Speciation and thermodynamic properties for cobalt chloride complexes in hydrothermal fluids at 35-440 °C and 600 bar: An in-situ XAS study

Aqueous Co(II) chloride complexes play a crucial role in cobalt transport and deposition in ore-forming hydrothermal systems, ore processing plants, and in the corrosion of special Co-bearing alloys. Reactive transport modelling of cobalt in hydrothermal fluids relies on the availability of thermodynamic properties for Co complexes over a wide range of temperature, pressure and salinity. Synchrotron X-ray absorption spectroscopy was used to determine the speciation of cobalt(II) in 0-6 m chloride solutions at temperatures between 35 and 440 °C at a constant pressure of 600 bar. Qualitative analysis of XANES spectra shows that octahedral species predominate in solution at 35 °C, while tetrahedral species become increasingly important with increasing temperature. Ab initio XANES calculations and EXAFS analyses suggest that in high temperature solutions the main species at high salinity (Cl:Co >> 2) is CoCl₄²⁻, while a lower order tetrahedral complex, most likely CoCl₂(H₂O)_2(aq), predominates at low salinity (Cl:Co ratios ∼2). EXAFS analyses further revealed the bonding distances for the octahedral Co(H₂O)₆²⁺ (^octCo-O = 2.075(19) Å), tetrahedral CoCl₄²⁻ (^tetCo-Cl = 2.252(19) Å) and tetrahedral CoCl₂(H₂O)_2(aq) (^tetCo-O = 2.038(54) Å and ^tetCo-Cl = 2.210(56) Å). An analysis of the Co(II) speciation in sodium bromide solutions shows a similar trend, with tetrahedral bromide complexes becoming predominant at higher temperature/salinity than in the chloride system. EXAFS analysis confirms that the limiting complex at high bromide concentration at high temperature is CoBr₄²⁻. Finally, XANES spectra were used to derive the thermodynamic properties for the CoCl₄²⁻ and CoCl₂(H₂O)_2(aq) complexes, enabling thermodynamic modelling of cobalt transport in hydrothermal fluids. Solubility calculations show that tetrahedral CoCl₄²⁻ is responsible for transport of cobalt in hydrothermal solutions with moderate chloride concentration (∼2 m NaCl) at temperatures of 250 °C and higher, and both cooling and dilution processes can cause deposition of cobalt from hydrothermal fluids.     

Experimental and numerical study of the behavior of a reinforced-earth wall subjected to a local load

The mechanical response of earth structures reinforced with steel strips to loads associated with rail traffic is not well known. In order to better understand the deformations induced by train loads, and to improve design methods, a full-scale structure was built, instrumented, and tested near Rouen, France. Finite-element analyses were performed both in plane-strain and three-dimensional conditions, using different approaches to account for the interaction between the strips and the backfill material. After calibration of the soil-strip interface parameters for a load of 90 kN, simulations of the wall response to a load of 850 kN provided results in good agreement with measurements. Numerical models showed that the soil-strip interface parameters have a major influence on the predicted behavior. The proposed approach could be used for further studies of the influence of geometric and mechanical parameters.     

Phyllosilicates synthesis: a way of accessing edges contributions in NMR and FTIR spectroscopies. Example of synthetic talc

A series of new-generation synthetic talcs were prepared by varying the hydrothermal synthesis duration from a few hours up to 2 months. Crystallinity and particle size analysis of the synthetic products were evaluated by photon correlation spectroscopy, transmission electron microscopy, differential thermal analysis and unoriented X-ray diffraction, and then analyzed by various spectroscopic methods such as Fourier transformed infrared spectroscopy and solid-state ¹H and ²⁹Si magic angle spinning nuclear magnetic resonance. The new process for preparing synthetic talcs allows to obtain single-phased particles which after few hours grow heterogeneously and simultaneously in both the c* direction and the (ab) plan. Fine particles, observed for whatever the synthesis duration, lead to the presence of numerous sheet edges surfaces due to particle size which represents the main difference with natural talc. Spectroscopy data show the influence of the fine particles on signals and highlight that synthetic talc characterization is a potential tool for better understanding crystal chemistry of natural talc.     

Structural inversion and its controls : examples from the Alpine foreland and the French Alps

Abstract

Positive structural inversion involves the uplift of rocks on the hanging-walls of faults, by dip slip or oblique slip movements. Controlling factors include the strike and dip of the earlier normal faults, the type of normal faults — whether they were listric or rotated blocks, the time lapsed since extension and the amount of contraction relative to extension. Steeply dipping faults are difficult to invert by dip slip movements; they form buttresses to displacement on both cover detachments and on deeper level but gently inclined basement faults. The decrease in displacement on the hanging-walls of such steep buttresses leads to the generation of layer parallel shortening, gentle to tight folds — depending on the amount of contractional displacement, back-folds and back-thrust systems, and short-cut thrust geometries — where the contractional fault slices across the footwall of the earlier normal fault to enclose a “floating horse”. However, early steeply dipping normal faults readily form oblique to strike slip inversion structures and often tramline the subsequent shortening into particular directions.

