Seismic and mechanical properties of Opalinus Clay: comparison between sandy and shaly facies from Mont Terri (Switzerland)

For the safe disposal of high-level radioactive waste, different host rocks are currently being considered. The favorable properties of clay are low permeability, some retention capacity concerning radionuclides, and the ability to self-seal cracks and fissures, e.g. by swelling or time-dependent compaction creep. In Switzerland, the Jurassic Opalinus Clay is envisaged as a potential host rock which—at Mont Terri—is subdivided into the sandy, shaly, and carbonate-rich facies, the latter being less abundant. For long-term safety assessments, the understanding of the relations of properties (e.g. mineralogical composition and microstructure) and performance (e.g. mechanical behavior) of clays and claystones is essential. In the case of the sandy Opalinus Clay, the mechanical strength increases with increasing carbonate content, because carbonates form the matrix. The mineralogical investigation of a set of sandy facies samples proved a significantly larger carbonate content (20–40 mass %) when compared to the shaly facies (10–20 mass %). The carbonates of the shaly Opalinus Clay, on the other hand, are mostly localized fossils aligned parallel to the bedding, acting as predetermined breaking points. Image analysis of SEM images of polished sections proved the determined microstructural differences. In addition, carbonate particles of the sandy facies are mostly isometric, whereas carbonate particles of the shaly facies cover a greater range of shapes. The mechanical tests were accompanied by investigations of the p- and s-wave velocities, which revealed that the anisotropy of the sandy facies is less pronounced than sedimentological analyses would suggest. The mechanical strength, which, for the first time, presents results of real triaxial tests of the sandy facies. The samples of the sandy facies exhibit a failure strength of σ _eff,B, approximately twice as high as was found for the shaly facies considering the deformation axis parallel to the bedding. Similar values were obtained when measuring perpendicularly to the bedding.     

Solute transport in formations of very low permeability: profiles of stable isotope and dissolved noble gas contents of pore water in the Opalinus Clay, Mont Terri, Switzerland

Pore water profiles of water, stable isotope, and dissolved noble gas content have been determined across the Opalinus Clay and adjacent formations at the rock laboratory at Mont Terri. We have found enhanced helium contents (up to [⁴He] = 1 × 10⁻⁴ cubic centimeters at standard pressure and temperature per gram of pore water) and argon isotope ratios (⁴⁰Ar/³⁶Ar ratios up to 334) due to accumulation of ⁴He and ⁴⁰Ar produced in situ. The helium profile was found to be in steady state with respect to in situ production and diffusive loss into the adjacent limestones where groundwater circulates. From this profile a representative mean value of the apparent diffusion coefficient for helium in the pore water of the whole formation was derived for the first time to be D_a = 3.5 × 10⁻¹¹ m² · s⁻¹, which is more than two orders of magnitude lower than the diffusion coefficient D₀ in free water. The stable isotope profile, however, indicates a component of fossil marine pore water, which has not yet been replaced by molecular diffusion of meteoric water from the adjacent limestone and shale formations over the past 10 million years.     

Mechanical validation of balanced cross-sections: The case of the Mont Terri anticline at the Jura front (NW Switzerland)

Close to north-west front of the Jura thrust belt in Switzerland, the Mont Terri anticline is located at a disruption of the Muschelkalk decollement, due to a former normal fault. Numerous geological data are available thanks to a motorway tunnel cross-cutting the fold and an underground laboratory. Original field data, two geological maps, a borehole, and a highway tunnel constrain the inner structure. The anticline is characterized by an overturned frontal limb, a steep back limb, and a ramp cross-cutting older normal faults. Three geological cross-sections and three 2D kinematic models are proposed as various combinations of fault-bend folding, fault-propagation folding and detachment folding. The main motivation of this study is to illustrate how geometrical constructions can be constrained by the kinematic approach of limit analysis, which determines the optimal deformation, including faults, that verifies mechanical equilibrium and the Coulomb criterion. Matching some of the important kinematic steps assumed in each three interpretations to the optimal deformation leads us to define the compatible ranges of values of the frictional parameters. These ranges allow us to assess the likeliness of the important kinematic steps of each kinematic model. The present analysis provides a quantitative link between the assumed evolution of the internal structure, and the concomitant evolutions of the topography and of the frictional parameters.     

