Contraintes apportées par les gaz rares sur les processus de dégazage des magmas en contexte explosif

Nous avons appliqué la géochimie des gaz rares à la dynamique des éruptions explosives. Eléments traces, inertes chimiquement, les gaz rares sont, en effet, de parfaits traceurs de sources mais aussi des processus physiques comme le dégazage des magmas. Premièrement, nous nous sommes intéressés au dégazage passif d un corps magmatique en période de repos à travers l étude de fluides hydrothermaux collectés à la surface. Deuxièmement, nous avons exploré le dégazage actif d un magma lors des explosions pliniennes à travers l étude de la phase gazeuse contenue dans les ponces. Dans ces différents types d échantillons, nous avons mesuré les fractionnements élémentaires et isotopiques des cinq gaz rares permettant ainsi d amener une contrainte temporelle sur ces processus. Le dégazage passif des magmas est étudié principalement sur le volcan de la Soufrière de Guadeloupe. Nous avons commencé par contraindre le fonctionnement du système hydrothermal en couplant les gaz rares aux méthodes de surveillances géochimiques actuelles. Puis, nous nous sommes affranchis de cette perturbation superficielle pour estimer la fréquence de remplissage de la chambre depuis la dernière éruption magmatique. Cette estimation, basée sur les flux d hélium émis par le volcan en lien avec les données géophysiques, nous a permis d émettre une nouvelle théorie sur l origine de la crise phréatique de 1976-77. Cette crise serait le résultat d une injection de magma entre 1959-1962 et d une obturation du système hydrothermal durant les années suivantes. L étude du dégazage actif des magmas lors des éruptions pliniennes a été réalisée en mesurant les concentrations et les rapports isotopiques des gaz rares contenus dans les vésicules fermées des pierres ponces. Nos mesures réalisées sur des ponces issues de zones de subduction et de points chauds sont indépendantes du contexte géodynamique, de l âge de la ponce et de la puissance de l éruption, mais similaires pour une même éruption. Nos ponces sont caractérisées par un enrichissement systématique en néon par rapport à l argon et par un rapport isotopique 38Ar/36Ar fractionné, corrélé au facteur de fractionnement du 84Kr. Nous proposons ici un modèle numérique de dégazage cinétique du magma avant sa fragmentation montrant la rapidité du processus : quelques minutes seulement semblent nécessaires. Nous avons appliqué ces nouvelles connaissances pour étudier les trois dernières éruptions pliniennes de la Montagne Pelée. Grâce à cette étude systématique, nous confirmons l utilisation des fractionnements élémentaires pour identifier à quelle éruption un affleurement peut être attribué. Nous démontrons que les différentes signatures en gaz rares des ponces sont le résultat de deux temps différents de dégazage. Nous sommes également capables de contraindre l efficacité de la fragmentation, dissociant fragmentation grossière et fragmentation fine.Noble gases are used to study magma degassing processes during explosive eruptions. They are trace elements and ideal tracers of physical processes since chemically inert. We essentially focused on the passive degassing of magma in quiescent volcanic periods and on the active degassing of magma during explosive eruptions. The passive degassing was investigated by measuring helium concentration and isotopic ratios in thermal springs and fumaroles in La Soufrière volcano (Guadeloupe, F.W.I.). By combining our study with carbon data, we are now able to understand the behaviour of the hydrothermal system. Then, 3He fluxes in both fumaroles and springs have been estimated and related to 3He content in the magma chamber. In order to maintain the 3He flux measured at the surface, we conclude that the magma chamber must be regularly fed by fresh magma batches. Using our new results and data from literature, we propose that the historical activity of la Soufrière volcano can be explained by both abnormal energy inputs from new magma batches in the chamber and cycles of clogging of the hydrothermal system. We propose a new scenario for the origin of the 1976-1977 crisis whereby a fresh batch of magma could have been emplaced possibly between 1959 and 1962 in the magma chamber. The resulting heat flux is not stored in the different aquifers but preferentially evacuated through fractures reactivated or created during the 1956 phreatic eruption. Only when the self-sealing of the hydrothermal system is sufficiently developed, can pressure and temperature within the aquifers rapidly increase to trigger a crisis. To assess active magma degassing processes during Plinian eruptions, noble gas abundances and isotopic ratios have been determined in preserved vesicles of pumices. All samples are characterized by a systematic enrichment in neon over argon and an isotopically fractioned 38Ar/36Ar ratio associated with the fractionation factor of 84Kr. These features do not depend on geological setting, or on pumice age, or eruption intensity. However, they are similar for pumices from the same eruption. We propose here a model of kinetic magma degassing before fragmentation. The model explains measurements and shows the rapidity of the magma degassing process in the conduit (few minutes). Then, we analysed samples coming from the last three Plinian eruptions of Montagne Pelée volcano (F.W.I.). Thanks to this new study, we confirmed the use of noble gas patterns to identify to which eruption an outcrop can be associated. We also demonstrate that the elemental and isotopic fractionations of noble gases in pumices are the result of two different degassing times. For the last Plinian eruption, we are also able to evaluate the efficiency of the fragmentation, separating coarse fragmentation and fine fragmentation.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Effect of water on the fluorine and chlorine partitioning behavior between olivine and silicate melt

