Role of carbonate minerals in the distribution of trace elements in marine clay formations

International audienceAnthropogenic radionuclides (RN) are generated by a wide range of industrial and medical activities. In the contexts of waste storage, the quantification of RN migration is of paramount importance. RN migration is partly ruled by the interaction of RN with the solid surfaces. Usually experiments are conducted at various scales from laboratory to the field in order to measure retention and retardation parameters of radiotracers. Whereas this experimental approach is mandatory to tackle the issue of RN migration, the understanding of the natural speciation of stable isotopes that are analog to RN brings additional useful information. In particular, the RN natural speciation sheds light on RN isotopic exchange and " irreversible " trapping mechanisms. This study aims at overviewing the association of natural trace elements (U, Th, Ni, I, Sr and Zn) with carbonate minerals in the Callovian-Oxfordian sedimentary formation that is under consideration for deep nuclear waste disposal in France. The combined use of sequential extraction techniques, microscopic and spectrometric techniques, as well as laser-ablation coupled to chemical analysis techniques made it possible to establish the distribution of I, Sr, U, Th and Ni in the various mineral and organic phases present in the clay rock. I and Sr and in a less extent U and Th are mainly carried by carbonates while Ni is distributed in a variety of phases including pyrite, sphalerite, chlorite, organic matter and muscovite

Thermodynamic evidence of giant salt deposit formation by serpentinization: an alternative mechanism to solar evaporation

International audienceThe evaporation of seawater in arid climates is currently the main accepted driving mechanism for the formation of ancient and recent salt deposits in shallow basins. However, the deposition of huge amounts of marine salts, including the formation of tens of metres of highly soluble types (tachyhydrite and bischofite) during the Aptian in the South Atlantic and during the Messinian Salinity Crisis, are inconsistent with the wet and warm palaeoclimate conditions reconstructed for these periods. Recently, a debate has been developed that opposes the classic model of evaporite deposition and argues for the generation of salt by serpentinization. The products of the latter process can be called "dehydratites". The associated geochemical processes involve the consumption of massive amounts of pure water, leading to the production of concentrated brines. Here, we investigate thermodynamic calculations that account for high salinities and the production of soluble salts and MgCl2-rich brines through sub-seafloor serpentinization processes. Our results indicate that salt and brine formation occurs during serpentinization and that the brine composition and salt assemblages are dependent on the temperature and CO2 partial pressure. Our findings help explain the presence and sustainability of highly soluble salts that appear inconsistent with reconstructed climatic conditions and demonstrate that the presence of highly soluble salts probably has implications for global tectonics and palaeoclimate reconstructions

Study of dolomite dissolution at various temperatures – Evidence for the formation of nanocrystalline secondary phases at dolomite surface and influence on dolomite interactions with other minerals

International audienceIn most clay-rock geological formation studied for the storage of nuclear waste,pore water compositions are expected to be at equilibrium with carbonate minerals, which are always included in predictive models for pore water composition calculations [1]. Among the carbonates known to be present, dolomite may be problematic in the pore water composition calculation because its solubility spans a large range of values as a function of its crystallinity in thermodynamic databases. In addition, the composition of dolomite minerals observed in clay-rock formations such as Callovian-Oxfordian or Opalinus clay formation differs from this of a pure dolomite: the Ca/Mg stoichiometry is not ideal, and the minerals contain minor amounts of Fe and traces of many other elements [2]. To understand the influence of secondary phases precipitation during dolomite dissolution on pore water chemistry, the dissolution of monocrystals of dolomite were investigated at 25 °C and at 80 °C in a pH range 3 to 8 for various time periods (30 minutes to 21 days) in sealed PTFE reactors. Solution analyses evidenced a stoichiometric release of Ca and Mg in solution during dolomite dissolution. Scanning Electron Microscopy (SEM), Raman and X-Ray Diffraction (XRD) analyses did not evidence secondary Mg-bearing minerals precipitation, but revealed the formation of Fe-bearing particles on the dolomite surface. Morphological characterizations performed with Small-angle X-ray scattering (SAXS)evidenced that the precipitation occurs along a specific crystallographic plane of the dolomite monocrystal. Thus, the precipitated nanoparticles clustered on specific surface sites, and are made of Fe-rich phases poorly crystallized (carbonates, oxides and hydroxides)

Study of dolomite dissolution at various temperatures – Evidence for the formation of nanocrystalline secondary phases at dolomite surface and influence on dolomite interactions with other minerals

