Search

Middle Jurassic Opalinus Clay (OPA) in Switzerland and Southern Germany is considered as host rock for the disposal of heat-generating radioactive waste. For the present study, core samples from the Mont Terri rock laboratory (Switzerland) were investigated using a facies-based approach including mineralogical and geochemical analyses. Special focus was on the analysis of the degree of ordering of irregular illite-smectite interstratified clay minerals (I-S) responsible for sorption of radionuclides and swelling properties of the rocks. The investigations support the classification of OPA into five main facies (instead of the established threefold division) and into further subfacies. The facies concept was extended to the Passwang Formation (PWF). The amount of clay fraction varies in samples from the different facies, which is consistent with variations in cation exchange capacity (CEC). X-ray diffraction (XRD) analyses of the <2 μm fraction revealed a homogenous composition of the main constituents, namely R1-ordered I-S, kaolinite, and illite. Crystal structure based Rietveld refinement indicates strong similarities in the nature of disorder of the interstratified illite-smectite minerals for all investigated facies. The I-S-phase can be described with the R1 model, which is together with the amount of illitic layers in the I-S an indicator for a maximum burial temperature between 90 and 105°C. The amount of illitic layer in the I-S varies from 73%–85% for all refinements and ordering types, and in all OPA and PW facies and subfacies. This, together with the homogenous composition of the clay size fraction points to (1) a homogenous overprint of the clay assemblage during burial and/or (2) a uniform siliciclastic sedimentary supply throughout the sedimentation period of the OPA. The results of this study highlight the need to combine lithofacial studies with mineralogical investigations in order to adequately capture the variability of clay-rich host rocks under consideration for radioactive waste disposal. • For the first time the structural parameters of I-S for all subfacies of OPA are characterized. • X-ray diffraction analyses revealed a homogenous composition of the <2 μm fraction. • The averaged proportion of illite layers in the I-S is 77% for the OPA, and 74% for the PWF. • The uniformity facilitates the calibration/extrapolation of mineralogical models or geophysical logs. • The transferability of these calibrations to other clay formation should be critically examined. [ABSTRACT FROM AUTHOR]

A five-year-old interface between a Low Alkali Concrete (LAC) formulation (CEM III/B containing 66% slag and 10% nano-silica) and Opalinus Clay (OPA) from a field experiment at Mont Terri Underground Rock Laboratory in Switzerland (Jenni et al., 2014) has been studied to decipher the textural, mineralogical and chemical changes that occurred between the two reacting materials. Reactivity between LAC concrete and OPA is found to be limited to a ∼1 mm thick highly porous (ca. 75% porosity) white crust developed on the concrete side. Quantitative mineralogical mapping of the white crust using an electron microprobe and infrared spectroscopy on the cement matrix provides evidence of a Mg-rich phase accounting for approximatively 25 wt % of the matrix associated with 11 wt % of calcite, calcium silicate hydrate (C-S-H) and other cement phases. EDX analyses and electron diffraction combined with transmission electron microscopy of the Mg-rich phase provide evidence for a tri-octahedral 2:1 phyllosilicate with mean composition: (Ca 0.5±0.2 ) (Mg 2.0±0.4 , Fe 0.2±0.1 , Al 0.5±03 , □ 0.3±0.3 ) (Al 0.9±0.2 , Si 3.1±0.2 ) O 10 (OH) 2 , where □ represents vacancies in the octahedral site. Apart from this reactive contact, textural, mineralogical and chemical modifications at the contact with the LAC concrete are limited. OPA mineralogy remains largely unmodified. X-ray micro-fluorescence and EPMA mapping of major elements on the OPA side also provides evidence for a Mg-enriched 300–400 μm thick layer. The cation exchange capacity (CEC) values measured in the OPA in contact with the LAC concrete range between 153 and 175 meq kg −1 of dry OPA, close to the reference value of 170 ± 10 meq kg −1 of dry OPA (Pearson et al., 2003). Changing cation occupancies at the interface with LAC concrete are mainly marked by increased Ca, Mg and K, and decreased Na. Leaching tests performed on OPA with deionized water and at different solid to water ratios strongly suggest that Cl and SO 4 have either conservative behaviour or are constrained by the solubility of a precipitated sulfate phase. The Cl and SO 4 concentrations measured at 2 cm from the interface are close to concentrations of undisturbed OPA pore waters (SO 4 : 4.5 ± 1.5 mmol kg −1 of dry OPA; Cl: 7.5 ± 2.1 mmol kg −1 of dry OPA), and increase towards the interface with the concrete. The SO 4 to Cl ratio also increases towards the interface, suggesting that the increasing anion concentrations are not related to porosity variations but rather to a concentration gradient and sulfate phase precipitation near the interface. [ABSTRACT FROM AUTHOR]