Halogens show a range from moderate (F) to highly (Cl, Br, I) volatile and incompatible behavior, which makes them excellent tracers for volatile transport processes in the Earth's mantle. Experimentally determined fluorine and chlorine partitioning data between mantle minerals and silicate melt enable us to estimate Mid Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB) source region concentrations for these elements. This study investigates the effect of varying small amounts of water on the fluorine and chlorine partitioning behavior at 1280 °C and 0.3 GPa between olivine and silicate melt in the Fe-free CMAS+F-Cl-Br-I-H2O model system. Results show that, within the uncertainty of the analyses, water has no effect on the chlorine partitioning behavior for bulk water contents ranging from 0.03 (2) wt% H2O (DCl(ol/melt) = 1.6 ± 0.9 × 10(-4)) to 0.33 (6) wt% H2O (DCl(ol/melt) = 2.2 ± 1.1 × 10(-4)). Consequently, with the effect of pressure being negligible in the uppermost mantle (Joachim et al. Chem Geol 416:65-78, 2015), temperature is the only parameter that needs to be considered for the determination of chlorine partition coefficients between olivine and melt at least in the simplified iron-free CMAS+F-Cl-Br-I-H2O system. In contrast, the fluorine partition coefficient increases linearly in this range and may be described at 1280 °C and 0.3 GPa with (R(2) = 0.99): [Formula: see text]. The observed fluorine partitioning behavior supports the theory suggested by Crépisson et al. (Earth Planet Sci Lett 390:287-295, 2014) that fluorine and water are incorporated as clumped OH/F defects in the olivine structure. Results of this study further suggest that fluorine concentration estimates in OIB source regions are at least 10% lower than previously expected (Joachim et al. Chem Geol 416:65-78, 2015), implying that consideration of the effect of water on the fluorine partitioning behavior between Earth's mantle minerals and silicate melt is vital for a correct estimation of fluorine abundances in OIB source regions. Estimates for MORB source fluorine concentrations as well as chlorine abundances in both mantle source regions are within uncertainty not affected by the presence of water

Halogens in Eclogite Facies Minerals from the Western Gneiss Region, Norway

Ultra-high-pressure (UHP) eclogites and ultramafites and associated fluid inclusions from the Western Gneiss Region, Norwegian Caledonides, have been analysed for F, Cl, Br and I using electron-probe micro-analysis, time-of-flight secondary ion mass spectrometry and neutron-irradiated noble gas mass spectrometry. Textures of multi-phase and fluid inclusions in the cores of silicate grains indicate formation during growth of the host crystal at UHP. Halogens are predominantly hosted by fluid inclusions with a minor component from mineral inclusions such as biotite, phengite, amphibole and apatite. The reconstructed fluid composition contains between 11.3 and 12.1 wt% Cl, 870 and 8900 ppm Br and 6 and 169 ppm I. F/Cl ratios indicate efficient fractionation of F from Cl by hydrous mineral crystallisation. Heavy halogen ratios are higher than modern seawater by up to two orders of magnitude for Br/Cl and up to three orders of magnitude for I/Cl. No correlation exists between Cl and Br or I, while Br and I show good correlation, suggesting that Cl behaved differently to Br and I during subduction. Evolution to higher Br/Cl ratios is similar to trends defined by eclogitic hydration reactions and seawater evaporation, indicating preferential removal of Cl from the fluid during UHP metamorphism. This study, by analogy, offers a field model for an alternative source (continental crust) and mechanism (metasomatism by partial melts or supercritical fluids) by which halogens may be transferred to and stored in the sub-continental lithospheric mantle during transient subduction of a continental margin

The Kallisti Limnes, carbon dioxide-accumulating subsea pools

International audienceNatural CO 2 releases from shallow marine hydrothermal vents are assumed to mix into the water column, and not accumulate into stratified seafloor pools. We present newly discovered shallow subsea pools located within the Santorini volcanic caldera of the Southern Aegean Sea, Greece, that accumulate CO 2 emissions from geologic reservoirs. This type of hydrothermal seafloor pool, containing highly concentrated CO 2 , provides direct evidence of shallow benthic CO 2 accumulations originating from sub-seafloor releases. Samples taken from within these acidic pools are devoid of calcifying organisms, and channel structures among the pools indicate gravity driven flow, suggesting that seafloor release of CO 2 at this site may preferentially impact benthic ecosystems. These naturally occurring seafloor pools may provide a diagnostic indicator of incipient volcanic activity and can serve as an analog for studying CO 2 leakage and benthic accumulations from subsea carbon capture and storage sites