Pervasive carbonation of peridotite to listvenite (Semail Ophiolite, Sultanate of Oman): clues from iron partitioning and chemical zoning

Earth's long-term cycling of carbon is regulated from mid-ocean ridges to convergent plate boundaries by mass transfers involving mantle rocks. Here we examine the conversion of peridotite to listvenite (magnesite + quartz rock) during CO2 metasomatism along the basal thrust of the Semail Ophiolite (Fanja, Sultanate of Oman). At the outcrop scale, this transformation defines reaction zones, from serpentinized peridotites to carbonated serpentinites and listvenites. Based on a detailed petrological and chemical study, we show that carbonation progressed through three main stages involving the development of replacive textures ascribed to early stages, whilst carbonate (± quartz) veining becomes predominant in the last stage. The pervasive replacement of serpentine by magnesite is characterized by the formation of spheroids, among which two types are identified based on the composition of their core regions: Fe-core and Mg-core spheroids. Fe zoning is a type feature of matrix and vein magnesite formed during the onset carbonation (Stage 1). While Fe-rich magnesite is predicted to form at low fluid XCO2 from a poorly to moderately oxidized protolith, our study evidences that the local non-redox destabilization of Fe oxides into Fe-rich magnesite is essential to the development of Fe-core spheroids. The formation of Fe-core spheroids is followed by the pervasive (over-)growth of Mg-rich spheroids and aggregates (Stage 2) at near-equilibrium conditions in response to increasing fluid XCO2. Furthermore, the compositions of carbonates indicate that most siderophile transition elements released by the dissolution of primary minerals are locally trapped in carbonate and oxides during matrix carbonation, while elements with a chalcophile affinity are the most likely to be leached out of reaction zones.</p

Non-basal dislocations should be accounted for in simulating ice mass flow

Prediction of ice mass flow and associated dynamics is pivotal at a time of climate change. Ice flow is dominantly accommodated by the motion of crystal defects – the dislocations. In the specific case of ice, their observation is not always accessible by means of the classical tools such as X-ray diffraction or transmission electron microscopy (TEM). Part of the dislocation population, the geometrically necessary dislocations (GNDs) can nevertheless be constrained using crystal orientation measurements via electron backscattering diffraction (EBSD) associated with appropriate analyses based on the Nye (1950) approach. The present study uses the Weighted Burgers Vectors, a reduced formulation of the Nye theory that enables the characterization of GNDs. Applied to ice, this method documents, for the first time, the presence of dislocations with non-basal [c][c] or 〈c+a〉〈c+a〉 Burgers vectors. These [c][c] or 〈c+a〉〈c+a〉 dislocations represent up to 35%35% of the GNDs observed in laboratory-deformed ice samples. Our findings offer a more complex and comprehensive picture of the key plasticity processes responsible for polycrystalline ice creep and provide better constraints on the constitutive mechanical laws implemented in ice sheet flow models used to predict the response of Earth ice masses to climate change

Centimetric CrSi2 crystal grown by the vertical gradient Freeze method

International audienc

Transparent Restoration

This paper investigates the application of structural glass in restoration and conservation practices in order to highlight and safeguard our built heritage. Cast glass masonry is introduced in order to consolidate a half-ruined historic tower in Greece, by replacing the original parts of the façade that are missing. Dry connections between the elements contribute to a completely reversible system that complies with the conservation guidelines suggested by the Venice Charter, while the interlocking nature of the glass units ensures the overall stability and the desired structural consolidation of the monument.Applied MechanicsStructural Design & Mechanic

Dislocation-driven recrystallization in AZ31B magnesium alloy imaged by quasi-in situ EBSD in annealing experiments

International audienceWe led a series of annealing experiments with quasi-in situ electron backscattered diffraction (EBSD) measurements to characterize the effect of the deformation microstructure on static recrystallization. Six samples of commercial purity AZ31B magnesium alloy were deformed under different temperature and strain rate conditions to produce microstructures with variable dislocation densities and arrangements, and then heated at 300°C (0.64 T m) for up to 6 h in several steps. All samples recrystallized by the growth of substructure-free grains, with nuclei mainly inherited from the deformed state. Recrystallization proceeded rapidly (minutes to hours), but remained incomplete in all cases. Using textural and microstructural proxies, we show that, under the studied experimental conditions, the stored energy associated with the dislocations controls the recrystallization ki-netics. We observe a positive correlation between the initial average kernel average misorientation (KAM) and the recrystallization kinetics of each sample and, to a lesser extent, the recrystallized fraction at a given time. We also present direct evidence on how the stored energy in the vicinity of the recrystallization front controls grain boundary migration kinetics. Yet, the reduction in the stored energy alone cannot explain the stagnation of the recrystallization front and incomplete recrystallization