Examples are given from the strongly inverted structures of the western Alps and the weakly inverted structures of the Alpine foreland. Extensional faulting developed during the Triassic to Jurassic, during the initial opening of the central Atlantic, while the main phases of inversion date from the end Cretaceous when spreading began in the north Atlantic and there was a change of relative motion between Europe and Africa. During the mid-Tertiary well over 100 km of Alpine shortening took place; Alpine thrusts, often detached along, or close to, the basement-cover interface, stacking the late Jurassic to Cretaceous sediments of the post-extensional subsidence phase. These high level detachments were joined and breached by lower level faults in the basement which, in the external zones of the western Alps, generally reactivated and rotated the earlier east dipping half-graben bounding faults. The external massifs are essentially uplifted half-graben blocks. There was more reactivation and stacking of basement sheets in the eastern part of this external zone, where the faults had been rotated into more gentle dips above a shallower extensional detachment than on the steeper faults to the west.

There is no direct relationship between the weaker inversion of the Alpine foreland and the major orogenic contraction of the western Alps; the inversion structures of southern Britain and the Channel were separated from the Alps by a zone of rifting from late Eocene to Miocene which affected the Rhone, Bresse and Rhine regions. Though they relate to the same plate movements which formed the Alps, the weaker inversion structures must have been generated by within plate stresses, or from those emanating from the Atlantic rather than the Tethyan margin.     

Les Montagnes Noires : cisaillement bordier méridional du bassin carbonifère de Châteaulin (Massif Armoricain,France). Caractéristiques structurales et métamorphiques

Résumé

L’évolution structurale et métamorphique de la zone des Montagnes Noires, qui borde vers le sud le bassin carbonifère de Châteaulin, est contrôlée par le fonctionnement d’un cisaillement trans-current dextre. Le caractère unique, mais progressif et inhomogène, de la déformation régionale d’âge carbonifère supérieur conduit à remettre en cause le concept d’une structuration hercynienne polyphasée et celui de « phase bretonne ».     

Contrôle morphostructural de l’histoire d’un réseau hydrographique : le site de capture de la Moselle

Résumé

La comparaison entre la structure tectonique des calcaires du Bajocien, la répartition des alluvions anciennes et la morphologie permet de retracer l’évolution du relief de côtes et l’évolution des axes de drainage depuis la fin du Tertiaire. Les alluvions conservées en inversion de relief sur le plateau d’érosion au sommet de la côte de Moselle, témoignent en faveur des cours divaguants de la « Paléo-Meurthe » et de la Haute-Moselle et du manque de vigueur du relief à la fin du Tertiaire. Le dégagement des surfaces structurales ne commence qu’au début du Pléistocène (stade 350 m) en relation avec un changement climatique. L’encaissement du réseau hydrographique et le déblaiement des dépressions argileuses ne font, depuis lors, que donner de l’amplitude au relief de côte malgré un recul qui a pu être chiffré entre 6 et 21 km, en fonction du contexte structural.

Les cours d’eau principaux sont presque totalement surimposés à la structure. Les affluents de rive gauche de la « Paléo- Meurthe », d’abord adaptés au cadre morphostructural, coulent dans la plaine argileuse du Callovien dans laquelle ils migrent et s’allongent en fonction du recul des côtes, avant de se surimposer dans les calcaires du Bajocien. Seuls apparaissent adaptés à la structure quelques petits ruisseaux orthoclinaux qui s’assèchent rapidement après incrustation dans les calcaires. N’étant pas alimentés par l’amont, ils s’enfoncent moins vite que les cours d’eau de rive gauche, qui drainent d’abord les argiles du Callovien avant de recouper la nappe des calcaires bajociens. La capture de la Moselle s’explique par l’adaptation au cadre morphostuctural du « Paléo-Terrouin », affluent de rive gauche de la « Paléo-Meurthe », s’écoulant à contre-pendage. Le recul important de la côte de Meuse dans le « synclinal » de Toul, passant d’un relief inverse à un relief conforme dans la cuvette de Dieulouard, a permis l’élargissement progressif de la plaine argileuse du Callovien et l’accroissement du bassin-versant du « Paléo-Terrouin ». Celui-ci a fini par capturer la « Rivière de la Haie Plaisante », puis par son intermédiaire, la Haute-Moselle elle-même. Dans l’avenir, il est probable qu’il capture également la Haute-Meuse. © Elsevier, Paris     

