Experimental simulation of magma mixing at high pressure

International audienceMagma mixing features are observed in many plutonic and volcanic environments. They result from the juxtaposition of two chemically contrasted magmas, usually during the replenishment of a magmatic reservoir, but also syn-eruptively within the conduit. Despite its ubiquity, only a few experimental studies have explored mixing between magmas. Existing data have been mostly acquired at atmospheric pressure and high shear rates (> 10- 1 s- 1), which differ from those accompanying magma mixing in reservoirs. To fill this gap, we performed high pressure mixing experiments at strain rates ranging from 4.10- 4 to 1.10- 3 s- 1. Layers of a synthetic crystal-free haplotonalite and a natural partially-molten basalt were juxtaposed in a Paterson apparatus at 300 MPa, and deformed between 900 and 1200 °C. The experiments shed light on the first stages of magma mixing and illustrate the role and behavior of crystals, either pre-existing or newly grown. Experiments evidence a rheological threshold for mafic material disruption, which sets in abruptly as its melt fraction exceeds 50%, which in the experiments occurs in the narrow temperature interval 1160-1170 °C. Below this threshold, plagioclase crystals in the mafic magma form a rigid touching network and all the deformation is accommodated by the less viscous felsic layer. Above it the crystal network collapses, allowing typical mingling/mixing features to appear altogether, such as enclaves, melt filaments or single xenocrysts isolated into the felsic end-member, coexisting with newly grown phases (plagioclase and pyroxene) whose compositions spread out over considerable ranges. The pre-existing fabric of the mafic magma is only slightly affected by deformation, altogether providing few clues on either the regime or geometry of applied deformation during the magmatic stage

Porosity redistribution enhanced by strain localization in crystal-rich magmas

International audienceMagma degassing, characterized by changes in permeability and porosity distribution, has a crucial control on the style of eruption. During ascent, magma might develop large porosities and crystallise while it is subjected to shear. Shear, in turn, enhances complex fabrics that result from the reorganization of the different phases (crystals, gas, melt). Such fabrics have not yet been evaluated experimentally on a 3-phase system. We performed torsion experiments on a synthetic crystal-rich hydrous magma at subsolidus conditions with 11 vol.% porosity to establish a link between strain partitioning and porosity redistribution. Crystals induce non-Newtonian deformation, resulting in localization of the shear strain. 3-D microtomography and 2-D Scanning Electron Microprobe (SEM) imaging show gas accumulation in local microstructures caused by shear-induced crystal fabric. Our data show that strain localization is a mechanism that could enable magma degassing at very low vesicularity

Segregation and extraction of late magmatic melt and fluids in mushes: experimental approach at high pressure

International audienceThe crustal mush (i.e., mixture of crystals and silicate liquid whose mobility is inhibited by a high fraction of solid particles) has acquired more importance in the last decade, replacing the traditional concept of magma chamber. It is still unclear how the deformation is localized in the mushes and how this localization can control the segregation and extraction of the melt from the crystalline network. In this study, we performed syn-deformational melt-extraction experiments on haplotonalitic magma. In order to create a hydrated mush (3 wt% H2O) composed of euhedral plagioclases crystals capable of covering the expected extraction window phase (Φ: 0.52 - 0.58, 700 ° C A pervasive SPO characterized by a main preferred orientation at 45° from the dextral shear direction develops with finite strains γ1, crystalline grains domains appear isolated, contacting and aligned alike, parallel to the shear direction (Y-bands) or to the main stress direction (C-bands). As the finite strain and strain rate increase, thickening of these crystalline grains domains occurs, resulting in the emergence of shear zones that stretch lengthwise with increasing strain and form an anastomosing array. Melt migration was not observed in any of the experiments, neither in the main pervasive fabric nor in the subsequently generated shear zones. We conclude that: 1) in discrete crystal orientation domains begins the initiation of strain localisation in magmatic mushes as the stress propagation over the crystal network is eased by the contact interactions between them; and 2) we were not able to get the melt extraction hence a possible requirement is higher crystal content

Segregation and extraction of late magmatic melt and fluids in mushes: experimental approach at high pressure

