Strain weakening enables continental plate tectonics

International audienceMuch debate exists concerning the strength distribution of the continental lithosphere, how it controls lithosphere-scale strain localization and hence enables plate tectonics. No rheological model proposed to date is comprehensive enough to describe both the weakness of plate boundary and rigid-like behaviour of plate interiors. Here we show that the duality of strength of the lithosphere corresponds to different stages of microstructural evolution. Geological constraints on lithospheric strength and large strain numerical experiments reveal that the development of layers containing weak minerals and the onset of grain boundary sliding upon grain size reduction in olivine cause strain localisation and reduce strength in the crust and subcontinental mantle, respectively. The positive feedback between weakening and strain localization leads to the progressive development of weak plate boundaries while plate interiors remai

Evolution in H2O contents during deformation of polycrystalline quartz: An experimental study

Accepted manuscript version, licensed CC BY-NC-ND 4.0. Published version available at https://doi.org/10.1016/j.jsg.2018.05.021.Shear experiments were performed in a Griggs-type apparatus at 800 °C and 1.5 GPa, at a strain rate of 2.1 × 10−5s−1 using different starting materials: (i) Powder (grain size 6–10 μm) of dry Brazil quartz with 0.15 wt% added H2O, (ii) “dry” Brazil quartz porphyroclasts (grain size ∼100–200 μm), devoid of fluid inclusions embedded in the same fine grained powder, and (iii) “wet” porphyroclasts (grain size ∼100–200 μm), containing initially a high density of μm-scale fluid inclusions embedded in the same powder. After hot pressing, samples were deformed to large shear strains (γ∼3 to 4.5), in order for the microstructures and H2O distribution to approach some state of “equilibrium”. The H2O content and speciation in quartz were analyzed by Fourier Transform Infra-Red (FTIR) spectroscopy before and after the experiments. Mechanical peak strength is generally lower in experiments with 100% hydrated matrix, intermediate in experiments incorporating wet porphyroclasts (with a proportion of 30 or 70%) and highest in those with dry porphyroclasts. All experiments with porphyroclasts show pronounced strain weakening, and the strengths of most samples converge to similar values at large strain. Wet porphyroclasts are pervasively recrystallized during deformation, while dry porphyroclasts recrystallize only at their rims and remain weakly deformed. Recrystallization of the initially fluid-inclusion-rich porphyroclasts results in a decrease in inclusion abundance and total H2O content, while H2O content of initially dry clasts increases during deformation. H2O contents of all high strain samples converge to similar values for matrix and recrystallized grains. In samples with wet porphyroclasts, shear bands with high porosity and fluid contents develop and they host the precipitation of euhedral quartz crystals surrounded by a free-fluid phase. These high porosity sites are sinks for collecting H2O in excess of the storage capacity of the grain boundary network of the recrystallized aggregate. The H2O storage capacity of the grain boundary network is determined as a H2O-boundary-film of ∼0.7 nm thickness

Nutrition in hemodialysis patients previously on a supplemented very low protein diet

Nutrition in hemodialysis patients previously on a supplemented very low protein diet.BackgroundNutritional safety of protein-restricted diets in patients with chronic renal failure is controversial. In the present study, we have assessed the evolution of nutritional status after initiation of hemodialysis in patients previously treated by a supplemented very low protein diet (SVLPD).MethodsNutritional data were prospectively collected during the first year of hemodialysis from 15 consecutive patients treated with a SVLPD (0.3 g protein/kg/day supplemented with essential amino acids, calcium, iron, and vitamins) and compared to 15 age- and gender-matched end-stage renal disease (ESRD) patients previously on a less-restricted diet (0.90 ± 0.21 g protein/kg/day) who started hemodialysis during the same period. Dual-energy x-ray absorptiometry (DEXA) was used to assess body composition at 0, 6, and 12 months. Hemodialysis prescriptions, biologic data and 3-day food records were collected every 3 months.ResultsProtein intake was higher than 1.2 g/kg/day in both groups as soon as 3 months after the start of hemodialysis. Albumin and prealbumin increased significantly during the first 6 months in all patients. Body mass index (BMI) increased in all patients (+0.97 ± 1.31 kg/m2; P < 0.001) reflecting a gain in fat mass in the overall population (+2.36 ± 2.94 kg/m2; P < 0.001) while lean body mass remained stable overall.ConclusionOnce on hemodialysis, SVLPD patients rapidly increased protein intake. Nutritional status improved in all patients, with a gain in fat mass in all, and a gain in lean body mass in SVLPD men only. These data indicate that treatment with a SVLPD prior to hemodialysis initiation is nutritionally safe

Quantification of lentiviral vector copy numbers in individual hematopoietic colony-forming cells shows vector dose-dependent effects on the frequency and level of transduction

Lentiviral vectors are effective tools for gene transfer and integrate variable numbers of proviral DNA copies in variable proportions of cells. The levels of transduction of a cellular population may therefore depend upon experimental parameters affecting the frequency and/or the distribution of vector integration events in this population. Such analysis would require measuring vector copy numbers (VCN) in individual cells. To evaluate the transduction of hematopoietic progenitor cells at the single-cell level, we measured VCN in individual colony-forming cell (CFC) units, using an adapted quantitative PCR (Q-PCR) method. The feasibility, reproducibility and sensitivity of this approach were tested with characterized cell lines carrying known numbers of vector integration. The method was validated by correlating data in CFC with gene expression or with calculated values, and was found to slightly underestimate VCN. In spite of this, such Q-PCR on CFC was useful to compare transduction levels with different infection protocols and different vectors. Increasing the vector concentration and re-iterating the infection were two different strategies that improved transduction by increasing the frequency of transduced progenitor cells. Repeated infection also augmented the number of integrated copies and the magnitude of this effect seemed to depend on the vector preparation. Thus, the distribution of VCN in hematopoietic colonies may depend upon experimental conditions including features of vectors. This should be carefully evaluated in the context of ex vivo hematopoietic gene therapy studies