Biogeochemical processes in a clay formation in situ experiment: Part E - Equilibrium controls on chemistry of pore water from the Opalinus Clay, Mont Terri Underground Research Laboratory, Switzerland

The chemistry of pore water (particularly pH and ionic strength) is an important property of clay rocks being considered as host rocks for long-term storage of radioactive waste. Pore waters in clay-rich rocks generally cannot be sampled directly. Instead, their chemistry must be found using laboratory-measured properties of core samples and geochemical modelling. Many such measurements have been made on samples from the Opalinus Clay from the Mont Terri Underground Research Laboratory (URL). Several boreholes in that URL yielded water samples against which pore water models have been calibrated. Following a first synthesis report published in 2003, this paper presents the evolution of the modelling approaches developed within Mont Terri URL scientific programs through the last decade (1997-2009). Models are compared to the composition of waters sampled during dedicated borehole experiments. Reanalysis of the models, parameters and database enabled the principal shortcomings of the previous modelling efforts to be overcome. The inability to model the K concentrations correctly with the measured cation exchange properties was found to be due to the use of an inappropriate selectivity coefficient for Na-K exchange; the inability to reproduce the measured carbonate chemistry and pH of the pore waters using mineral-water reactions alone was corrected by considering clay mineral equilibria. Re-examination of the measured Ca/Mg activity ratios and consideration of the mineralogical composition of the Opalinus Clay suggested that Ca/Mg cation exchange rather than dolomite saturation may control the ratio of these ions in solution. This re-examination also suggests that the Ca/Mg ratio decreases with increasing pore-water salinity. Several possible reasons for this are proposed. Moreover, it is demonstrated that feldspar equilibria must not be included in Opalinus Clay modelling because feldspars are present only in very small quantities in the formation and because Na/K ratios measured in pore water samples are inconsistent with feldspar saturation. The principal need to improve future modelling is additional or better data on rock properties, in particular: (i) a more detailed identification of phases in the Opalinus Clay that include redox-sensitive elements together with evaluation of their thermodynamic properties; (ii) an improved understanding of the distribution of celestite throughout the Opalinus Clay for Sr/SO₄ concentrations control; (iii) improvements in analytic and thermodynamic data for Ca-Mg rock cation exchange and mineral chemical properties and (iv) the measurement of composition and stability constants of clay minerals actually present in the formation.     

Biogeochemical processes in a clay formation in situ experiment: Part A - Overview, experimental design and water data of an experiment in the Opalinus Clay at the Mont Terri Underground Research Laboratory, Switzerland

An in situ test in the Opalinus Clay formation, termed porewater chemistry (PC) experiment, was carried out for a period of 5 years. It was based on the concept of diffusive equilibration whereby a traced water with a composition close to that expected in the formation was continuously circulated and monitored in a packed-off borehole. The main original focus was to obtain reliable data on the pH/pCO₂ conditions of the porewater, but because of unexpected microbiologically-induced redox reactions, the objective was extended to elucidate the biogeochemical processes occurring in the borehole and to understand their impact on pH/pCO₂ and porewater chemistry in the low permeability clay formation.The behaviour of the conservative tracers ²H and Br⁻ could be explained by diffusive dilution in the clay and moreover the results showed that diffusive equilibration between the borehole water and the formation occurred within about 3 year’s time. However, the composition and pH/pCO₂ conditions differed considerably from those of the in situ porewater. Thus, pH was lower and pCO₂ was higher than indicated by complementary laboratory investigations. The noted differences are explained by microbiologically-induced redox reactions occurring in the borehole and in the interfacial wall area which were caused by an organic source released from the equipment material. The degradation of this source was accompanied by sulfate reduction and - to a lesser extent - by methane generation, which induced a high rate of acetogenic reactions corresponding to very high acetate concentrations for the first 600 days. Concomitantly with the anaerobic degradation of an organic source, carbonate dissolution occurred and these processes resulted in high pCO₂ and alkalinities as well as drop in pH. Afterwards, the microbial regime changed and, in parallel to ongoing sulfate reduction, acetate was consumed, leading to a strong decrease in TOC which reached background levels after about 1200 days. In spite of the depletion of this organic perturbation in the circuit water, sulfate reduction and methanogenesis continued to occur at a constant rate leading to near-to-constant concentrations of sulfate and bicarbonate as well as pH/pCO₂ conditions until the end of the experiment. The main sink for sulphur was iron sulfide, which precipitated as FeS (am) and FeS₂.The chemical and isotopic composition was affected by the complex interplay of diffusion, carbon degradation rates, mineral equilibria and dissolution rates, iron sulfide precipitation rates, and clay exchange reactions. The ¹³C signals measured for different carbon species showed significant variations which could only be partly explained. The main cations, such as Na, Ca and Mg remained remarkably constant during the experiment, thus indicating the strong buffering of the formation via cation and proton exchange as well as carbonate dissolution/precipitation reactions.     

