sandstone (UK).These experiments show that both Pseudomonas denitrificans and Pseudomonas aeruginosa appear to survive and thrive in pressurized flow-through column experiments which utilized host rock materials of relevance to radioactive waste disposal. Indeed, despite there being a difference in the numbers of organisms introduced into both biotic experiments, numbers appear to stabilize at ∼105 ml−1 at their completion. Post experimental imaging has highlighted the distinct differences in biofilm morphology, for the chosen rock types and bacteria, with Pseudomonas aeruginosa derived biofilms completely covering the surface of the sandstone host and Pseudomonas denitrificans forming biofilament structures. Regardless of substrate host or choice of microbe, microbial activity results in measurable changes in permeability. Such activity appears to influence changes in fluid flow and suggests that the transport of radionuclides through the far-field will be complicated by the presence of microbes.

High Spatial Resolution Quantitative Imaging by Cross-calibration Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry and Synchrotron Micro-X-ray Fluorescence Technique

High spatial resolution, quantitative chemical imaging is of importance to various scientific communities, however high spatial resolution and robust quantification are not trivial to attain at the same time. In order to achieve microscopic chemical imaging with enhanced quantification capabilities, the current study links the independent and complementary advantages of two micro-analytical techniques - Synchrotron Radiation-based micro X-ray Fluorescence (SR-microXRF) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). A cross-calibration approach is established between these two techniques and validated by one experimental demonstration. In the presented test case, the diffusion pattern of trace level Cs migrating into a heterogeneous geological medium is imaged quantitatively with high spatial resolution. The one-dimensional line scans and the two-dimensional chemical images reveal two distinct types of geochemical domains: calcium carbonate rich domains and clay rich domains. During the diffusion, Cs shows a much higher interfacial reactivity within the clay rich domain, and turns out to be nearly non-reactive in the calcium carbonate domains. Such information obtained on the micrometer scale improves our chemical knowledge concerning reactive solute transport mechanism in heterogeneous media. Related to the chosen demonstration study, the outcome of the quantitative, microscopic chemical imaging contributes to a refined safety assessment of potential host rock materials for deep-geological nuclear waste storage repositories.

Formation and transfer of stoped blocks into magma chambers: The high-temperature interplay between focused porous flow, cracking, channel flow, host-rock anisotropy, and regional deformation

Magmatic stoping, i.e., the formation, transfer into, and movement through magma of older plutonic and metamorphic host-rock xenoliths, was widespread in the Mesozoic Sierra Nevada batholith (California, United States). However, the prevailing view that stoped blocks form by rapid thermal shattering and collapse into chambers may not be the dominant process of block formation and displacement into chambers in the Sierra Nevada. In detailed studies in and around the Tuolumne Batholith and Jackass Lakes pluton, we found evidence for the following history of block formation in slightly older, fairly isotropic plutonic host rocks: (1) low stress sites developed, leading to planar zones of increased porosity; (2) focused porous flow of first felsic melts followed by intermediate melts led to growth of magma fingers, which in turn led to increased porosity and loss of host-rock cohesion; and (3) connection of magmatic fingers resulted in the formation of dike-like channels in which flow facilitated removal of all host-rock material in these planar zones. Once formed, blocks were initially displaced by repeated magma injections along these channels, often resulting in unidirectional growth in these zones creating local magmatic sheeted complexes along block margins. Free block rotation occurred when sufficient nonlayered magma surrounded the host block; in some cases, segments of former sheeted zones remain attached to rotated blocks.

Formation of tabular plutons - results and implications of centrifuge modelling

Geophysical investigations reveal that many granitoid plutons possess a tabular shape: either laccolithic, lopolithic or phacolithic. In this study, the results of a centrifuge experiment are used to understand the formation mechanisms of these features. The model was build of a sequence of 14 differently coloured plasticine layers. Two buoyant layers - with a volume of c. 40 cm(3) each - were incorporated into the model stratigraphy at different depths to investigate, whether the rise and emplacement of buoyant material at different levels results in different intrusion structures. After centrifuging for 30 min at 700 G, both the buoyant layers had formed two lenticular sills (phacoliths) with aspect ratios (length/thickness) of 6 and 3.4 for the upper and lower phacoliths, respectively, directly above both pre-existing perturbations in the buoyant layers. During their movement, the buoyant phacoliths had pushed their roof plasticine upward. Simultaneously, their plasticine had subsided (bottom sinking). Subsidence of the material had choked the inflow of further buoyant material into the feeder channel of the developing sills and inhibited their further lateral growth. The observed forced downward movement of the plasticine of the forming PDMS (polydimethylsiloxane) phacoliths resembles the so-called floor of host rock material around an emplacing tabular pluton. Floor depression is supposed to be a very important vertical material transfer process, which provides space for the construction of lopo- and phacoliths. The subsidence of host material made space for the developing buoyant phacoliths, but also restricted their growth to a certain time slot before the influx of new buoyant material into the feeder dyke of the tabular intrusive body was shut off. Similarly, in nature, the growth of a tabular pluton might be limited not only by the rate of magma ascent and its physical properties, but also by the emplacement processes of the evolving pluton.