Stress evolution and associated microstructure during transient creep of olivine at 1000–1200 °C

International audienceWe study the mechanical response and correlated microstructure of axial deformed fine-grained olivine aggregates as a function of incremental finite strains. Deformation experiments were conducted in uniaxial compression in an internally heated gas-medium deformation apparatus at temperatures of 1000 and 1200 °C, at strain rates of 10−6 s−1 to 10−5 s−1 and at confining pressure of 300 MPa. Sample volumes are around 1.2 cm3. Finite strains range from 0.1 to 8.6% and corresponding maximal (final) differential stresses range from 80 to 1073 MPa for deformation at 1000 °C and from 71 to 322 MPa for deformation at 1200 °C. At 1200 °C, samples approach steady state deformation after about 8% of strain. At 1000 °C, significant strain hardening leads to stresses exceeding the confining pressure by a factor of 3.5 with brittle deformation after 3% of strain. Deformed samples were characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). EBSD maps with step sizes as low as 50 nm were acquired without introducing analytical artifacts for the first time. The grain size of deformed samples ranges from 2.1 to 2.6 µm. Despite clear strain hardening, texture or microstructure do not change as a function of stress or finite strain. This observation is supported by a constant texture strength (J-index) and symmetry (BA-index), constant grain shape and aspect ratio, constant density of geometrically necessary dislocations, grain orientation spread, and constant subgrain boundary spacing and misorientation in between samples. TEM shows that all samples exhibit unambiguous dislocation activity but with a highly heterogeneous dislocation distribution. Olivine grains display evidence of [1 0 0] and [0 0 1] slip activity, but there is no evidence of interaction between the dislocations from the different slip systems. Several observations of grain boundaries acting as dislocation sources have been found. We find no confirmation of increasing dislocation densities as the cause for strain hardening during transient creep. This suggests other, yet not fully understood mechanisms affecting the strength of deformed olivine. These mechanisms could possibly involve grain boundaries. Such mechanisms are relevant for the deformation of uppermost mantle rocks, where the Si diffusion rate is too slow and dislocation glide must be accommodated in another way to fulfill the von Mises criterion

Structural glass: A new remedial tool for the consolidation of historic structures

This research investigates the potential of glass as a new design tool to highlight and safeguard our historic structures. Current restoration and conservation treatments with traditional materials bear the risk of conjecture between the original and new elements, whereas the high consolidation demands often result in visually invasive and irreversible solutions. Nowadays, aspects of materiality and aesthetics appear as integral parts of the restoration practices, indicating new materials and technologies in the form of ambiguous gestures rather than absolute and permanent manifestations that prevail over the historic structures. The inherent transparent properties render glass a distinct material that enables the simultaneous perception of the monument in both its original and ruinous state. The emerging technologies have set the ground for using glass in a structural way minimizing the need for substructure and maximizing transparency, while protecting the sensitive historic materials. The paper explores the feasibility of this concept addressing aspects of structural compatibility, reversibility and aesthetics, through a review of realized examples. Finally, a methodology is developed to relate the glass products, available in the market today, to the possible consolidation treatments in respect to the degree of intervention and representativeness, stressing the potential of using and considering glass as a promising restorative material

Dry interlayers out of cast polyurethane rubber for interlocking cast glass structures: experimental exploration and validation

A novel, reversible structural system comprising interlocking, dry-assembled cast glass components is currently being developed at the TU Delft Glass &amp; Transparency Lab. This paper, in continuation of the research conducted by (Oikonomopoulou et al. 2018a), investigates the mechanical properties of different materials that function as dry interlayers for interlocking cast glass structures. The interlayers should be preferably transparent, able to be pre-formed to the desired shapes, and resistant to UV-radiation-induced colour shifts, long-term compressive loads and creep. Based on the above criteria, polyurethane (PU) rubber with a shore hardness between 60A - 80A is chosen as the most suitable material. Accordingly, different readily available PU interlayers are selected and cast in the desired shape. Each interlayer is introduced between two interlocking osteomorphic cast glass components (bricks) and the assembly is tested under compression in series of 3 specimens. The experiments indicate that for the harder interlayer variants, failure mainly occurs due to peak stresses occurring at the shortest section of the brick, where the manufacturing tolerances of the concave-convex surface are the highest, leading to mismatch, i.e. incomplete contact at that area of the interlayer with the glass units. The stiffer interlayers further contribute to the failure due to the increased shear stresses induced at the edges of the interlocking surface while they are deforming. This is evident by the radial breaking pattern of the failed glass blocks. Interlayer variants with low tear resistance fail due to the perforation of the interlayer leading to glass-to-glass contact. Still, all specimens with interlayer in between presented a considerably higher failure stress than an assembly with no interlayer, highlighting the critical contribution of the PU to the structural performance of the system

