253 research outputs found
Diversity of eruptive styles inferred from the microlites of Mt Pelée andesite (Martinique, Lesser Antilles).
International audienceWe have studied the compositional and textural characteristics of plagioclase microlites from several dome-forming eruptions (dome, block-and-ash flows, and surges) and plinian fallouts of the recent period of activity of Mt Pelée in order to infer magma ascent dynamics. Dome-forming and plinian samples display a wide range of plagioclase microlite compositions, from An35 to An90 and An55 to An90, respectively. Microlites with compositions > An50-60 crystallized from a mafic melt that intruded the andesitic reservoir in proportion of ~10 vol %. During magma ascent in the conduit, the fraction, number density, and morphology of the decompression-induced microlites (compositions < An50-60) suggest that the plinian magmas experienced a nucleation-dominated crystallization regime with high undercooling. In contrast magmas from domes and block-and-ash flows were dominated by a growth regime with low undercooling, and the surges were influenced by both regimes. According to these crystallization regimes and new experimental data on H2O content of the matrix glasses, we propose magma relative ascent rates and fragmentation pressures for the different eruptive styles observed at Mt Pelée that may explain why surges are explosive events and why they may or may not be followed by a plinian event
In situ high-pressure and high-temperature bubble growth in silicic melts
We present the first investigation of in situ high-pressure and high-temperature bubble growth in silicic melts. In a hydrothermal diamond-anvil cell, a haplogranite melt (79 wt% SiO2) is hydrated then subjected to cooling and decompression. With decreasing pressure, water exsolves from the melt and bubbles grow. The whole experiment is observed through an optical microscope and video-recorded, so that bubble nucleation, bubble growth, and the glass transition are directly monitored. Bubbles nucleate and expand in melt globules having radii from 15 to 70 ÎŒm. Bubbles reached 3.6â9.1 ÎŒm in radius within 6.1â11.7 s (until the glass transition is attained) while temperature decreases from 709â879°C to 482â524°C, corresponding to decompressions from 7.0â21.9 to 3.4â15.2 kbar. Bubbles nucleated either in a single event occurring within the first second or in successive pulses over a period of up to 7 s when the melt globules are in contact with a diamond culet of the cell. In these experiments, bubble growth can be fitted to the cube root or a logarithm of time, mainly ascribable to the combination of large water oversaturations due to rapid cooling and decompression. At pressures of 3.4â15.2 kbar, we measure glass transition temperatures that are 20â80°C higher than those calculated at atmospheric pressure
Experimental Phase Equilibria: Data and Models
Equilibria between silicate melts, crystalline and vapour phases essentially control major and trace element compositions of natural magmas and play an essential role in partial melting as well as in differentiation processes. The development of models of phase equilibria is a prerequisite for assessing the dynamics of magmatic processes, such as magma storage in a reservoir or magma ascent in a conduit. The interest of experimental phase equilibria is essentially twofold. (1) Experimental phase equilibrium results can put precise constraints on the magmatic conditions, either in the magma storage region or in the conduit. Knowledge of pre-eruptive parameters (P, T, H2O in melt, fO2, ...) is necessary for eruption models and consequently for the evaluation of volcanic risk. (2) Experimental phase equilibria constitute our main source of information for the calibration of the mixing properties of multicomponent silicate melts, and for the construction of thermodynamic models which are our tomorrow's tools for the simulation of magmatic processes
Un regard qualitatif sur le cheminement d'adolescentes en Centre jeunesse : usage de substances psychoactives et autres comportements qualifiés de déviants
Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal
Eruption dynamics inferred from microlite crystallization experiments: Application to Plinian and dome-forming eruptions of Mt Pelée (Martinique, Lesser Antilles).