Large-scale tracer profiles in a deep claystone formation (Opalinus Clay at Mont Russelin,Switzerland): Implications for solute transport processes and transport properties of the rock

Natural tracers (Cl^? and stable water isotopes) in pore water of the Opalinus Clay and adjacent formations were studied in the motorway tunnel at Mont Russelin, Switzerland. The Opalinus Clay occurs in the core of an anticline which is cut by a complex system of thrust faults. Concentration profiles of natural tracers were taken from 17 boreholes along a 363 m long section. Pore waters of drillcore samples were analysed with indirect and direct methods. The Cl^? and stable water isotope distribution in the pore water shows a regular and well defined profile, with a conspicuous decrease towards the overlying Dogger limestone aquifer. The highest Cl^? values (approximately 23,000 mg/L) are found in the core of the anticline in Liassic claystones underlying the Opalinus Clay. To quantify the large-scale transport properties of the Opalinus Clay formation, a 2D transport model was constructed and used to reproduce the observed concentration profiles. The calculations indicate that the observed tracer distributions are consistent with diffusion as the dominant transport process. Groundwater flow in the overlying Dogger aquifer was initiated about 2–4 Ma ago, which is long after the folding of the Jura Mountains and probably coincides with the exposure of the aquifer to freshwater recharge following continued erosion of the anticline. The calculations suggest that tracer distributions are controlled by 1) the timing of freshwater recharge in the overlying limestone aquifer, 2) the shape of the anticline and 3) the magnitude and the anisotropy ratio of diffusion coefficients.     

Faults and fractures in the Gallery 04 of the Mont Terri rock laboratory: Characterization,simulation and application

This paper deals with the reconstruction of a fracture network observed in an underground gallery of a tunnel buried in clay stones in Switzerland, below Mont Terri. The trace maps of the Gallery 04 and of the EZ-G niche of this site have first been digitized and used in various ways to characterize the data. The traces have been divided into two groups, i.e., the pre-existing faults in the gallery and the fractures of the excavated damaged zone (EDZ) in the niche. Each group has been analyzed individually. The number of data for the faults has turned out to be statistically significant, while it was very limited for the EDZ fractures. For both cases, the major statistical characteristics of the traces have been extracted and a reconstruction procedure developed and tested. A full example has been worked out; a gallery immersed in a reconstructed fractured porous medium has been meshed, and the electrical field created by a dipole at the wall calculated by solving the three dimensional Laplace equations in the fractures and in the porous medium.     

Features of mineral and chemical composition of the Khamambettu Carbonatites, Tamil Nadu

The paper presents mineralogical features and EPMA results of the Khamambettu carbonatites. The mineralogical data suggest that these rocks have been generated in magmatic and hydrothermal stages. Mineral geothermometer for carbonatite give temperatures of 790°–980°C. Fluid inclusion measurements in monazite (hydrothermal stage) give temperatures of 220°–290°C. One of the features of the carbonatites is high content of magnesia that is defined by the presence of dolomite, olivine, spinel, phlogopite, Mg-rich ilmenite. Chloritization, serpentinization, amphibolization, silicification processes and occurrence of barite, monazite-(Ce), strontianite, celestine are related to hydrothermal stage. Hydrothermal minerals at the Khamambettu were formed by recrystallization of primary carbonatite minerals in the presence of Ba, (SO₄)^2?, REE and Si carried in solution by the hydrothermal fluid.     

Modeling of Cs+ diffusion and retention in the DI-A2 experiment (Mont Terri). Uncertainties in sorption and diffusion parameters

