Editorial for Special Issue “Mantle Strain Localization—How Minerals Deform at Deep Plate Interfaces”

International audienceUnderstanding Earth’s interior dynamics, the origin and factors of which maintain the present-day plate-like behavior of the lithosphere on our planet, is one of the main goals of geosciences [...

La localisation de la déformation dans le manteau sous-continental (origine à travers l étude du massif de Ronda (Espagne) et implications sur la résistance de la lithosphère)

L actuelle définition rhéologique du manteau sous-continental ne rend pas compte de sa faible résistance estimée sous les régions déformées et 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 (< 800C), 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 chute majeure 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.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.RENNES1-BU Sciences Philo (352382102) / SudocRENNES-Géosciences (352382209) / SudocSudocFranceF

Unraveling static olivine grain growth properties in the Earth's upper mantle

International audienceGrain size in the Earth's upper mantle is a fundamental parameter that has crucial implications on large-scale processes, such as the permeability and the rheology of rocks. However, grain size is constantly evolving with time, where static grain growth implies an increase of the average grain size whereas dynamic recrystallization contributes to its decrease. Static grain growth is most dominant in grain size-sensitive deformation regimes and is classically defined by a grain growth law of the form:rfn - rin = k twith rf and ri, the final and initial grain radii, n the grain size exponent, t the duration, k the grain growth rate. These growth parameters are highly dependent on the value of n, which has considerable implications when extrapolating from laboratory to geological length and time scales. Here, we will show that there is no clear n value that can be extracted from grain growth experiments and that this value must be fixed based on the appropriate theoretical background. We have therefore investigated static grain growth of olivine aggregates where the intergranular medium is dry, wet or contains melt. Grain growth experiments were performed and modeled by considering different growth mechanisms (i.e. diffusion-limited and interface reaction-limited). We have established the dry grain growth law from previously published experiments at 1-atm and high-temperature conditions. Grain growth rates for these samples are limited by Si diffusion at grain boundaries (GB), implying n = 2. On the contrary, experiments on melt- and H2O-bearing aggregates indicate faster growth rates than for dry samples, regardless of the liquid fraction (i.e. >0%). We propose a general grain growth law, which takes into account dry GB as well as wetted grain-grain interfaces, by using the wetting properties of the liquid phase as shown by our high-resolution images. We show that our unified grain growth law considerably deviates from the classical grain growth law, with critical differences at geological time scales. We expect that our law will help unravel physical properties that are dependent on processes happening at the GB scale, such as rheology, diffusion or permeability

Unraveling static olivine grain growth properties in the Earth's upper mantle

International audienceGrain size in the Earth's upper mantle is a fundamental parameter that has crucial implications on large-scale processes, such as the permeability and the rheology of rocks. However, grain size is constantly evolving with time, where static grain growth implies an increase of the average grain size whereas dynamic recrystallization contributes to its decrease. Static grain growth is most dominant in grain size-sensitive deformation regimes and is classically defined by a grain growth law of the form:rfn - rin = k twith rf and ri, the final and initial grain radii, n the grain size exponent, t the duration, k the grain growth rate. These growth parameters are highly dependent on the value of n, which has considerable implications when extrapolating from laboratory to geological length and time scales. Here, we will show that there is no clear n value that can be extracted from grain growth experiments and that this value must be fixed based on the appropriate theoretical background. We have therefore investigated static grain growth of olivine aggregates where the intergranular medium is dry, wet or contains melt. Grain growth experiments were performed and modeled by considering different growth mechanisms (i.e. diffusion-limited and interface reaction-limited). We have established the dry grain growth law from previously published experiments at 1-atm and high-temperature conditions. Grain growth rates for these samples are limited by Si diffusion at grain boundaries (GB), implying n = 2. On the contrary, experiments on melt- and H2O-bearing aggregates indicate faster growth rates than for dry samples, regardless of the liquid fraction (i.e. >0%). We propose a general grain growth law, which takes into account dry GB as well as wetted grain-grain interfaces, by using the wetting properties of the liquid phase as shown by our high-resolution images. We show that our unified grain growth law considerably deviates from the classical grain growth law, with critical differences at geological time scales. We expect that our law will help unravel physical properties that are dependent on processes happening at the GB scale, such as rheology, diffusion or permeability