International audienceIn most clay-rock geological formation studied for the storage of nuclear waste,pore water compositions are expected to be at equilibrium with carbonate minerals, which are always included in predictive models for pore water composition calculations [1]. Among the carbonates known to be present, dolomite may be problematic in the pore water composition calculation because its solubility spans a large range of values as a function of its crystallinity in thermodynamic databases. In addition, the composition of dolomite minerals observed in clay-rock formations such as Callovian-Oxfordian or Opalinus clay formation differs from this of a pure dolomite: the Ca/Mg stoichiometry is not ideal, and the minerals contain minor amounts of Fe and traces of many other elements [2]. To understand the influence of secondary phases precipitation during dolomite dissolution on pore water chemistry, the dissolution of monocrystals of dolomite were investigated at 25 °C and at 80 °C in a pH range 3 to 8 for various time periods (30 minutes to 21 days) in sealed PTFE reactors. Solution analyses evidenced a stoichiometric release of Ca and Mg in solution during dolomite dissolution. Scanning Electron Microscopy (SEM), Raman and X-Ray Diffraction (XRD) analyses did not evidence secondary Mg-bearing minerals precipitation, but revealed the formation of Fe-bearing particles on the dolomite surface. Morphological characterizations performed with Small-angle X-ray scattering (SAXS)evidenced that the precipitation occurs along a specific crystallographic plane of the dolomite monocrystal. Thus, the precipitated nanoparticles clustered on specific surface sites, and are made of Fe-rich phases poorly crystallized (carbonates, oxides and hydroxides)

Effects of magnesium minerals representative of the Callovian-Oxfordian claystone on borosilicate glass alteration

La dissolution de verres borosilicatés en présence de minéraux magnésiens a été étudiée. Ces minéraux (dolomite, illite, smectite…) appartiennent à la couche géologique (Callovo-Oxfordien) destinée à accueillir le stockage des déchets nucléaires vitrifiés en France. Ils contiennent du magnésium, élément capable d'entretenir l'altération du verre lorsqu'il est disponible en solution. Dans les milieux confinés du stockage, la réactivité des solides contrôle la composition de la solution et peut être la force motrice de l'altération des verres nucléaires. Les expériences montrent que les carbonates magnésiens (hydromagnésite, dolomite) entretiennent l'altération du verre : la précipitation de silicates de magnésium empêche la recondensation du silicium dans la couche passivante en surface du verre. Plus le minéral magnésien est soluble, plus l'altération du verre est importante. Les phases argileuses purifiées (illite, smectite…) du Callovo-Oxfordien (COx) augmentent également l'altération du verre. La moitié du magnésium échangeable de ces phases a été remplacée par du sodium lors du protocole de purification. Dans ces conditions, l'effet des phases argileuses sur l'altération du verre est en partie dû au pH acide qu'elles imposent. Le modèle d'altération des verres GRAAL implémenté dans le code de transport réactif HYTEC a permis de confirmer et de quantifier les mécanismes identifiés à partir des expériences en système fermé. Des expériences en cellule de diffusion, deux compartiments séparés par une barrière diffusive inerte, ont permis de valider une modélisation du transport réactif. Ces expériences, plus représentatives des conditions de stockage, où le bloc de verre sera séparé du COx par les produits de corrosion des aciers, illustrent le ralentissement des cinétiques attendu compte tenu de l'éloignement du verre et des minéraux réactifs.Borosilicate glasses dissolution has been studied in presence of magnesium minerals. Those minerals (dolomite, illite, smectite…) belong to the Callovo-Oxfordian (COx) claystone layer, studied in France as a potential site for nuclear waste disposal. Such minerals contain magnesium, an element able to sustain glass alteration when it is available in solution. In the confined media of the wastes disposal, thesolids reactivity controls the solution composition and can be the driving force of nuclear glass alteration. Experiments show that magnesium carbonates (hydromagnesite and dolomite) increase in the glass alteration: the precipitation of magnesium silicates consumes silicon which slows down the formation of the glass passivating layer. The lower the magnesium mineral solubility, the lower the glass alteration.The purified clay phases (illite, smectite…) from the COx layer increase the glass alteration. Half the magnesium was remplaced by sodium during the purification process. In such conditions, the effect of clay phases on glass alteration is in part due to the acidic pH-buffering effect of the clay fraction. The GRAAL model implemented in the geochemical transport code HYTEC has confirmed and quantified the mechanisms put in evidence in the experiments. Cells diffusion experiments where the two solids were separated by an inert diffusion barrier allow to valid reactive transport modelling. Such experiments are more representative of the glass package which will be separated from the COx by corrosion products. They show that glass alteration rate is reduced when solids are not close