International audienceThe crustal mush (i.e., mixture of crystals and silicate liquid whose mobility is inhibited by a high fraction of solid particles) has acquired more importance in the last decade, replacing the traditional concept of magma chamber. It is still unclear how the deformation is localized in the mushes and how this localization can control the segregation and extraction of the melt from the crystalline network. In this study, we performed syn-deformational melt-extraction experiments on haplotonalitic magma. In order to create a hydrated mush (3 wt% H2O) composed of euhedral plagioclases crystals capable of covering the expected extraction window phase (Φ: 0.52 - 0.58, 700 ° C A pervasive SPO characterized by a main preferred orientation at 45° from the dextral shear direction develops with finite strains γ1, crystalline grains domains appear isolated, contacting and aligned alike, parallel to the shear direction (Y-bands) or to the main stress direction (C-bands). As the finite strain and strain rate increase, thickening of these crystalline grains domains occurs, resulting in the emergence of shear zones that stretch lengthwise with increasing strain and form an anastomosing array. Melt migration was not observed in any of the experiments, neither in the main pervasive fabric nor in the subsequently generated shear zones. We conclude that: 1) in discrete crystal orientation domains begins the initiation of strain localisation in magmatic mushes as the stress propagation over the crystal network is eased by the contact interactions between them; and 2) we were not able to get the melt extraction hence a possible requirement is higher crystal content

Segregation and extraction of late magmatic melt and fluids in mushes: experimental approach at high pressure

International audienceThe crustal mush (i.e., mixture of crystals and silicate liquid whose mobility is inhibited by a high fraction of solid particles) has acquired more importance in the last decade, replacing the traditional concept of magma chamber. It is still unclear how the deformation is localized in the mushes and how this localization can control the segregation and extraction of the melt from the crystalline network. In this study, we performed syn-deformational melt-extraction experiments on haplotonalitic magma. In order to create a hydrated mush (3 wt% H2O) composed of euhedral plagioclases crystals capable of covering the expected extraction window phase (Φ: 0.52 - 0.58, 700 ° C A pervasive SPO characterized by a main preferred orientation at 45° from the dextral shear direction develops with finite strains γ1, crystalline grains domains appear isolated, contacting and aligned alike, parallel to the shear direction (Y-bands) or to the main stress direction (C-bands). As the finite strain and strain rate increase, thickening of these crystalline grains domains occurs, resulting in the emergence of shear zones that stretch lengthwise with increasing strain and form an anastomosing array. Melt migration was not observed in any of the experiments, neither in the main pervasive fabric nor in the subsequently generated shear zones. We conclude that: 1) in discrete crystal orientation domains begins the initiation of strain localisation in magmatic mushes as the stress propagation over the crystal network is eased by the contact interactions between them; and 2) we were not able to get the melt extraction hence a possible requirement is higher crystal content

Mesures d'échantillons issus du démantèlement de G1 par autoradiographie digitale (MAUD)

International audiencePour le suivi des chantiers en assainissement démantèlement, les appareils de caractérisation sur le terrain sont essentiels. De nombreuses caméras dédiées à la mesure de rayonnements gamma, sont déjà industrialisées ou bien existent à l'état de prototype. Par contre, les développements de caméras capables de détecter les rayonnements alpha ou bêta sont beaucoup plus rares. Dans le cadre d'un projet de recherche soutenu par un PIA Andra, un démonstrateur industriel appelé MAUD qui utilise des détecteurs SiPM (Silicon Photomultipliers) couplés à des scintillants organiques a été développé pour répondre à ce manque. Les travaux présentés dans cette étude montreront l'utilisation sur le terrain de l'appareil MAUD. Le réacteur G1, construit en 1955 sur le site de Marcoule est à l'arrêt depuis 1968. Afin de préparer son démantèlement, un programme d'investigations et de prélèvements des différentes parties du réacteur est en cours. Ainsi, 48 carottes graphites du cœur du réacteur ont été extraites. La détermination de l'activité des radioélements présents(majoritairement des émetteurs beta pur), est généralement réalisée de manière destructive dans les laboratoires d'analyse. Ces opérations sont couteuses et très chronophages. L'appareil MAUD a été installé dans une boite à gant afin d'analyser la cinquantaine de carottes en graphites ainsi que des échantillons métalliques prélevés sur le réacteur en quelques semaines. L'interprétation des données a permis de -Classer les échantillons en fonction de leur niveau radioactif, -Caractériser la répartition de la radioactivité sur les différentes faces de la carotte (coté canal, bloc et sur la tranche)-Vérifier la présence de contamination en surface. En effet, l'analyse des spectres a permis d'identifier de manière qualitative certains radionucléides présents uniquement dans les échantillons les plus contaminés. Cette première étude a permis d'orienter la sélection des échantillons les plus pertinents à être examinés de façon destructive en laboratoire de chimie analytique. Elle contribue à moindre coût à optimiser le programme analytique