In the DI-A2 experiment several non-reactive and reactive tracers were injected as a pulse in a packed-off borehole in the Opalinus Clay. Unlike the previous DI-A1 test, the design of the Teflon filter in the injection borehole forced the water to flow through the filter and the open space between the filter and the borehole wall (the filter itself did not act as a diffusion barrier between the circulating solution and the rock). The decrease in tracer concentration in the liquid phase was monitored during a period of a year. Afterwards, the borehole section was overcored and the tracer profiles in the rock were analyzed. A main interest of this experiment was to understand the chemical behavior of sorbing tracers: Cs⁺ (stable), ⁸⁵Sr²⁺, ⁶⁰Co²⁺ and Eu³⁺ (stable). The complete dataset (except for Eu³⁺ because of strong sorption to experimental equipment) was analyzed in a previous study with a 2D diffusion–reaction model and the derived diffusion and sorption parameters were compared with laboratory data. As in DI-A1, a difference by a factor of about 2 for sorption (magnitude of the Freundlich isotherm) was obtained between in situ and laboratory batch sorption experiments.Recent experimental and modeling studies have shown equivalent Cs⁺ sorption on intact and disaggregated Opalinus Clay samples. In view of these developments, new modeling of Cs⁺ diffusion and retention in the DI-A2 experiment has been performed using CrunchFlow. The calculations include transport by diffusion and a multisite cation exchange model to account for the retention of Cs⁺. The new results show that upscaling of Cs⁺ sorption from laboratory to field is no longer required. However, a difference in sorption by a factor of about 2 is still explained by the use of different versions of the same cation exchange model (a small difference in the selectivity coefficient for one type of site). This uncertainty in sorption leads to an uncertainty in the effective diffusion coefficient (D_e) for Cs⁺, also by a factor of 2 (2–4 × 10⁻¹⁰ m²/s). Clearly, the values of D_e obtained are correlated with the strength of sorption in the model, with stronger sorption leading to larger D_e values. Discrimination between the two versions of the exchange model is not possible when using only the results of the in situ test. Additionally, during early times (t < 10 days) the drop in Cs⁺ concentration in the circulation system is slower than expected. Due to the experimental setup, this slow decrease in concentration cannot be caused by the filter in the contact between borehole and rock. Poor mixing in the circulation system could explain this effect.     

Specific chemical and mineral composition of ferromanganese nodules from the Chukchi Sea

The specific chemical and mineral composition of discoid, cake-shaped, and platy ferromanganese nodules (FMNs) from the Chukchi Sea are considered. The main ore components of these FMNs are Fe, Mn, and P. The contents of trace elements (except for Ba and Sr) do not exceed hundredths of percent. Maximum concentrations of most of these elements are specific for the cake-shaped nodules. In general, Mn accumulates most intensely in FMNs. Next (in order of decrease) are Ag(?), Co, and Pb; Sr, Fe, P, Y, and Ca; and Ni, La, Zn, and Cu. As for Ba, Cr, Mg, and K, they do not accumulate in the FMNs. “Dilution” is typical of Si, Al, Na, and Ti. The main ore phases are strengite and amorphous Mn hydroxides. It has been revealed for the first time for the Chukchi Sea that Cu, Zn, Sn, Ni, Pb, W, Bi, Cr, Fe, Ti, Ag, Au, Y, Zr, and La–Nd lanthanides form individual mineral microphases in FMNs: native elements, intermetallic compounds, oxides, and, much more seldom, tungstates, silicates, and phosphates. Accessory ore mineralization is the best pronounced and most diverse in the platy nodules. Though the FMNs from the Chukchi Sea are diagenetic, high-temperature fluids are, most likely, the source of microinclusions of various accessory ore minerals.     

Rocks control the chemical composition of surface water from the high Alpine Zermatt area (Swiss Alps)

The study presents composition data of 87 surface water samples from high alpine catchments of the Zermatt area (Swiss Alps). The investigated area covers 170 km². It was found that the surface runoff acquires the dissolved solids mostly by reaction of precipitation water with the minerals of the bedrock. Total dissolved solids (TDS) vary from 6 to 268 mg L^?1. All collected water shows a clear chemical signature of the bedrock mineralogy. The contribution of atmospheric input is restricted to small amounts of ammonium nitrate and sodium chloride. NH₄ is a transient component and has not been detected in the runoff. Evaporation is not a significant mechanism for TDS increase in the Zermatt area. The chemical composition of the three main types of water can be related to the mineralogy of the dominant bedrock in the catchments. Specifically, Ca-HCO₃ (CC) waters develop from metamorphic mafic rocks and from carbonate-bearing schists. Mg-HCO₃ water originates from serpentinites and peridotites. Ca-SO₄ (CS) waters derive from continental basement rocks such as pyrite-rich granite and gneiss containing oligoclase or andesine. The collected data suggest that, together with reaction time, modal sulfide primarily controls and limits TDS of the waters by providing sulfuric acid for calcite (CC waters) and silicate (CS waters) dissolution. If calcite is present in the bedrock, its dissolution neutralizes the acid produced by sulphide weathering and buffers pH to near neutral to weakly alkaline conditions. If calcite is absent, the process produces low-pH waters in gneiss and granite catchments. The type of bedrock and its mineral assemblage can be recognized in water leaving very small catchments of some km² area. The large variety of water with a characteristic chemical signature is an impressive consequence of the richly diverse geology and the different rock inventory of the local catchments in the Zermatt area.     

