Evaluation of T2G band intensity distribution across a surface of an UO2 ceramic

International audienc

The record of deep fluid pressure in veins : a new method based on quartz geochemistry

International audienceFluids are a primary control on deformation processes, in particular in the upper, brittle portion of the crust. In the mechanical framework of poroelasticity or friction, used to describe brittle rock behavior, the influence of fluid is integrated through the fluid pressure. High fluid pressure reduce the deviatoric stress necessary for slip ; for example during seismic slip, the temperature rise due to frictional work in the fault core might result in a large drop in resistance to further slip and constitutes therefore a very efficient lubricating process. Another example of the influence of fluid pressure is observed in deep slow slip events in subduction zones, where the slipping portion of the plate interface and domains of high fluid pressure migrate conjointly.While models and observations highlight the large mechanical role of fluid pressure, measurements of fluid pressure below a few kilometers of depths are very indirect and plagued by large uncertainties. Veins constitute one of the ubiquitous by-products of the fluid-rock interaction during deformation at depths. Vein-forming mineral, such as quartz and calcite, trap, as inclusions, the fluid that was present during crystal growth. Fluid inclusions constitute therefore one of the very few record of the physicochemical conditions of the deep fluid.We examined in this work three examples of syn-deformation quartz veins, from a japanese accretionary complex. The crystals within veins show growth rims, bringing to light the time evolution of the rock-fluid system. Many fluid inclusions are trapped within the growth rims ; in particular methane-rich fluid inclusions, which minimize the problem of late-stage reequilibration and therefore unravel the fluid pressure at the time of trapping. In parallel, those growth rims can be divided into two types, with either low or large content in trace elements (in particular aluminum).We correlated the median fluid pressure recorded in fluid inclusions with the average Al concentration in quartz : High/low fluid pressure correspond to low/high Al concentration, respectively. Based on literature data about crystal growth in hydrothermal and magmatic contexts, it appears that the higher incorporation of impurities can be accounted for by rapid, out-of-equilibrium growth of quartz. We propose therefore a model of vein evolution with repetitions of large fluid pressure drop, where crystal grew rapidly and incorporated a large concentration in Al, alternating with longer period of slower growth, at higher fluid pressure, with a reduced incorporation of Al. The highest fluid pressure variations are of the order of 70MPa, and the corresponding Al concentration variations of the order of 0.28wt%.Quartz veins are abundant in most, if not all tectonic contexts. In addition, Al concentration in quartz is preserved throughout exhumation, unlike fluid inclusions signal, which is in many cases questionable because of reequilibration. In conclusion, quartz geochemistry can be considered as a promising sensor of fluid pressure variations, which can provide access to the conditions of the fluid attending deformation of the brittle crust

Fluid pressure changes recorded by trace elements in quartz

International audienceFluid pressure is a key parameter in earthquake mechanics, controlling seismic failure and plate coupling in convergent zones. Yet fluid pressure is also extremely difficult to quantify at seismogenic depth, which limits our knowledge of the stress state in accretionary prisms. Here, we show that the geochemical record of exhumed hydrothermal quartz veins may be used to place quantitative bounds on fluid pressure variations in subduction zones. The studied veins come from sediments accreted and exhumed by plate convergence in southwestern Japan. Quartz in veins displays growth rims of contrasted bright blue/dark brown cathodoluminescence (CL) colors, high/low Al concentrations, and low/high fluid inclusion densities. Because Si-Al substitution (and charge compensation by Li) strongly depends on the rate of quartz precipitation and Si solubility, Al-Li concentrations must be sensitive to fluid pressure. This is confirmed by fluid inclusions, the density of which, converted into trapping pressures, record fluid pressure drops by up to ∼70 MPa from CL-brown, Al-Li-poor rims to CL-blue, Al-Li-rich quartz rims. CL-blue rims grow at a fast rate, high Si supersaturation and low fluid pressure whereas CL-brown rims grow at a slower pace, lower Si supersaturation, and higher pressure. Quartz trace element chemistry thus offers a promising tool to quantify deep fluid pressure variations and their relationships to earthquakes