Étude de la dissolution de verres borosilicatés en présence de minéraux magnésiens modèles représentatifs des minéraux de l'argilite du Callovo-Oxfordien

Borosilicate glasses dissolution has been studied in presence of magnesium minerals. Those minerals (dolomite, illite, smectite…) belong to the Callovo-Oxfordian (COx) claystone layer, studied in France as a potential site for nuclear waste disposal. Such minerals contain magnesium, an element able to sustain glass alteration when it is available in solution. In the confined media of the wastes disposal, thesolids reactivity controls the solution composition and can be the driving force of nuclear glass alteration. Experiments show that magnesium carbonates (hydromagnesite and dolomite) increase in the glass alteration: the precipitation of magnesium silicates consumes silicon which slows down the formation of the glass passivating layer. The lower the magnesium mineral solubility, the lower the glass alteration.The purified clay phases (illite, smectite…) from the COx layer increase the glass alteration. Half the magnesium was remplaced by sodium during the purification process. In such conditions, the effect of clay phases on glass alteration is in part due to the acidic pH-buffering effect of the clay fraction. The GRAAL model implemented in the geochemical transport code HYTEC has confirmed and quantified the mechanisms put in evidence in the experiments. Cells diffusion experiments where the two solids were separated by an inert diffusion barrier allow to valid reactive transport modelling. Such experiments are more representative of the glass package which will be separated from the COx by corrosion products. They show that glass alteration rate is reduced when solids are not close.La dissolution de verres borosilicatés en présence de minéraux magnésiens a été étudiée. Ces minéraux (dolomite, illite, smectite…) appartiennent à la couche géologique (Callovo-Oxfordien) destinée à accueillir le stockage des déchets nucléaires vitrifiés en France. Ils contiennent du magnésium, élément capable d'entretenir l'altération du verre lorsqu'il est disponible en solution. Dans les milieux confinés du stockage, la réactivité des solides contrôle la composition de la solution et peut être la force motrice de l'altération des verres nucléaires. Les expériences montrent que les carbonates magnésiens (hydromagnésite, dolomite) entretiennent l'altération du verre : la précipitation de silicates de magnésium empêche la recondensation du silicium dans la couche passivante en surface du verre. Plus le minéral magnésien est soluble, plus l'altération du verre est importante. Les phases argileuses purifiées (illite, smectite…) du Callovo-Oxfordien (COx) augmentent également l'altération du verre. La moitié du magnésium échangeable de ces phases a été remplacée par du sodium lors du protocole de purification. Dans ces conditions, l'effet des phases argileuses sur l'altération du verre est en partie dû au pH acide qu'elles imposent. Le modèle d'altération des verres GRAAL implémenté dans le code de transport réactif HYTEC a permis de confirmer et de quantifier les mécanismes identifiés à partir des expériences en système fermé. Des expériences en cellule de diffusion, deux compartiments séparés par une barrière diffusive inerte, ont permis de valider une modélisation du transport réactif. Ces expériences, plus représentatives des conditions de stockage, où le bloc de verre sera séparé du COx par les produits de corrosion des aciers, illustrent le ralentissement des cinétiques attendu compte tenu de l'éloignement du verre et des minéraux réactifs

Experimental study of water-extractable sulphate in Opalinus Clay and implications for deriving porewater concentrations