磷灰石化学组成研究进展: 成岩成矿过程示踪及对矿产勘查的指示

磷灰石广泛分布于各类岩石和矿床中, 根据其F、OH以及Cl阴离子种类可分为氟磷灰石、氯磷灰石和羟基磷灰石。磷灰石对其自身结构的变形以及化学取代有良好的耐受性, 一系列元素可以替代磷灰石中的Ca、P、F、Cl和OH, 因此其化学组成复杂。磷灰石中Ca位存在两种不同的阳离子位置, 影响不同元素在这两种位置取代的因素包括离子半径、价态以及磷灰石的种类等。岩浆过程中, 稀土(REE)、Y、Sr是磷灰石中的相容元素, 而F、Cl、OH的相容性随物理化学条件不同而变化。相较于元素在磷灰石与熔体相间的分配研究, 目前对于元素在磷灰石与流体相间的分配研究较少。磷灰石化学组成可以判别母岩类型: 如F、Cl、As、V含量可以区分Ⅰ型与S型花岗岩; REE、Sr、Y、Mn、As、Th的含量、轻稀土(LREE)的富集程度和球粒陨石标准化稀土元素配分模式可以用于判别碳酸岩、二辉橄榄岩、镁铁质岩石等岩石类型。利用磷灰石的F、Cl和CO₃等挥发分的含量, 并结合Sr-Nd同位素组成, 可以识别地幔源区及其流体交代作用。磷灰石Sr/Th与La/Sm的变化可用于指示岩浆来自脱水板块或熔融沉积物。磷灰石的主微量元素(Mn、Eu)含量或元素比值(F/Cl)能够反映岩浆的分异程度, 而且Ce/Pb和Th/U比值可以反映岩浆形成过程中流体参与程度。磷灰石中的变价元素如Eu、Ce、Ga和S含量及δEu和δCe可用于推断岩浆的氧化还原状态。由于磷灰石对热液交代作用非常敏感, 所以磷灰石的结构和化学组成已广泛用于成矿过程研究。在与花岗岩有关的关键金属矿床中, 磷灰石的主、微量元素含量和Sr-Nd同位素比值常常用于区分不同花岗质岩浆的源区特征, δCe、δEu也可以用于指示花岗质侵入体的氧化还原状态; 斑岩有关的多金属矿床F、Cl、S、Mn含量以及F/Cl比值能够指示成矿岩浆-热液的来源及演化, δCe、δEu可以用于指示岩浆热液的氧逸度; 铁氧化物-铜-金和铁氧化物-磷灰石矿床中磷灰石结构、共生稀土矿物结合Na、F、Cl以及REE含量能够反映与成矿有关的岩浆-流体作用以及流体性质。磷灰石中Mn、Fe、REE、Y、F、Cl、SO₃的含量或比值的差异可用于判别矿化与未矿化的岩石或者蚀变类型, 并用于区分不同矿床类型等, 对矿产勘查具有一定指示意义。磷灰石化学组成结合机器学习方法, 可以更好地识别磷灰石的来源。综上所述, 磷灰石作为一种新兴的指示矿物, 除了在成岩和成矿过程示踪方面体现出独特优势, 在矿产勘查应用方面也具有重要的应用前景。

     

Influence of the surface energy on the kinetics of chemical reactions of mineral nanoparticles

Doklady Earth Sciences -     

Mesozoic (Lower Jurassic) red stromatactis limestones from the Southern Alps (Arzo, Switzerland): calcite mineral authigenesis and syneresis-type deformation

The Broccatello lithological unit (Lower Jurassic, Hettangian to lower parts of Upper Sinemurian) near the village of Arzo (southern Alps, southern Switzerland) is a mound-shaped carbonate deposit that contains patches of red stromatactis limestone. Within the largely bioclastic Broccatello unit, the stromatactis limestone is distinguished by its early-diagenetic cavity system, a relatively fine-grained texture, and an in-situ assemblage of calcified siliceous sponges (various demosponges and hexactinellids). A complex shallow subsurface diagenetic pathway can be reconstructed from sediment petrography in combination with comparative geochemical analysis (carbon and oxygen isotopes; trace and rare earth elements, REE + Y). This pathway includes organic matter transformation, aragonite and skeletal opal dissolution, patchy calcification and lithification, sediment shrinkage, sagging and collapse, partial REE remobilization, and multiple sediment infiltration. These processes occurred under normal-marine, essentially oxic conditions and were independent from local, recurring syn-sedimentary faulting. It is concluded that the stromatactis results from a combination of calcite mineral authigenesis and syneresis-type deformation. The natural stromatactis phenomenon may thus be best explained by maturation processes of particulate polymer gels expected to form in fine-grained carbonate sediments in the shallow subsurface. Conditions favorable for the evolution of stromatactis appear to be particularly frequent during drowning of tropical or subtropical carbonate platforms.