Subseismic to Seismic Slip in Smectite Clay Nanofoliation

Smectite clays are the main constituent of slipping zones found in subduction zone faults at shallow depth (e.g., <1-km depth in the Japan Trench) and in the decollements of large landslides (e.g., 1963 landslide, Vajont, Italy). Therefore, deformation processes in smectite clays may control the mechanical behavior from slow creep to fast accelerations and slip during earthquakes and landslides. Here, we use (1) laboratory experiments to investigate the mechanical behavior of partly water-saturated smectite-rich gouges sheared from subseismic to seismic slip rates V and (2) nanoscale microscopy to study the gouge fabric. At all slip rates, deformation localizes in volumes of the gouge layer that contain a \u201cnanofoliation\u201d consisting of anastomosing smectite crystals. \u201cSeismic\u201d nanofoliations produced at V = 0.01, 0.1, and 1.3 m/s are similar to \u201csubseismic\u201d nanofoliations obtained at V = 10 125 m/s. This similarity suggests that frictional slip along water-lubricated smectite grain boundaries and basal planes may occur from subseismic to seismic slip rates in natural smectite-rich faults. Thus, if water is available along smectite grain boundaries and basal planes, nanofoliations can develop from slow to fast slip rates. Still, when nanofoliations are found highly localized in a volume, they can be diagnostic of slip that occurred at rates equal or larger than 0.01 m/s. In such a case, they could be markers of past seismic events when found in natural fault rocks

Higher education and unemployment in Europe : an analysis of the academic subject and national effects

This paper examines the impact of an academic degree and field of study on short and long-term unemployment across Europe (EU15). Labour Force Survey (LFS) data on over half a million individuals are utilised for that purpose. The harmonized LFS classification of level of education and field of study overcomes past problems of comparability across Europe. The study analyses (i) the effect of an academic degree at a European level, (ii) the specific effect of 14 academic subjects and (iii) country specific effects. The results indicate that an academic degree is more effective on reducing the likelihood of short-term than long-term unemployment. This general pattern even though it is observed for most of the academic subjects its levels show significant variation across disciplines and countries

Forming Cohesive Calcium Oxalate Layers on Marble Surfaces for Stone Conservation

Batch experiments were conducted with Carrara marble cubes to examine the replacement of calciteby calcium oxalate, a proposed method of protection for marble used as building stone. Coherent oxalate coatings formed on the marble surface during reactions with &gt;10 mM oxalic acid. The replacement rim contained an inner layer that remained attached to the marble surface and was composed of submicron-sized, rounded grains of calcium oxalate with minimal interconnected porosity, although open fluid pathways (inherited grain boundaries from the underlying marble) were present. In contrast, the outer rim comprises large, individual crystals and is easily removed. Raman spectroscopyidentified the mineral in both layers as whewellite (CaC2O4·H2O). Raman mapping revealed that the rims have zones of different crystallographic orientations contributing to the friability of the outer layer. Mapping of 18O incorporation into the replacement rim indicates that the outer layer formed from the inner layer via a fluid-mediated dissolution-reprecipitation mechanism. This suggests that the textures of precipitated oxalates could be tailored to different marble protection applications through changes insolution chemistry

Multiscale Imaging in Understanding Pore Structure and Alignment of Seal Rock for Long Term CO₂ Containment

Carbon capture and storage (CCS) provides a viable option to reduce CO2 emissions into the atmosphere. In CCS, CO2 is planned to be safely stored in suitable subsurface formations, providing sufficient storage capacity and sealing integrity. Analysing caprock properties like porosity and permeability is important in ensuring long-term CO2 containment. Caprocks are mainly composed of mudrocks that are characterised by nanometre to micron scale structures (pores and particles). A combination of medical and micro-CT scanning, Scanning Electron Microscopy (SEM), Focused-Ion-Beam (FIB), and Broad-Ion-Beam (BIB) are available to produce high-resolution 3D and 2D images. These can help to determine porosity down to a nanometre resolution. They can however also be used in pore network modelling which allows the computation of flow properties. In this study, core samples were CT scanned to determine the presence of fractures. This was followed by micro-CT and SEM scanning for higher-resolution images of the larger pores. To improve understanding of the porosity and permeability of the mudrocks analysed, samples were milled by BIB and imaged by SEM producing high-resolution 2D images (up to 5 nm) and allowing for detailed microstructural analysis at a scale of 2 mm2. We then identified different areas representing sample heterogeneity in terms of pore structure and permeability. These areas were then analysed using FIB-SEM with resolution down to 10 nm to obtain 3D models. Image analysis was done to identify the pore structure and alignment with the principal stress directions. The total porosity (within the resolution of the measurements) has been integrated with other experimental measurements and will become important parameters in the correlation of data determined on drill cuttings with core data and well logs. We hypothesize that upscaling from cuttings and core samples to well scale is feasible if an improved understanding of sample heterogeneity, method limitation, and the integration of experimental data with pore scale and pore network modelling is achieved. This will support de-risking of CO2 leakage across low permeability sealing formations.</p