Growing Negative Pressure in Dissolved Solutes: Raman Monitoring of Solvent-Pulling Effect

International audienceNegative pressure in liquids is both an experimental fact and a usually-neglected state of condensed matter. Using synthetic fluid inclusions, namely closed vacuoles fabricated inside one solid host by hydrothermal processes, a Raman study was performed to examine how a superheated solvent (under negative pressure) interacts with its dissolved solutes. As a result, this contribution not only illustrates this well-known tensile state, but also displays evidence that a stretched solvent is able to pull on its dissolved solutes and put them also under a stretched state. The dielectric continuum hypothesis may lead to expect a stretching effect in solutes similar to the solvent’s, but our measurements evidence a damping mechanical effect (growing with tension), most probably related to solvation shells. One practical consequence is that the (experimentally known) super-solvent properties of superheated solutions are certainly related to the change of the chemical potential of solutes which results from the damping effect. This change can determine as well a change in the thermodynamic driving force of the superheated solution towards bubble nucleation. A more complex than usual picture of the aqueous solution physical chemistry emerges from this study

Frictional melting during seismic rupture? A new Raman Spectroscopy approach to detect short-lived heat pulses

International audienceWhether seismic rupture propagates over large distances to generate earthquakes or on the contrary slows down quickly, is heavily dependent on the slip processes operating within the fault core. One possible scenario is that during seismic slip, the frictional work induces a local and transient release of heat up to reach the melting of the rock. This melt-lubrication of the fault plane results in resistance drop and promotes further propagation of the fault. Nonetheless, assessing the occurrence of flash melting has turned problematic, especially in the metasediments that constitute a large fraction of seismically active collision or subduction zones.In this work, we explore the effects of short-lived intense heating on the crystallinity of the carbonaceous particles present in the fault core. For this purpose, we carried out flash-heating experiments on pellets of natural sediments. Using a pair of lasers, the sample temperature was raised to 1400°C for durations ranging from 0.5 to 60 seconds, resulting in partial to total melting. The carbonaceous particles were then analyzed by Raman Spectroscopy. The spectroscopic signal of particles intensely heated for a short period of time present an atypical shape, with a large D3 band centered around 1500cm-1. The D3/Gsl. ratio in Flash-heating experiments show an evolution from 0.2 for the starting material up to 0.7 after a couple of seconds of Flash-heating. Following this experimental work, we analyzed with Raman spectroscopy several independent examples of short-lived intense heating of carbon-bearing rocks: static heating, stick-slip, high-velocity-friction experiments, In all these cases, we observed the presence of a prominent D3 band and a D3/Gsl. ratio larger than reference material. Based on these observations, we established a new parameter, the D3/Gsl. ratio, as sensitive to short-lived intense heating.Finally, we applied this new Raman parameter in association with micro-structural observations to discriminate the formation process of five Black Fault Rocks (BFR) from the Shimanto and the Kodiak Accretionary Complex. Microstructures are in several cases ambiguous as to the occurrence of melting in the BFR. However, the D3/Gsl. ratio shows a large increase in the Kure and the Mugi BFR while the values are close to 0.2 in the host-rock. In contrast, Nobeoka, Okitsu and Kodiak BFR show similar values in comparing the BFR veins and the host-rocks. Accordingly, the Mugi and Kure BFR are associated with a molten origin when the three others BFR are the result of mechanical wear solely, without evidence for large temperature increase.In summary, the D3/Gsl. ratio is a parameter that can be easily retrieved in most fault rocks cutting across sediments and that efficiently tracks the occurrence of short-lived intense heating. The use of this parameter appears as a promising approach to decipher the dynamics of faulting and to discriminate faults with intense frictional work from faults where temperature increase was much more limited, either because of slow creep or inhibiting processes (e.g. fluid vaporization during slip)

Quartz Stressing and Fracturing by Pore Pressure Dropping Down to Negative Pressure