The Middle Jurassic Burgundy carbonate platform occupied a central part of the Central European Epeiric Sea during the Middle Jurassic. The facies architecture of the oolitic calcarenite bodies was affected by tidal currents on the platform and relative sea-level changes. The δ13C-values of inorganic carbonates, sampled in biostratigraphic-defined intervals, do not match very well between closely spaced sections and, hence, are of restricted use for stratigraphic purposes. It appears that the platform interior might have been decoupled from the global carbon pool. Although deposited in a rapidly accumulating setting, the recorded isotope signatures might be affected by some local stratigraphic gaps. Nonetheless, the carbon isotope data imply lateral changes of the platform waters; these appear to be related to the position on the platform and to the sediment dispersal pattern, as evidenced by clay minerals. Adjacent to the eastern margin of the platform, detrital chlorite and illite occur in considerable proportions, both ascribed to a boreal source to the east and the north-east. In contrast, smectite-rich mixed-layer clay mineral content increases significantly towards the platform interior, pointing to a delivery from the north-west. All these data are suggestive of an overall clockwise current pattern in the Central European Epeiric Sea during the Middle Jurassic. [ABSTRACT FROM AUTHOR]

Abstract: Interactions between climate and soil remain ambiguous, particularly when silicate weathering and clay mineral formation and transformation rates are considered in relation to global climate changes. Recent studies suggest that climate affects weathering rates much less than previously thought. Here we show that the climate in the central European Alps has a significant, but indirect, influence on the weathering of soils through vegetation. The pattern of element leaching and mineral transformations is not only due to precipitation and temperature. Element leaching was greatest in subalpine forests near the timberline; weathering is lessened at higher and lower altitudes. Vegetation, therefore, contributes significantly to weathering processes. The highest accumulation of organic matter was found in climatically cooler sites (subalpine range) where the production of organic ligands, which enhance weathering, is greatest. Patterns of smectite formation and distribution had strong similarities to that of the elemental losses of Fe and Al (R =0.69; P <0.01) or base cations (R =0.58; P <0.05). Higher precipitation rates and the production of organic chelating compounds in the soil promoted the appearance of smectites. The relationship between climate, element leaching (Fe, Al, Ca, Mg, K, Na), and smectite formation is strongly nonlinear and driven by the podzolisation process, which is more pronounced near the timberline because of the bioclimatic constellation. Climate warming will probably, in the future, lead to a decrease in SOM stocks in the subalpine to alpine range because of more favourable conditions for biodegradation that would also affect weathering processes. [Copyright &y& Elsevier]

Abstract: High-resolution clay-mineral analyses were performed on upper Hauterivian to lower Aptian sediments along a platform-to-basin transect through the northern Tethyan margin from the Neuchâtel area (Switzerland), to the Vocontian Trough (France) in order to investigate links between climate change, carbonate platform evolution, and fractionation patterns in clay minerals during their transport. During the Hauterivian, the northern Tethyan carbonate platform developed in a heterozoan mode, and the associated ramp-like topography facilitated the export of detrital material into the adjacent basin, where clay-mineral assemblages are dominated by smectite and kaolinite is almost absent, thereby suggesting dry-seasonal conditions. During the Late Hauterivian Balearites balearis ammonite zone, a change to a more humid climate is documented by the appearance of kaolinite, which reaches up to 30% of the clay fraction in sediments in the Vocontian Trough. This prominent change just preceded the Faraoni Oceanic Anoxic Event and the onset of the demise of the Helvetic Carbonate Platform, which lasted to the late early Barremian. From the Late Barremian onwards, the renewed growth of the northern Tethyan carbonate platform in a photozoan mode and the associated development of a marginally confined platform topography fractionated the clay-mineral assemblages exported into hemipelagic settings: kaolinite particles were preferentially retained in proximal, platform settings, due to their size and their relatively high specific weight. In the inner platform environment preserved in the Swiss Jura, an average of 32% of kaolinite in the clay fraction is observed during the latest Barremian–earliest Aptian, whereas clay-mineral assemblages of coeval sediments from deeper depositional settings are dominated by smectite and show only minor amounts of kaolinite. This signifies that besides palaeoclimate conditions, the morphology and ecology of the carbonate platform had a significant effect on the distribution and composition of clay assemblages during the Late Hauterivian–Early Aptian along the northern Tethyan margin. [Copyright &y& Elsevier]