Biogenic and abiogenic low-Mg calcite (bLMC and aLMC): Evaluation of seawater-REE composition, water masses and carbonate diagenesis

The ΣREE and shale-normalized (PAAS) REE SN values of modern brachiopods (biogenic low-Mg calcite: bLMC) represented by several species from high- to low latitudes, from shallow- to deep waters and from warm- and cold-water environments, define three distinct average ‘seawater’ trends. The warm- and cold-water brachiopods define two indistinguishable ( p < 0.050) groups that mimic open-ocean seawater REE chemistry, exhibiting the typical LREE enrichment with a slightly positive to negative Ce anomaly followed by an otherwise invariant series. Other recent brachiopods from an essentially siliciclastic seabed environment are distinct in both ΣREE and REE SN trends from the previous two populations, showing a slight enrichment in the MREEs and an increasing trend in the HREEs. Other groups of modern brachiopods are characterized by elevated REE SN trends relative to the ‘normal’ groups as well as by complexity of the series trends. The most characteristic feature is the decrease in the HREEs in these brachiopods from areas of unusual productivity (i.e., such as upwelling currents, fluvial input and aerosol dust deposition). Preserved brachiopods from the Eocene and Silurian exhibit REE SN trends and Ce anomalies similar to that of the ‘open-ocean’ modern brachiopods, although, their enriched ΣREE concentrations suggest precipitation of bLMC influenced by extrinsic environmental conditions. Preservation of the bLMC was tested by comparing the ΣREE and REE SN trends of preserved Eocene brachiopods to those of Oligocene brachiopods that were altered in an open diagenetic system in the presence of phreatic meteoric-water. The altered bLMC is enriched by approximately one order of magnitude in both ΣREE and REE SN trends relative to that in bLMC of their preserved counterparts. Similarly, the ΣREE and REE SN of preserved Silurian brachiopod bLMC were compared to those of their enclosing altered lime mudstone, which exhibits features of partly closed system, phreatic meteoric-water diagenesis. Despite these differences in the diagenetic alteration systems and processes, the ΣREEs and REE SN trends of the bLMC of altered brachiopods and of originally mixed mineralogy lime mudstones (now diagenetic low-Mg calcite) are enriched by about one order of magnitude relative to those observed in the coeval and preserved bLMC. In contrast to the changes in ΣREE and REE SN of carbonates exposed to phreatic meteoric-water diagenesis, are the REE compositions of late burial calcite cements precipitated in diagenetically open systems from burial fluids. The ΣREE and REE SN trends of the burial cements mimic those of their host lime mudstone, with all showing slight LREE enrichment and slight HREE depletion, exhibiting a ‘chevron’ pattern of the REE SN trends. The overall enrichment or depletion of the cement REE SN trends relative to that of their respective host rock material reflects not only the openness of the diagenetic system, but also strong differences in the elemental and REE compositions of the burial fluids. Evaluation of the (Ce/Ce*) SN and La = (Pr/Pr*) SN anomalies suggests precipitation of the burial calcite cements essentially in equilibrium with their source fluids.

Hydro-mechanical, geochemical and mineralogical characteristics of the bentonite buffer in a heater experiment: The HE-B project at the Mont Terri Rock Laboratory

A small scale heater experiment was realized at the underground laboratory in Mont Terri, Switzerland. The effects of heat-producing waste in a repository for high-level waste and spent fuel were simulated by a heater element of 10 cm diameter, held at a constant surface temperature of 100 °C. The heater element was embedded in an engineered barrier of compacted bentonite blocks that were water saturated for 35 months before the 18 months heating phase was started. The host rock is a highly consolidated stiff Jurassic clay stone (Opalinus Clay). Extensive on-site and laboratory tests were performed to determine the hydro-mechanical, geochemical and mineralogical characteristics of the bentonite buffer to identify variations in the material properties in response to heating. Other studies deal with the host rock material. The results indicate that the bentonite shows only very weak modifications during the heater experiment. Whereas mineralogical alterations (e.g. transformations and formation of new phases) could not be measured, very weak changes in layer charge density and interlayer chemical composition of the smectite were found. Changes in the porosity and surface area of the bentonite may be explained as result of cementing or aggregation processes. These processes affect some THM characteristics like porosity, water uptake and thermal properties.

The development of bone-shaped structures in initially segmented layers during layer-parallel extension: numerical modelling and parameter sensitivity analysis

In orogenic zones the geometric occurrence of specific tectonic structures can often be related to contrasting rheological properties of rock materials. One of such tectonic structures is layer-perpendicular naturally fractured layers with vein infill that underwent subsequent deformation. Internal deformation by coaxial extension of these segmented layers, with host-rock material less competent than the vein material, gives rise to the development of bone-shaped structures (i.e. dogbones). In this paper, the formation of such dogbones is modelled using finite element techniques. A parameter sensitivity analysis of the model demonstrates that the degree of extension of the host-rock segments, their initial aspect ratio and the competence contrast between vein material and host-rock are the controlling parameters for the final shape of the dogbones during coaxial extension. The results, moreover, suggest that the specific geometry of the dogbones can serve to quantify competence contrasts between the host-rock and vein material during flow at geological strain rates.