Unraveling static olivine grain growth properties in the Earth's upper mantle

International audienceGrain size in the Earth's upper mantle is a fundamental parameter that has crucial implications on large-scale processes, such as the permeability and the rheology of rocks. However, grain size is constantly evolving with time, where static grain growth implies an increase of the average grain size whereas dynamic recrystallization contributes to its decrease. Static grain growth is most dominant in grain size-sensitive deformation regimes and is classically defined by a grain growth law of the form:rfn - rin = k twith rf and ri, the final and initial grain radii, n the grain size exponent, t the duration, k the grain growth rate. These growth parameters are highly dependent on the value of n, which has considerable implications when extrapolating from laboratory to geological length and time scales. Here, we will show that there is no clear n value that can be extracted from grain growth experiments and that this value must be fixed based on the appropriate theoretical background. We have therefore investigated static grain growth of olivine aggregates where the intergranular medium is dry, wet or contains melt. Grain growth experiments were performed and modeled by considering different growth mechanisms (i.e. diffusion-limited and interface reaction-limited). We have established the dry grain growth law from previously published experiments at 1-atm and high-temperature conditions. Grain growth rates for these samples are limited by Si diffusion at grain boundaries (GB), implying n = 2. On the contrary, experiments on melt- and H2O-bearing aggregates indicate faster growth rates than for dry samples, regardless of the liquid fraction (i.e. >0%). We propose a general grain growth law, which takes into account dry GB as well as wetted grain-grain interfaces, by using the wetting properties of the liquid phase as shown by our high-resolution images. We show that our unified grain growth law considerably deviates from the classical grain growth law, with critical differences at geological time scales. We expect that our law will help unravel physical properties that are dependent on processes happening at the GB scale, such as rheology, diffusion or permeability

Unraveling static olivine grain growth properties in the Earth's upper mantle

International audienceGrain size in the Earth's upper mantle is a fundamental parameter that has crucial implications on large-scale processes, such as the permeability and the rheology of rocks. However, grain size is constantly evolving with time, where static grain growth implies an increase of the average grain size whereas dynamic recrystallization contributes to its decrease. Static grain growth is most dominant in grain size-sensitive deformation regimes and is classically defined by a grain growth law of the form:rfn - rin = k twith rf and ri, the final and initial grain radii, n the grain size exponent, t the duration, k the grain growth rate. These growth parameters are highly dependent on the value of n, which has considerable implications when extrapolating from laboratory to geological length and time scales. Here, we will show that there is no clear n value that can be extracted from grain growth experiments and that this value must be fixed based on the appropriate theoretical background. We have therefore investigated static grain growth of olivine aggregates where the intergranular medium is dry, wet or contains melt. Grain growth experiments were performed and modeled by considering different growth mechanisms (i.e. diffusion-limited and interface reaction-limited). We have established the dry grain growth law from previously published experiments at 1-atm and high-temperature conditions. Grain growth rates for these samples are limited by Si diffusion at grain boundaries (GB), implying n = 2. On the contrary, experiments on melt- and H2O-bearing aggregates indicate faster growth rates than for dry samples, regardless of the liquid fraction (i.e. >0%). We propose a general grain growth law, which takes into account dry GB as well as wetted grain-grain interfaces, by using the wetting properties of the liquid phase as shown by our high-resolution images. We show that our unified grain growth law considerably deviates from the classical grain growth law, with critical differences at geological time scales. We expect that our law will help unravel physical properties that are dependent on processes happening at the GB scale, such as rheology, diffusion or permeability

Low incidence of SARS-CoV-2, risk factors of mortality and the course of illness in the French national cohort of dialysis patients

International audienceThe aim of this study was to estimate the incidence of COVID-19 disease in the French national population of dialysis patients, their course of illness and to identify the risk factors associated with mortality. Our study included all patients on dialysis recorded in the French REIN Registry in April 2020. Clinical characteristics at last follow-up and the evolution of COVID-19 illness severity over time were recorded for diagnosed cases (either suspicious clinical symptoms, characteristic signs on the chest scan or a positive reverse transcription polymerase chain reaction) for SARS-CoV-2. A total of 1,621 infected patients were reported on the REIN registry from March 16th, 2020 to May 4th, 2020. Of these, 344 died. The prevalence of COVID-19 patients varied from less than 1% to 10% between regions. The probability of being a case was higher in males, patients with diabetes, those in need of assistance for transfer or treated at a self-care unit. Dialysis at home was associated with a lower probability of being infected as was being a smoker, a former smoker, having an active malignancy, or peripheral vascular disease. Mortality in diagnosed cases (21%) was associated with the same causes as in the general population. Higher age, hypoalbuminemia and the presence of an ischemic heart disease were statistically independently associated with a higher risk of death. Being treated at a selfcare unit was associated with a lower risk. Thus, our study showed a relatively low frequency of COVID-19 among dialysis patients contrary to what might have been assumed