Intracrystalline melt migration in deformed olivine revealed by trace element compositions and polyphase solid inclusions

International audienceMelt transport mechanisms have an important impact on the chemical composition of the percolated host rock and the migrating melts. Melt migration is usually assumed to occur at grain boundaries. However, microstructural studies revealed the occurrence of polyphase inclusions along dislocations, subgrain boundaries and microcracks in single mineral grains. The inclusions are interpreted as crystallized melt pockets suggesting that melts can migrate within deformed crystals. Intracrystalline melt migration and diffusive re-equilibration can lead to significant mineral trace element enrichments when associated with dissolution–precipitation reactions. In this contribution, we study a body of replacive troctolites associated with the Erro-Tobbio ophiolitic mantle peridotites (Ligurian Alps, Italy). The replacive formation of the olivine-rich troctolite involved extensive impregnation of a dunitic matrix, i.e. partial dissolution of olivine and concomitant crystallization of interstitial phases. The olivine matrix is characterized by two distinct olivine textures: (i) coarse deformed olivine, representing relicts of the pre-existing mantle dunite matrix (olivine1), and (ii) fine-grained undeformed olivine, a product of the melt–rock interaction process (olivine2). Previous studies documented a decoupling between olivine texture and trace element composition, namely enriched trace element compositions in olivine1 rather than in olivine2, as would be expected from the dissolution–precipitation process. Notably, the trace element enrichments in deformed olivines are correlated with the occurrence of elongated 10 µm size polyphase inclusions (clinopyroxene, Ti-pargasite, chromite) preferentially oriented along olivine crystallographic axes. These inclusions show irregular contacts and have no crystallographic preferred orientation with the host olivine, and the phases composing the inclusions show similar chemical compositions to the vermicular phases formed at the grain boundaries during late-stage reactive crystallization of the troctolite. This suggests that the investigated inclusions did not form as exsolutions of the host olivine but rather by input of metasomatic fluids percolating through the deformed olivine grains during closure of the magmatic system. We infer that strongly fractionated volatile-rich melts were incorporated in oriented microfractures within olivine1 and led to the crystallization of the polyphase inclusions. The presence of intracrystalline melt greatly enhanced diffusive re-equilibration between the evolved melt and the percolated olivine1, in turn acquiring the enriched character expected in neoformed olivine crystals. Intracrystalline melt percolation can have strong geochemical implications and can lead to efficient re-equilibration of percolated minerals and rocks

The complex hydrothermal history of granitic rocks: Multiple feldspar replacement reactions under subsolidus conditions

Recurring subsolidus re-equilibration of granitic feldspars induced by fluid infiltration events provides a record of fluid-rock interactions that affect large volumes of the Earth's continental crust. This has a direct bearing on the interpretation of the present-day granitic rock mineralogy and geochemistry. We examine Palaeoproterozoic grey and red-stained granitoids from the Simpevarp and Laxemar areas in SE Sweden, particularly focusing on consecutive feldspar replacement reactions, to provide an in-depth understanding of subsolidus re-equilibration of granitic rocks with hydrothermal fluids. The apparently most unaltered grey granitoids contain highly porous oligoclase grains that enclose crystallographically continuous microcline relicts. This texture suggests that the oligoclase is already secondary and may be a replacement product of original microcline. Oligoclase is progressively replaced by albite (~An9) along polysynthetic twinning and intragranular fractures. The features of this replacement are characteristic of a dissolution-reprecipitation mechanism. Fine-grained mica (sericite) is closely associated with the albite porosity within micron-sized pores observable with scanning electron microscopy as well as in nanopores imaged with transmission electron microscopy. The reddening phenomenon in the vicinity of fractures is contemporaneously related to the K-feldspathization of sericite, which is restricted to the altered oligoclase. Submicron size hematite precipitation within orthoclase pores at the replacement front results in the red coloration. The complex associations between the fluid-feldspar reactions indicate that the replacement reactions may be due to sequential fluid infiltration events and that the granitoids have undergone extensive subsolidus re-equilibration, changing the original magmatic mineralogy. Therefore, the effects of large-scale re-equilibrations of granitic rocks through hydrothermal convection systems should be more closely considered. © The Author 2009. Published by Oxford University Press. All rights reserved