International audienceDecompression experiments have been conducted to simulate syn-eruptive crystallization in the volcanic conduit, in order to infer magma ascent rates and conditions during dome-forming and Plinian eruptions of silicic arc volcanoes. The experiments were carried out starting from Mt Pelée rhyolitic interstitial melt (76 wt. % SiO2) and consisted in three consecutive steps: hydration, decompression, and annealing. Hydration (saturated and undersaturated) was performed at 850°C and 200 MPa and followed by isothermal decompression, either linearly or stepwise, to a final pressure, Pf, of 30 or 5-10 MPa. Decompression rates range from 0.003 to 25 MPa/min (decompression durations of 15 min to 40 days). Two samples were cooled by 25°C and 50°C during a 3-days step at Pf. Subsequent to decompression, the samples were held up to 15 days at Pf. The experiments generated three types of crystals: pre-, syn-, and post-decompression crystallization. The experiments basically differ from previous studies in that they are specifically designed to discriminate crystal nucleation from growth and to evaluate the influence of pre-decompression crystals on the decompression-induced crystallization. The effects of pre-decompression crystals, decompression rate, undercooling (Pf), and terminal cooling have been determined on plagioclase nucleation, growth, morphology, and composition. The main results i) suggest a positive correlation between decompression rate and the number density of plagioclases nucleated at Pf and ii) highlight the effect of pre-decompression crystals in further decompression-induced crystallization. The relations between the decompression conditions and the plagioclase characteristics have been used to infer Mt Pelée eruption dynamics, suggesting that i) Plinian magmas ascend from the reservoir within less than 1 hour (1-10 m/s), ii) dome and block-and-ash flows magmas ascend within more than 2-5 days, giving time for syn-decompression crystallization around pre-existent microlites, iii) dome magmas evidence long stagnation and cooling at low pressure, and iv) surge magmas ascend without significant crystallization (within less than ~4 days) and massively nucleate plagioclase at very low pressure. The extent and violence of dome destruction may depend on the size/age of the dome, with large/old domes favouring mildly-explosive BAFs, whereas small/young protodomes may generate highly-explosive surges
Localisation et caractérisation d'un élément de réponse à la triiodothyronine (T3) dans la partie promotrice du gÚne de la synthétase des acides gras (FAS) chez l'oie
L'obésité est maintenant perçue comme un problÚme de santé majeur dans notre société. C'est pourquoi la compréhension des mécanismes moléculaires liés au développement de cette maladie est primordiale. Des études comparatives entre des poulets de lignées grasses et des poulets de lignées maigres ont montré que la lipogénÚse hépatique était plus élevée chez les animaux gras. La Synthétase des Acides Gras (FAS) est connue comme étant responsable de la synthÚse des lipides saturés à longues chaßnes. Des études antérieures ont démontré que ce gÚne est régulé à la fois par l'état nutritionnel et hormonal. Dans ce contexte, l'objectif de la présente étude était d'examiner le rÎle de l'hormone thyroïdienne (T3), et aussi d'un mélange de T3 et d'insuline, sur l'activité transcriptionnelle du gÚne aviaire de la FAS. Certains des éléments agissant au niveau de la réponse à l'insuline ont déjà été identifiés chez l'humain, le poulet, la souris et le rat. Afin d'identifier l'effet de la T3 et d'un mélange de T3 et d'insuline, un fragment d'ADN de 1,6 Kpb de la partie promotrice de FAS a été isolé, séquencé et cloné dans un vecteur contenant le gÚne rapporteur codant pour la chloramphénicol acétyl transférase (CAT). Cette construction nommée 1.6FAS-pJFCAT1 a ensuite été transfectée de façon transitoire dans les cellules HepG2 et les hépatocytes embryonnaires de poulet. Les analyses d'expression ont démontré une augmentation de l'activité du gÚne rapporteur codant pour CAT de 2.5 fois, suite à une stimulation de 48 heures avec 1.6”M de T3. Ce promoteur posséderait donc un élément de réponse nécessaire à la modulation par cette hormone. Des constructions contenant des délétions successives de ce grand fragment d'ADN et des alignements de séquences ont permis l'identification de la région interne localisée entre \ud
-902/ -577 pb oĂč se retrouve l'Ă©lĂ©ment de rĂ©ponse Ă la T3, appelĂ© TRE pour 'ThyroĂŻd Hormone Response Element'. Des analyses de retard sur gel (EMSA) utilisant comme TRE un oligonuclĂ©otide contenant les bases -732 Ă -692 ont ensuite permis d'identifier des protĂ©ines se fixant sur ce fragment d'ADN en rĂ©ponse Ă la T3, renforçant ainsi la prĂ©sence possible d'un TRE. Une augmentation de l'activitĂ© CAT de 2.3 fois a d'ailleurs aussi Ă©tĂ© obtenue aprĂšs 48 heures de stimulation Ă la T3 lors d'expĂ©riences de transfections transitoires avec la construction contenant le fragment -765/-663 placĂ© devant le promoteur TK. Le TRE correspondrait donc Ă la sĂ©quence CGCCCTgtggTAACCT, sĂ©quence trĂšs diffĂ©rente du consensus Ă©tabli et trĂšs diffĂ©rente des T3RE retrouvĂ©s dans la partie promotrice du gĂšne de l'enzyme malique. Il a toutefois une forte homologie avec un TRE retrouvĂ© dans le gĂšne FAS chez l'humain, le poulet, la souris et le rat. Ceci semble dĂ©montrer la forte variation de ces Ă©lĂ©ments de rĂ©ponse. L'analyse par retard sur gel rĂ©vĂšle qu'en prĂ©sence de T3 seulement il y a fixation du rĂ©cepteur TR alors que lorsque l'insuline est aussi prĂ©sente, il semble que le facteur de transcription RXR se fixe aussi. Ceci suggĂ©rerait l'idĂ©e que la formation d'hĂ©tĂ©rodimĂšres TR/RXR serait grandement favorisĂ©e en prĂ©sence d'insuline. Ces informations, en combinaison avec les rĂ©sultats rĂ©cents obtenus par d'autres Ă©quipes sur la rĂ©gulation de la FAS, serviront Ă la comprĂ©hension des mĂ©canismes de rĂ©gulation de la transcription d'un gĂšne clĂ© de la lipogenĂšse hĂ©patique. ______________________________________________________________________________ MOTS-CLĂS DE LâAUTEUR : Hormone thyroĂŻdienne, TRE, SynthĂ©tase des acides gras, FAS, LipogenĂšse