A database of plagioclase crystal preferred orientations (CPO) and microstructures - implications for CPO origin, strength, symmetry and seismic anisotropy in gabbroic rocks

Corrigendum to this article was published in Solid Earth, 5, 509-509, 2014, doi:10.5194/se-5-509-2014. This study presents a unique database of 172 plagioclase Crystallographic Preferred Orientations (CPO) of variously deformed gabbroic rocks. The CPO characteristics as a function of the deformation regime (magmatic or crystal-plastic) are outlined and discussed. The studied samples are dominantly from slow-and fast-spread present-day ocean crust, as well as from the Oman ophiolite. Plagioclase is the dominant mineral phase in the studied samples. Plagioclase CPOs are grouped into three main categories: Axial-B, a strong point alignment of (010) with a girdle distribution of [100]; Axial-A, a strong point maximum concentration of [100] with parallel girdle distributions of (010) and (001); and P-type, point maxima of [100], (010), and (001). A majority of CPO patterns are Axial-B and P-type, in samples showing either magmatic or crystal-plastic deformation textures. Axial-A CPOs are less common; they represent 21% of the samples deformed by crystal-plastic flow. Although fabric strength (ODF J index) does not show any consistent variation as a function of the CPO patterns, there is a significant difference in the relationship between the ODF and pole figures J indices; the magmatic type microstructures have high (010) pole figures J indices, which increase linearly with ODF J index, whereas the high [100] pole figures J indices of plastically deformed samples vary in a more scattered manner with ODF J index. The multistage nature of plastic deformation superposed on a magmatic structure compared with magmatic flow, and the large number of possible slip-systems in plagioclase probably account for these differences. Calculated seismic properties (P wave and S wave velocities and anisotropies) of plagioclase aggregates show that anisotropy (up to 12% for P wave and 14% for S wave) tends to increase as a function of ODF J index. In comparison with the olivine 1998 CPO database, the magnitude of P wave anisotropy for a given J index is much less than olivine, whereas it is similar for S wave anisotropy. Despite a large variation of fabric patterns and geodynamic setting, seismic properties of plagioclase-rich rocks have similar magnitudes of anisotropy. There is a small difference in the aggregate elastic symmetry, with magmatic microstructures having higher orthorhombic and hexagonal components, whereas plastic deformation microstructures have a slightly higher monoclinic component, possibly correlated with predominant monoclinic simple shear flow in plastically deformed samples. Overall, plots for CPO strength (ODF J index), pole figure strength, CPO symmetry and seismic anisotropy show significant scattering. This could be related to sampling statistics, although our database is a factor of ten higher than the olivine database of 1998, or it could be related to the low symmetry (triclinic) structure of plagioclase resulting in the addition of degrees of freedom in the processes creating the CPOs.32 page(s

Dry interlayers out of cast polyurethane rubber for interlocking cast glass structures: experimental exploration and validation

A novel, reversible structural system comprising interlocking, dry-assembled cast glass components is currently being developed at the TU Delft Glass &amp; Transparency Lab. This paper, in continuation of the research conducted by (Oikonomopoulou et al. 2018a), investigates the mechanical properties of different materials that function as dry interlayers for interlocking cast glass structures. The interlayers should be preferably transparent, able to be pre-formed to the desired shapes, and resistant to UV-radiation-induced colour shifts, long-term compressive loads and creep. Based on the above criteria, polyurethane (PU) rubber with a shore hardness between 60A - 80A is chosen as the most suitable material. Accordingly, different readily available PU interlayers are selected and cast in the desired shape. Each interlayer is introduced between two interlocking osteomorphic cast glass components (bricks) and the assembly is tested under compression in series of 3 specimens. The experiments indicate that for the harder interlayer variants, failure mainly occurs due to peak stresses occurring at the shortest section of the brick, where the manufacturing tolerances of the concave-convex surface are the highest, leading to mismatch, i.e. incomplete contact at that area of the interlayer with the glass units. The stiffer interlayers further contribute to the failure due to the increased shear stresses induced at the edges of the interlocking surface while they are deforming. This is evident by the radial breaking pattern of the failed glass blocks. Interlayer variants with low tear resistance fail due to the perforation of the interlayer leading to glass-to-glass contact. Still, all specimens with interlayer in between presented a considerably higher failure stress than an assembly with no interlayer, highlighting the critical contribution of the PU to the structural performance of the system.Accepted Author ManuscriptStructural Design & MechanicsApplied Mechanic