In northern Switzerland, the Opalinus Clay, a Jurassic claystone formation, is foreseen as host rock for a deep geological repository for radioactive waste. Characterizing its porewater is of particular importance for assessing the mobility of radionuclides and the stability of the engineered barriers. Although the porewater composition of the Opalinus Clay is fairly well known, there is still controversy on the sources of sulphate obtained by different porewater characterization methods. A striking observation is that sulphate concentrations from aqueous extraction and recalculated to in-situ conditions are consistently much higher than sulphate concentrations measured in borehole waters, squeezed waters and advectively displaced waters (“excess sulphate”). Accordingly, the main objective of this study is to better investigate the processes affecting dissolved sulphate concentrations during aqueous extraction and, thus, to reduce uncertainties in predicting the concentrations of this compound in the Opalinus Clay porewater. To this end, a series of extraction experiments were conducted using variable solid/liquid ratios, extraction times and extract solutions. In order to suppress sulphide-mineral oxidation, all the experiments were performed in a glovebox under oxygen-free conditions (atmosphere and solutions). Measurements of the sulphur and oxygen isotope composition of the dissolved sulphate in aqueous extracts are aimed to further constrain the source of the “excess sulphate”. Finally, the plausibility of the SO4 data from extraction experiments in terms of their representativeness for in-situ conditions was evaluated by simple geochemical modelling. The modelling shows that SO4 concentrations from aqueous extracts recalculated to in-situ conditions imply dissolved and exchangeable cation concentrations which are not consistent with measured data, thus, attesting non-conservative behaviour for sulphate during aqueous extraction. However, the various extraction experiments showed that pyrite oxidation was successfully suppressed during the experiments and neither contributions from e.g. organic material, congruent calcite dissolution and/or sulphate mineral dissolution provide enough SO4 to explain the “excess sulphate”. Ultimately, the various extraction experiments failed to definitely identify the source of the “excess sulphate” in aqueous extracts. However, the good agreement found between the δ18O and δ34S values of dissolved SO4 in aqueous extracts and those of borehole waters suggest that the “excess sulphate” might be weakly bound to mineral surfaces

An improved thermodynamic model based on Pitzer’s equations to describe the chemical behaviour of complex chloride-type brines up to salt solubility and elevated temperatures, for geothermal and economic geology applications

International audienceThe context of global warming has changed our perception of the Earth’s subsurface, which is now seen as a source of renewable energy and of valuable elements contained in geothermal waters, as well as a vast geological reservoir for storing energy vectors or gases. Therefore, the description of the chemical processes occurring in deep and hot geological environments has recently gained renewed interest and contributes to the understanding of geological phenomena, including ore forming processes and the set-up of dehydrate formation layers1.Deep waters are most often salty and hot, which makes the description of their chemical behaviour a challenge. The present contribution focuses on developing a new thermodynamic model for the H-Li-Na-K-Ca-Mg-Cl-H2O chemical system, from dilute solutions up to salt solubility, and for temperatures up to 250°C. This model relies on the Helgeson-Kirkham-Flowers (HKF) and the Pitzer equations, and considers the partial dissociation of the CaCl2 electrolyte. It has been implemented in the PhreeSCALE2 geochemical calculation software to compute properties such as osmotic coefficient (Figure), heat capacity, enthalpy and density of chloride-bearing multi-electrolyte solutions in conditions relevant for the production of geothermal energy.The model is also able to predict the formation of highly soluble salts like tachyhydrite [CaCl2·2MgCl2·12H2O], observed in giant salts deposits, below the deep ocean’s crust, in the passive margin of the Atlantic Ocean. Recent works suggest that the formation of such salts may be related to the serpentinization process of the primary mantle peridotite that consumes important amounts of pore water and leads to extremely concentrated brines in hot environments.These encouraging results suggest that the model should be further extended and used for other applications, like the transport of dissolved metals from deep hot fluids to areas where metals in solution will be extracted from these fluids, and concentrated to make ore bodies.This study was done in the framework of the REFLECT Project, funded by the European Union’s Horizon 2020 research and innovation programme (grant agreement No 850626)