Tectonometamorphic history of the Gruf complex (Central Alps): exhumation of a granulite–migmatite complex with the Bergell pluton

We describe field occurrences of sapphirine-bearing granulites, charnockites and migmatites in the Gruf complex, Central Alps and present a new geological map and a structural analysis of the entire Gruf complex for the first time. We have carried out an accurate analysis of the relationships between granulite facies metamorphism, migmatisation and deformation within the complex, in relation to the intrusion of the Bergell pluton. Granulites and charnockites display fabrics different from those defined by the regional foliation and lineation, which are, typically for migmatites, disordered on the mesoscale. On a regional scale, strike variations are also related to the structural complexity of migmatites within which no major antiform could be identified. Irregular interfingering of sub-parallel leucosome veins and back-veining along the contact between the Gruf migmatites and the Bergell tonalite are evidence for contemporaneous emplacement and crystallisation at about 740 °C and 6.5–7.5 kbar in Oligocene times (ca 30 Ma). Metamorphic conditions in the charnockites and granulites (>920 °C for 8.5–9.5 kbar) largely exceed these regional metamorphic conditions and are dated at 282–260 Ma. We propose that the ascending Bergell pluton entrained the polymetamorphic, granulitic lower crust enclosed within the peripheral migmatitic Gruf complex.     

Neutron absorption constraints on the composition of 4 Vesta

Global maps of the macroscopic thermal neutron absorption cross section of Vesta's regolith by the Gamma Ray and Neutron Detector (GRaND) on board the NASA Dawn spacecraft provide constraints on the abundance and distribution of Fe, Ca, Al, Mg, and other rock‐forming elements. From a circular, polar low‐altitude mapping orbit, GRaND sampled the regolith to decimeter depths with a spatial resolution of about 300 km. At this spatial scale, the variation in neutron absorption is about seven times lower than that of the Moon. The observed variation is consistent with the range of absorption for howardite whole‐rock compositions, which further supports the connection between Vesta and the howardite, eucrite, and diogenite meteorites. We find a strong correlation between neutron absorption and the percentage of eucritic materials in howardites and polymict breccias, which enables petrologic mapping of Vesta's surface. The distribution of basaltic eucrite and diogenite determined from neutron absorption measurements is qualitatively similar to that indicated by visible and near infrared spectroscopy. The Rheasilvia basin and ejecta blanket has relatively low absorption, consistent with Mg‐rich orthopyroxene. Based on a combination of Fe and neutron absorption measurements, olivine‐rich lithologies are not detected on the spatial scales sampled by GRaND. The sensitivity of GRaND to the presence of mantle material is described and implications for the absence of an olivine signature are discussed. High absorption values found in Vesta's “dark” hemisphere, where exogenic hydrogen has accumulated, indicate that this region is richer in basaltic eucrite, representative of Vesta's ancient upper crust.     

Miocene to Recent exhumation of the central Himalaya determined from combined detrital zircon fission-track and U/Pb analysis of Siwalik sediments, western Nepal

Fission‐track (FT) analysis of detrital zircon from synorogenic sediment is a well‐established tool to examine the cooling and exhumation history of convergent mountain belts, but has so far not been used to determine the long‐term evolution of the central Himalaya. This study presents FT analysis of detrital zircon from 22 sandstone and modern sediment samples that were collected along three stratigraphic sections within the Miocene to Pliocene Siwalik Group, and from modern rivers, in western and central Nepal. The results provide evidence for widespread cooling in the Nepalese Himalaya at about 16.0±1.4 Ma, and continuous exhumation at a rate of about 1.4±0.2 km Myr^?1 thereafter. The ～16 Ma cooling is likely related to a combination of tectonic and erosional activity, including movement on the Main Central thrust and Southern Tibetan Detachment system, as well as emplacement of the Ramgarh thrust on Lesser Himalayan sedimentary and meta‐sedimentary units. The continuous exhumation signal following the ～16 Ma cooling event is seen in connection with ongoing tectonic uplift, river incision and erosion of lower Lesser Himalayan rocks exposed below the MCT and Higher Himalayan rocks in the hanging wall of the MCT, controlled by orographic precipitation and crustal extrusion. Provenance analysis, to distinguish between Higher Himalayan and Lesser Himalayan zircon sources, is based on double dating of individual zircons with the FT and U/Pb methods. Zircons with pre‐Himalayan FT cooling ages may be derived from either nonmetamorphic parts of the Tethyan sedimentary succession or Higher Himalayan protolith that formerly covered the Dadeldhura and Ramgarh thrust sheets, but that have been removed by erosion. Both the Higher and Lesser Himalaya appear to be sources for the zircons that record either ～16 Ma cooling or the continuous exhumation afterwards.