Abstract The Chuos Formation of Namibia is the sedimentary product of the Neoproterozoic Sturtian ( c. 720–660 Ma) glaciation and contains massive diamictites intercalated with finely laminated iron formation. Similar Sturtian glacially associated iron formations are found globally. The iron formations are laminated and generally very pure. The diamictites are massive, contain abundant clasts and can be highly ferruginous. These two lithofacies are repeatedly interbedded with no facies transition. The iron formations preserve the rare earth element geochemistry of their contemporaneous seawater and contain rare Ce and Eu anomalies. The geochemistry does not implicate a hydrothermal influence. The Chuos iron formation is interpreted to have been deposited in an ice-proximal glacio-marine setting in a sub-ice shelf environment. Oxygenated fluids, such as sea ice brines and glacial meltwater, are invoked as a mechanism to precipitate iron oxides due to mixing with ferruginous seawater. The iron formation accumulates under an ice shelf with little clastic input. Episodic movement of the grounding line reworks the sediments into ferruginous diamict. Glaciogenic debris flows are intercalated with the iron formations. Palaeobathymetric depressions and accompanying brine pools increased the preservation potential of these iron formations. This model explains the relationship between glaciation and iron formation in the Neoproterozoic. Supplementary material: The full set of geochemical data is available at https://doi.org/10.6084/m9.figshare.c.4031125.v1

Abstract The lower cap carbonate (Rasthof Formation) overlies Neoproterozoic glacial deposits (Chuos Formation) and is exposed in the Khowarib-Warmquelle area in Northern Namibia. The basal 14.2 m part of the Rasthof Formation (total about 220 m) consists of the carbonate rhythmite. The rhythmite part of the Rasthof Formation contains 1 m cycles of dark- and light-coloured rhythmites. Alizarin-red staining of thin sections and elemental mapping of polished samples indicate that the dark-coloured parts are rich in calcite, whereas the light-coloured parts are dolomite-rich. On a 1 cm scale, a reddish clay layer is intercalated in each calcite rich dark-coloured rhythmite part. These cycles of reddish clays as well as some associated major turbidites can be well correlated between columns up to about 20 km distance. Furthermore, at one locality (K4), rip-up clasts occur in a turbidite bed. Their lithology consists of dark- and light-coloured rhythmite and a reddish clay layer and can be judged to have been derived from underlying horizons. Because the clasts are elastically deformed, it is strongly suggested that the difference in lithology observed within the basal part of the Rasthof Formation existed when clasts were ripped-up shortly after sedimentation. This suggests that the cycle involving dolomite is synsedimentary, and not a diagenetic feature. Direct precipitation of dolomite does not occur in present day open marine seawater. Hurtgen et al. (2002) suggest that seawater was depleted in sulphate after Neoproterozoic glaciation. It is proposed here that some possible depositional models of cycles that consist of calcite-rich and dolomite-rich parts as well as reddish clay beds in rhythmites of the Rasthof cap carbonate.

Abstract Lunar mare basalts are spatially unevenly distributed, and their abundances differ between the nearside and farside of the Moon. Although mare asymmetry has been attributed to thickness variations in the low-density anorthositic crust, the eruptive mechanism of lunar magma remains unknown. In this study, we investigate the relationship between mare distribution and crustal thickness using geological and geophysical data obtained by the SELENE (Kaguya) and the Gravity Recovery and Interior Laboratory spacecraft, and quantitatively re-evaluate the influence of the anorthositic crust on magma eruption. We identify a lateral heterogeneity in the upper limit of crustal thickness that allows magma extrusion to the surface. In the Procellarum KREEP Terrane, where the surface abundances of heat-producing elements are extremely high, magmas can erupt in regions of crustal thickness below about 30 km. In contrast, magma eruptions are limited to regions of crustal thickness below about 20 km in other nearside regions, around 10 km in the South Pole–Aitken Basin and approximately 5 km in the farside Felspathic Highland Terrane. Such heterogeneity may result from lateral variations in magma production in the lunar mantle and/or crustal density.

Abstract Bentonite-based buffer materials play an important safety role in engineered barriers planned for use in geological disposal repositories for radioactive high-level waste (HLW) in Japan. The effectiveness of buffer materials is dependent on the status of groundwater saturation during resaturation of the repository. Accordingly, it is important to determine the behaviour of buffer materials during saturation and predict post-saturation conditions such as the distribution of residual dry density and chemical alteration. In this study, the rate of groundwater uptake into a buffer material was determined to clarify the behaviour of the material during the saturation process. As mechanical changes and chemical alteration of buffer materials are generated by groundwater permeation, knowledge of the water uptake rate is necessary for the prediction of post-permeation conditions. In the experiment reported here, one-dimensional permeation by distilled water and a NaCl water solution at a constant rate was monitored over a period of more than seven years. The results indicated that the seepage and saturation front moved in proportion to the square root of the seepage time. The coefficient of the relationships between the seepage and the saturation fronts with time of the reference bentonite used in Japan was determined.

Abstract Long-term gas migration through clays cannot be simulated by conventional two-phase flow models alone owing to the presence of material deformation. In this article, an extended two-phase flow model that incorporates mechanical effects is proposed. The model allows the formation of preferential pathway and considers the relation between pore moisture and pore deformation. It was carried out with the intention of avoiding the complexity of a fully coupled thermal, hydraulic and mechanical modelling. In the new model, porosity, permeability, swelling pressure and pathways formation threshold depend on the water saturation. The model is validated on different gas injection experiments with controlled flow rate and controlled pressure. Some experiments are well known in the literature; some are new. In each case, an inverse approach is used to identify the model parameters. The results confirm that, depending on the type of bentonite (MX80, Avonlea, KunigelV1), modelling the gas migration could require the existence of a pressure-induced saturation-depending preferential pathway. In laboratory-scale experiments, the model leads to an accurate evaluation of the long-term gas migration trends, including not just the gas migration stage but also the water re-saturation level. In a field-scale experiment, the behaviour of the model in a realistic context is revealed.