International audienceIn water-bearing porous rocks, pore pressure variations play a major role in deformation, through dissolution−precipitation and fracturing processes. An often-overlooked variation where pressure falls to negative pressure or tension can operate whenever aquifer formations dry out, for instance, in deep storage (nuclear or industrial wastes, long-term CO 2 mitigation, short-term energetic resources, etc.). This can generate capillary tension within the aquifers. This study investigates the mechanical effect of such in-pore tension in the surrounding crystal field, through laboratory experiments at the one-pore scale. Microthermometric procedures were carried out on synthetic fluid inclusions to generate large tensile stress and were combined with Raman microspectrometry to visualize the resulting stress fields in the host quartz. For comparison, we numerically modeled the stress field by linear elasticity theory. The experiments demonstrate that significant damage is produced in crystalline materials by the pore tension. Despite the induced stress measured by micro-Raman spectrometry to remain moderate, it is able to fracture the quartz. The volume of the cavity is a prominent controlling parameter for the stress amplitude. The crystalline heterogeneities of the solid are another major parameter for localizing the mean weak stress and accumulating overstress. Our results call for bringing pore-scale micromechanics into the safety assessment of the geological storage of various wastes inside depleted aquifers. They also show the magnifying effect of heterogeneities on propagating stress and localizing it along certain directions, promoting the final failure of water-bearing minerals, rocks, or pore networks

Raman imaging and principal component analysis-based data processing on uranium oxide ceramics

A ceramic sample of uranium dioxide is probed by Raman imaging followed by a combined Lorentzian fitting – Principal Component Analysis process. This allows to evidence structural or chemical inhomogeneities of the material, which affect Raman line intensities but also line positions, evidencing local symmetry lowering. The inhomogeneities were observed not only between grain cores and boundaries, but also inside grain cores themselves. Only a part of these intensity inhomogeneities is as expected due to different orientations of the ceramic grains. Besides, a zone noticeably differing from others has been distinguished, presumably due to local strains or to chemical nature (oxygen stoichiometry), showing the sensitivity of the analysis. This underlines the importance to perform Raman analysis in such ceramic materials at least on several points and better, in imaging mode

h − ErMnO 3 absorbance, reflectivity, and emissivity in the terahertz to mid-infrared from 2 to 1700 K: Carrier screening, Fröhlich resonance, small polarons, and bipolarons

International audienc

The influence of Al2O3, CaO, MgO and TiO2 content on the early-age reactivity of GGBS in blended cements, alkali-activated materials and supersulfated cements

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Study of iron-bearing dolomite dissolution at various temperatures: Evidence for the formation of secondary nanocrystalline iron-rich phases on the dolomite surface

International audienceWe investigated the dissolution of a natural Fe-containing dolomite [Ca 1.003 Mg 0.972 Fe 0.024 Mn 0.002 (CO 3) 2 ] under acidic conditions (pH 3-5.5) with atomic force microscopy (AFM) at 20 °C, and with batch dissolution experiments at 80 °C. Dolomite dissolution proceeded by two identified mechanisms: removal of dolomite layers through spreading and coalescence of etch pits nucleated at defect points, and stepped retreat from surface edges. The dolomite dissolution rate increased when pH decreased (from 0.410 nm s-1 at pH 3 to 0.035 nm s-1 at pH 5). Rates calculated from edge retreat (v edges) and from etch-pit spreading rates (v sum) were consistent; the etch-pit digging rate was almost 10 times slower than its spreading rate. Nanocrystalline secondary phases precipitated in the course of dolomite dissolution at pH 3 and 80 °C were identified as (nano)hematite, ferrihydrite and an ankerite like mineral using X-ray diffraction, transmission electron microscopy, Raman and X-ray photoelectron spectrometry. In addition, Mg enrichment of the surface layer was observed at 80 °C, due to preferential release of Ca in solution. The characterizations performed at a nanocrystalline scale highlighted the role played by impurities in the dolomite dissolution/precipitation scheme and proved that two mechanisms explain the incongruent dolomite dissolution: secondary phase precipitation and preferential release of Ca over Mg