Modelling of the COx/glass interactions: case of the long term MVE experiment

International audienceHigh-level nuclear waste (HLW) is confined in a glass matrix packaged into stainless steel canister and carbon steel overpack. The Callovian–Oxfordian (COx) claystone layer located in the north-eastern Parisian basin is currently investigated as a potential host-rock. As the COx contains minerals that can feed the near-field with soluble elements (e.g. Mg) that can enhance the glass alteration, the study of the COx/glass interactions is of primary importance to evaluate the containment capacity of the glass over the time period required to the decrease of the radioactive elements. Several studies focused on clay behavior in temperature (Gailhanou et al., 2017), on the glass behavior (Gin et al., 2012) and on integrated system: glass/iron/clay (Schlegel et al., 2016) but no modelling study of the COx/glass interactions in situ at the temperature of the COx formation had already been made.This work aims at studying the glass alteration in presence of the COx in purely diffusive conditions. The studied glass is called SON68 and is the inactive analogous of the of the French R7T7 HLW glass. The vertical descending borehole is twelve meters deep and contains a two meters high test chamber with a series of three stacked clay blocks. Those blocks were drill in order to be filled with glass powder separated from the COx by sintered stainless steel filters (Linard et al., 2015). The test interval was then closed and saturated with synthetic porewater representative of the host-rock (Gaucher et al., 2009). To reach diffusive conditions, the pressure in the test interval was rose to 40 bars close to the expected pressure of the COx formation. A flow rate equal to 15 mL/minute between the interval and a module located in the drift is maintained. The pH, Eh, electrical conductivity, temperature and mass are measured online and samples can be collected at chosen time.A diffusive transport model based on the model developed by Appelo et al. (2008) was built. The hydrological parameters were implemented according to this work and to the particularity of the test (Linard, 2010). The model considers the COx whose parameters (minerals, exchangers) are based on the model published by Gaucher et al. (2009) and the glass (SON68). A glass alteration model based on the work of Frugier et al. (2008) has been implemented in Phreeqc. First the deuterium tracer was modeled with a porewater diffusion coefficient of 8 10-10 m2.s-1. The porosity of the formation was set to 0.18. Because of the complexity of the perturbation that occurred during the whole test, every external change influences the test is not yet considered in the modelling. Thus, the current model does not enable to reproduce the higher deuterium and iodine concentrations measured at 546 days (Figure 1.a) but match all the bromide data. Despite the difficulties on the hydrological parameters, the pH reaches a steady state in the modelling once the COx buffered the pH (Figure 1.b). The initial high pH is close to 9 that is the value imposed by the glass dissolution. This gap occurs because the system is closed in the model the first 427 days of reaction. However, it appears that several water entrance from the formation lower the pH in the system toward values usually encountered in the clay (pH 7.2). The modeled chlorine agrees with the measured one (Figure 1.b). This element is only influenced by the porewater chemistry and is not modified during the test. A difference appears for the sodium that is underestimated and remains at the value of the porewater injected at the beginning of the test. This discrepancy can be attributed to an underestimation of the water coming from the formation that is richer in sodium than the injected water and/or from the glass dissolution that is currently underestimated (Figure 1.b). The first explanation is assumed because the porewater measured in the field varies in function of the samples (Vinsot et al., 2008), so a difference with the injected water can occur. The second hypothesis is linked to the first one as a higher solution renewal will lead to higher glass dissolution and then to higher boron, lithium and sodium release in solution. Furthermore, desaturation process occurring during the drilling and leading to salt precipitation cannot be discarded. Those salt will dissolve during the entrance of the water coming from the formation (Vinsot et al., 2013). It is worth noting that the Si concentration is in equilibrium with cristobalite. Furthermore, modelling tests proved that secondary phases usually considered like hydroxyapatite or sepiolite highly influence the results and cannot be considered at thermodynamic equilibrium otherwise glass dissolution is overestimated.This model is a first step to investigate the interaction between the COx and the glass SON68. The several perturbations occurring during the test (water entrance, leakages, and thermal variations) have to be considered more accurately because they highly influence the system and then the glass dissolution. Improvements concerning these matters are already under progress

Mechanisms involved in the increase of borosilicate glass alteration by interaction with the Callovian-Oxfordian clayey fraction

International audienceThe influence of clay minerals of the Callovian-Oxfordian claystone on the alteration of international simple glass (ISG) was investigated through batch experiments conducted at 90 °C and via geochemical modeling. Several clay/glass weight ratios ranging from 0.9 to 91 were tested. These experiments revealed that the mechanisms controlling glass alteration were indirectly dependent on that ratio. The solution and solid characterizations indicated the presence of non-clay minerals such as pyrite, Mg-calcite, quartz, dolomite, and magnesite in the treated clay material, which influenced the results. The clay minerals were poorly reactive. The carbonates and clay exchanger released reactive magnesium in solution. Furthermore, it is worth noting that the reactors did not prevent air-solution interactions. The simulations demonstrated that O 2 contribution needs to be considered to explain the clay-glass interactions. The mechanisms controlling clay-glass interactions were deciphered based on mineralogical characterizations and geochemical modeling. At high clay/glass ratio, glass dissolution was sustained by pyrite oxidation and possibly iron silicate precipitation, while at low clay/glass ratio the passivation of the glass was prevented by the precipitation of magnesium silicate minerals