Aqueous–solid solution thermodynamic model of U(VI) uptake in C–S–H phases

Reliable thermodynamic models assessing the interaction of radionuclides with cementitious materials are important in connection with long-term predictions of the safe disposal of radioactive waste in cement-based repositories. In this study, a geochemical model of U(VI) interaction with calcium silicate hydrates (C–S–H phases), the main component of hardened cement paste (HCP), has been developed. Uranium(VI) sorption isotherms on C–S–H phases of different Ca:Si ratios (C:S) and structural data from spectroscopic studies provided the indispensable set of experimental data required for the model development. This information suggested that U(VI) is neither adsorbed nor incorporated in the Ca–O octahedral layers of the C–S–H structure, but rather is located in the interlayer, similar to Ca²⁺ and other cations. With a view to the high recrystallisation rates and the cryptocrystalline ‘gel-like’ structure of the C–S–H phases, these observations indicated a U(VI) uptake driven by the formation of a solid solution.     

Compressibility of Ca3Al2Si3O12 perovskite up to 55 GPa

Compressibility of perovskite-structured Ca₃Al₂Si₃O₁₂ grossular (GrPv) was investigated at high pressure and high temperature by means of angle-dispersive powder X-ray diffraction using a laser-heated diamond anvil cell. We observed the Pbnm orthorhombic distortion for the pure phase above 50 GPa, whereas below this pressure, Al-bearing CaSiO₃ perovskite coexists with an excess of corundum. GrPv has a bulk modulus (K ₀ = 229 ± 5 GPa; \(K_{0}^{{\prime }}\) fixed to 4) almost similar to that reported for pure CaSiO₃ perovskite. Its unit-cell volume extrapolated to ambient conditions (V ₀ = 187.1 ± 0.4 Å³) is found to be ~2.5 % larger than for the Al-free phase. We observe an increasing unit-cell anisotropy with increasing pressure, which could have implications for the shear properties of Ca-bearing perovskite in cold slabs subducted into the Earth’s mantle.     

Elemental and isotopic (Si and O) tracing of glass alteration mechanisms

To better understand glass alteration mechanisms, especially alteration layers formation, leaching experiments of a borosilicate glass (SON68) doped with a different rare earth element (La, Ce, or Nd) with solutions rich in ²⁹Si and ¹⁸O were carried out. The coupled analyses of glass, alteration products, and solution led to a complete elemental and isotopic (²⁹Si and ¹⁸O) budget. They revealed different behaviours of elements that depend not only on their structural role in the glass, but also on their affinity for alteration products (gel, phyllosilicates, phosphates). However, analyses of both glass and solution are not sufficient to describe the real exchanges occurring between glass and solution. The use of ²⁹Si and ¹⁸O tracers gives new insights on the formation of alteration layers. During alteration the phyllosilicates records the isotopic variations of the leaching solution. Their isotopic signatures highlight a mechanism of dissolution/precipitation, which implies equilibrium between the secondary phases and the solution. On the other hand the gel isotopic signature, that is intermediate between the glass and the solution, substantiates the hypothesis that the gel is formed by hydrolysis/condensation reactions. This mechanism can thus explain the influence of the gel formation conditions (alteration conditions, solution saturation) on the structure (reorganisation) and texture (porosity) of this layer and on its protective effect. These hydrolysis/condensation reactions are also certainly involved in the aluminosilicate glass alteration and in the formation of palagonite.     

Behaviour of lithium and its isotopes during weathering in the Mackenzie Basin, Canada

We report Li isotopic compositions, for river waters and suspended sediments, of about 40 rivers sampled within the Mackenzie River Basin in northwestern Canada. The aim of this study is to characterize the behaviour of Li and its isotopes during weathering at the scale of a large mixed lithology basin. The Mackenzie River waters display systematically heavier Li isotopic compositions relative to source rocks and suspended sediments. The range in δ⁷Li is larger in dissolved load (from +9.3‰ to +29.0‰) compared to suspended sediments (from −1.7‰ to +3.2‰), which are not significantly different from δ⁷Li values in bedrocks. Our study shows that dissolved Li is essentially derived from the weathering of silicates and that its isotopic composition in the dissolved load is inversely correlated with its relative mobility when compared to Na. The highest enrichment of ⁷Li in the dissolved load is reported when Li is not or poorly incorporated in secondary phases after its release into solution by mineral dissolution. This counterintuitive observation is interpreted by the mixing of water types derived from two different weathering regimes producing different Li isotopic compositions within the Mackenzie River Basin. The incipient weathering regime characterizing the Rocky Mountains and the Shield areas produces ⁷Li enrichment in the fluid phase that is most simply explained by the precipitation of oxyhydroxide phases fractionating Li isotopes. The second weathering regime is found in the lowland area and produces the lower δ⁷Li waters (but still enriched in ⁷Li compared to bedrocks) and the most Li-depleted waters (compared to Na). Fractionation factors suggest that the incorporation of Li in clay minerals is the mechanism that explains the isotopic composition of the lowland rivers. The correlation of boron and lithium concentrations found in the dissolved load of the Mackenzie Rivers suggests that precipitation of clay minerals is favoured by the relatively high residence time of water in groundwater. In the Shield and Rocky Mountains, Li isotopes suggest that clay minerals are not forming and that secondary minerals with stronger affinity for ⁷Li appear.Although the weathering mechanisms operating in the Mackenzie Basin need to be characterized more precisely, the Li isotope data reported here clearly show the control of Li isotopes by the weathering intensity. The spatial diversity of weathering regimes, resulting from a complex combination of factors such as topography, geology, climate and hydrology explains, in fine, the spatial distribution of Li isotopic ratios in the large drainage basin of the Mackenzie River. There is no simple relationship between Li isotopic composition and chemical denudation fluxes in the Mackenzie River Basin.     

Seismic stratigraphy of the NW Moroccan Atlantic continental shelf and Quaternary deformation at the offshore termination of the southern Rif front

A recent high resolution seismic survey was carried out along the North-West Moroccan Atlantic continental shelf to perform sequence stratigraphy and to better understand the link between the onshore observations across the western termination of the Rif front and the multibeam bathymetric data previously collected across the Gulf of Cadiz. Seismic interpretation clearly shows evidence of deformation affecting the Pleistocene sedimentary cover. This is mainly located between the Lalla Zahra ridge and the Neogene nappes that outcrop at the seabed to the North of Larache, where the identified structures bound the upper part of a gravitationally spreading lobe and are interpreted to be developed by activation of the major tectonic structures located at the southern front of the Rif Cordillera. The offshore extension of the Lalla Zahra Ridge is characterised by an east-west trending major folded and faulted corridor. This structure marks the active front of Prerifan area and corresponds to a major Quaternary kinematic boundary in the western prolongation of the ENE-WSW trending Jebha fault.