Experimental mixing of hydrous magmas

International audienceDeformation experiments involving hydrous magmas of different compositions (basalt andhaplotonalite) have been performed in a Paterson press at 300 MPa, in the temperature range600°C-1020°C, with water-saturated melts, during 2-4 hours. Prior to deformation the twoend-member magmas were annealed at either 950°C or 1000°C, yielding magmas with crystalcontents in the range 31-53 wt% and 2 sets of viscosity contrasts. Under the experimentalconditions investigated (i.e. moderate shear rates 950°C. In the temperature range 950-985°C a few mixing and minglingtextures occur, though both end-members essentially retain their physical integrity. It is onlyat, or above, 1000°C that a dramatic jump in mingling efficiency happens, corresponding to acrystal fraction of 45 vol%. Textures include entrainment of mafic crystals into the felsicmagma, mafic-felsic banding, enclave formation, diffusion-induced interface, the latter only over limited distances (< 300 microns) due to the short run durations. In the most strainedparcels of interacting magmas, complex mixing/mingling textures were produced, similar tothose observed in volcanic and plutonic rocks in arc settings. The experiments show thatmixing between hydrous felsic and mafic magmas takes place at around 1000°C, atemperature which is almost 200°C lower than mixing under dry conditions. Magma mixingis commonly invoked as a trigger for volcanic eruptions; our experiments suggest that sucheruptions can be driven by small (~15°C) temperature fluctuation in the reservoir. Our resultsalso suggest that slow replenishment of a felsic reservoir by mafic inputs will likely result instratification between end-members rather than in a homogeneous mixture

On the conditions of mafic-felsic magmas mixing and its bearing on andesite production in the crust

International audienceMixing between magmas is thought to affect a variety of processes, from the growth of continental crust to the triggering of volcanic eruptions, but its thermophysical viability remains unclear. Here, using high pressure mixing experiments, we show that mixing only occurs at low viscosity contrast, when the touching crystal network of the more viscous magma breaks down. Using thermal calculations, we show that hybridization requires injection of high proportions of the replenishing magma during short periods. The incremental growth of crustal reservoirs limits the production of hybrids to the waning stage of pluton assembly and to small portions of it. Large scale mixing appears to be more efficient at lower crustal conditions, but requires higher proportions of mafic melt, hence produces hybrids more mafic than in shallow reservoirs. Altogether, hybrid arc magmas correspond to periods of enhanced magma production at depth

On the conditions of magma mixing and its bearing on andesite production in the crust

International audienceMixing between magmas is thought to affect a variety of processes, from the growth of continental crust to the triggering of volcanic eruptions, but its thermophysical viability remains unclear. Here, by using high-pressure mixing experiments and thermal calculations, we show that hybridization during single-intrusive events requires injection of high proportions of the replenishing magma during short periods, producing magmas with 55–58 wt% SiO2 when the mafic end-member is basaltic. High strain rates and gas-rich conditions may produce more felsic hybrids. The incremental growth of crustal reservoirs limits the production of hybrids to the waning stage of pluton assembly and to small portions of it. Large-scale mixing appears to be more efficient at lower crustal conditions, but requires higher proportions of mafic melt, producing more mafic hybrids than in shallow reservoirs. Altogether, our results show that hybrid arc magmas correspond to periods of enhanced magma production at depth