Decompression-induced crystallization in hydrated silica-rich melts: Empirical models of experimental plagioclase nucleation and growth kinetics
International audienceIsothermal and isobaric crystallization of plagioclase in a water-saturated synthetic rhyolitic melt is investigated through time series of decompression experiments. The experimental variables are the rate at which samples are initially decompressed (30, 150, and 1200 MPa/h) from 200 MPa and 875°C, final pressure (25 to 160 MPa), and holding time at final pressure (up to 17 days). Through textural measurements of the crystals, plagioclase crystallization kinetics has been characterized in terms of nucleation lag and rates of nucleation and growth. Plagioclase crystallization is markedly dependent on effective undercooling, ïTeff, and holding time at crystallization pressure. With ïTeff increasing from 55 to 110°C, (i) nucleation lag decreases from 1-2 days to ~15 min, (ii) maximum nucleation rates increase from ~10-3 to 10-2 mm-2.s-1, and (iii) maximum growth rates decrease from ~10-6 to 5x10-7 mm.s-1. The initial decompression rate (30, 150, and 1200 MPa/h) has no systematic control on crystallization at final pressure, except for the 1200-MPa/h series in which samples show nucleation difficulties. From the experimental data of ïTeff-constrained plagioclase number density, proportion, and morphology, we provide means to assess conditions of nucleation and growth of natural plagioclase microlites from rapidly-ascended rhyolitic melts, through the determination of plagioclase liquidus curve and ïTeff prevailing during crystallization
Expérimentation d'une nouvelle méthode culturale sur billons (BED) pour produire des pommes de terres de semences destinées à l'exportation
Affiche prĂ©sentĂ©e dans le cadre du Colloque de l'ARC, «Des racines et des ailes pour la recherche collĂ©giale», dans le cadre du 85e CongrĂšs de lâAcfas, UniversitĂ© McGill, MontrĂ©al, les 8 et 9 mai 2017.Pour le marchĂ© de lâexportation, la taille de la semence de pomme de terre est un important critĂšre de qualitĂ© : un tubercule entier mesurant entre 38 et 55 mm est recherchĂ©. Lâobjectif du projet Ă©tait dâĂ©valuer lâimpact de la taille et de la densitĂ© de plantation sur la taille des tubercules produits. Deux variĂ©tĂ©s, Chieftain et Harmony, ont Ă©tĂ© expĂ©rimentĂ©es avec deux distances entre les rangs, trois distances sur le rang et deux grosseurs de plantons. La production en plateforme a Ă©tĂ© comparĂ©e Ă la production traditionnelle en rang unique. Pour la variĂ©tĂ© Harmony, la production de tubercules de 38 Ă 55 mm a Ă©tĂ© significativement supĂ©rieure avec une distance entre les rangs de 40 cm, une distance sur le rang de 20 cm et un planton de 100 g. Le systĂšme en plateforme a gĂ©nĂ©rĂ© un rendement total supĂ©rieur et a produit davantage de tubercules de la taille recherchĂ©e. Pour la variĂ©tĂ© Chieftain, la production de tubercules de 38 Ă 55 mm a Ă©tĂ© considĂ©rablement plus Ă©levĂ©e avec une distance entre les rangs de 40 cm, des distances sur le rang de 20 et 25 cm et un planton de 100 g. Le projet a dĂ©montrĂ© que le systĂšme en plateforme est plus performant pour la production de semences de la taille recherchĂ©e pour lâexportation, que la densitĂ© de plantation, tant entre les rangs que sur le rang, a un impact sur la taille des tubercules produits, et enfin, que les variĂ©tĂ©s rĂ©pondent diffĂ©remment aux traitements de densitĂ© de plantation
Phase equilibrium constraints on the viscosity of silicic magmas with implications for mafic-silicic mixing processes
Isobaric crystallization paths obtained from phase equilibrium experiments show that, whereas in rhyolitic compositions melt fraction trends are distinctly eutectic-like, dacitic and more mafic compositions have their crystallinities linearly correlated with temperature. As a consequence, the viscosities of the latter continuously increase on cooling, whereas for the former they remain constant or even decrease during 80% of the crystallisation interval, which opens new perspectives for the fluid dynamical modeling of felsic magma chambers. Given the typical dike widths observed for basaltic magmas, results of analogue modelling predict that injection of mafic magmas into crystallizing intermediate to silicic plutons under pre-eruption conditions cannot yield homogeneous composition. Homogenization can occur, however, if injection takes place in the early stages of magmatic evolution (i.e. at near liquidus conditions) but only in magmas of dacitic or more mafic composition. More generally, the potential for efficient mixing between silicic and mafic magmas sharing large interfaces at upper crustal levels is greater for dry basalts than for wet ones. At the other extreme, small mafic enclaves found in many granitoids behave essentially as rigid objects during a substantial part of the crystallization interval of the host magmas which implies that finite strain analyses carried out on such markers can give only a minimum estimate of the total amount of strain experienced by the host pluton. Mafic enclaves carried by granitic magmas behave as passive markers only at near solidus conditions, typically when the host granitic magma shows near-solid behavior. Thus they cannot be used as fossil indicators of direction of magmatic flow
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