Emerging forms of food anxieties among Western contemporary Societies : an analysis and reconstruction of discourses and practices among food chain protagonists

Dans cette étude nous nous interrogeons sur les raisons qui, au moment où il semble exister une meilleure sécurité sanitaire des aliments, font que les peurs alimentaires occupent une place aussi importante dans les discours qu’ils émanent des mangeurs ordinaires, des professionnels de la filière agroalimentaire ou des experts. Nous analysons ici les discours relatifs aux peurs alimentaires afin d’en déterminer les fondements et justifications. Nous postulons que ces peurs doivent être considérées comme des métaphores sociales. L’alimentation est un lieu d’expression et de cristallisation des crises identitaires des différents acteurs. Dans une perspective compréhensive, nous reconstituons des récits de vie du point de vue des peurs alimentaires, puis nous les confrontons les uns aux autres, et enfin nous les articulons avec le contexte social et historique au sein duquel ils ont émergés. En déconstruisant les discours nous reconstruirons la logique de ces peursWhile today the sanitary security of food is largely secured, we examine the reasons why food fears occupy such an important place in discourses by ordinary eaters, food industry professionals as well as experts. We attempt here to analyze such fear-related discourses so as to determine grounds and justifications. We defend the hypothesis that such fears should be considered as social metaphors - food revealing the expression and crystallisation of the diverse protagonists’ identity crisis.In a comprehensive perspective, we reconstruct life stories focusing on food fears then we confront them; finally we articulate them with the social and historical context they are stemming from. By deconstructing discourses, we reconstruct the intrinsic logic of such fears

Sociologie appliquée à l’alimentation : ouvrir l’Université et la recherche sur l’extérieur

Interview réalisée à Tours par Frédéric Précigout Frédéric Précigout : Jean-Pierre Corbeau, tu es un des sociologues français les plus respecté dans le champ de l’alimentation. Par ailleurs tu fais partie de ceux qui, depuis longtemps, plaident pour une ouverture de l’Université et de la recherche sur l’extérieur, qu’il s’agisse d’institutions publiques ou privées. C’est à ce titre que tu nous accordes  un entretien aujourd’hui pour le numéro sur les conditions de la recherche en sciences hum..

La localisation de la déformation dans le manteau sous-continental:<br />origine à travers l'étude du massif de Ronda (Espagne) et implications sur la résistance de la lithosphère.

The predicted rheology for the sub-continental mantle does not account for both the weak mantle strength beneath deforming regions and processes of lithosphere-scale strain localization. In order to better constrain this rheology, we used structural geology, tectonics and numerical modeling through the study of the Ronda peridotites. The structural study of these peridotites shows that their deformations were formed beneath a back-arc continental rift, before they were inserted into the internal Betics at the early Miocene. Just below the extending crust, a ductile strain gradient occurs in response to a process that implies the dominance of the dryGBS creep during dynamic olivine grain size reduction. Numerical quantifications demonstrate that this ductile process can trigger large strain localization and a drop of strength for the highly deformed peridotites at low temperature (< 800 °C). At larger scale, the results of a 2-D numerical model show also that such a weakening process promotes high strain localization in the sub-continental mantle, which leads to trigger continental necking. This strain localization provokes consistently an important drop of strength in the sub-continental mantle, as observed beneath deforming regions. Based on this “localizing” rheology, our results allow us to propose a new definition of the mantle rheology that accounts for the evolution of the mantle strength during the deformation of the lithosphere.L'actuelle définition rhéologique du manteau sous-continental ne rend ni compte de sa faible résistance estimée sous les régions déformées, ni des processus de localisation de la déformation qui le caractérisent. Pour mieux contraindre sa rhéologie, nous avons donc utilisé la tectonique et la modélisation numérique à travers l'étude des péridotites de Ronda. L'étude structurale de ces péridotites montre que leurs déformations sont liées à une extension continentale arrière-arc, juste avant qu'elles soient intégrées dans les Bétiques internes au Miocène inférieur. Au cours de cette extension intervient la formation d'un gradient de déformation ductile et sous-continental, qui serait initié par un processus impliquant l'action dominante du fluage dryGBS pendant la réduction dynamique de taille de grains de l'olivine. La quantification numérique de ce processus ductile montre qu'il peut provoquer, à basse température (< 800 °C), une intense localisation de la déformation et une chute de résistance des péridotites intensément déformées. À plus grande échelle, ce processus est aussi capable de localiser la déformation dans le manteau sous-continental, permettant, d'après nos résultats numériques 2-D, d'initier la formation d'un rift continental étroit. La conséquence majeure de cette localisation se traduit par une intense chute de résistance du manteau au cœur du rift, comme observée dans les régions déformées. Cette rhéologie « localisante » et ces résultats nous permettent donc de proposer une nouvelle définition de la rhéologie du manteau, qui tient compte de l'évolution de sa résistance pendant la déformation de la lithosphère