The “Fani Maoré” eruption off the coasts of Mayotte has been intensively monitored by applying methods similar to those used for subaerial eruptions. Repeated high-resolution bathymetric surveys and dredging, coupled with petrological analyses of time-constrained samples, allowed tracking the evolution of magma over the whole submarine eruptive sequence. Indeed, after one year of direct ascent (Phase 1), basanitic magma switched to a different pathway that sampled a tephri-phonolitic subcrustal reservoir (Phase 2). Later, the magma pathway shifted again in the crust resulting in a new eruption site located 6 km northwest of the main edifice (Phase 3). The petrological signature of lava flows reveals both an evolution by fractional crystallization and syn-eruptive mixing with a tephri-phonolitic magma. We demonstrate that high-flux eruption of large volumes of basanitic magma from a deep-seated reservoir can interact with shallower reservoirs and remobilize eruptible magma. This has significant hazards implications with respect to the capacity of such large eruptions to reactivate shallow-seated inactive reservoirs from a transcrustal magmatic system that could be located potentially at a distance from the high-flux eruptive site.

Five effusive eruptions of Piton de la Fournaise (La Réunion) are analyzed to investigate temporal trends of erupted mass and sulfur dioxide (SO₂) emissions. Daily SO₂ emissions are acquired from three ultraviolet (UV) satellite instruments (the Ozone Monitoring Instrument (OMI), the Ozone Mapping and Profiler Suite (OMPS), and the Tropospheric Monitoring Instrument (TROPOMI)) and an array of ground-based UV spectrometers (Network for Observation of Volcanic and Atmospheric Change (NOVAC)). Time-averaged lava discharge rates (TADRs) are obtained from two automatic satellite-based hot spot detection systems: MIROVA and MODVOLC. Assuming that the lava volumes measured in the field are accurate, the MIROVA system gave the best estimation of erupted volume among the methods investigated. We use a reverse petrological method to constrain pre-eruptive magmatic sulfur contents based on observed SO₂ emissions and lava volumes. We also show that a direct petrological approach using SO₂ data might be a viable alternative for TADR estimation during cloudy weather that compromises hot spot detection. In several eruptions we observed a terminal increase in TADR and SO₂ emissions after initial emission of evolved degassed magma. We ascribe this to input of deeper, volatile-rich magma into the plumbing system towards the end of these eruptions. Furthermore, we find no evidence of volatile excess in the five eruptions studied, which were thus mostly fed by shallow degassed magma.

The 2018–2020 lava flow field that built the submarine Fani Maoré volcano (Mayotte, north Mozambique Channel) at a water depth of 3,300 m was extensively sampled. We use the petrological analysis of samples collected at high spatial and temporal resolution to provide information on the textural evolution of the submarine lavas and its impacts on lava rheology, and thus on flow emplacement. Although the crystal content is lower than 10%, we observed two populations of crystals: (a) the micro‐phenocrysts which mostly display skeletal shapes suggesting rapid growth upon magma ascent most likely due to decompression‐driven crystallization, (b) the microlites resulting from post‐emplacement groundmass crystallization. Combining both petrological and geochemical analyses, and applying rheological models, we show that lava viscosity increases from 158 ± 21 to 608 ± 134 Pa·s considering rigid bubbles or from 36 ± 5 to 485 ± 130 Pa·s considering deformable bubbles. Together with the waning ascent velocity and the decreasing effusion rates, this correlates with the evolution of lava flow emplacement dynamics, from long sheet lava flows to shorter flows exhibiting pillow mounts and multiple breakouts.

De l'ascension du magma à l'emplacement des coulées de laves sur le plancher océanique : étude de l'éruption sous-marine de 2018 - 2020 de Fani Maoré (Mayotte, Canal du Mozambique): Le volcanisme sous-marin représente environ 75 % de l'activité volcanique sur Terre. Cependant, en raison de la difficulté d'accès aux éruptions sous-marines, celles-ci restent peu détectées, et rarement observées. Suite à la crise sismique qui a impacté Mayotte (canal du Mozambique) en mai 2018, un événement magmatique prenant racine dans la lithosphère a donné naissance à un volcan sous-marin, Fani Maoré, à 50 km de l'île. Cette éruption sous-marine effusive, qui dura environ deux ans, est la plus importante depuis le Laki (1783), avec plus de 6 km3 de lave basanitique émise à 3300 m de profondeur.Plusieurs campagnes océanographiques ont fourni un grand nombre d'échantillons couvrant l'ensemble des coulées de lave émises entre 2018 et 2020 à des résolutions spatiales et temporelles élevées. Ce jeu d'échantillons provient aussi bien du sommet du volcan que de ses coulées distales, offrant une occasion unique de comprendre les processus à l'origine de cette éruption et de la mise en place d'un volume conséquent de magmas alcalins riches en éléments volatils dans un environnement sous-marin. Pour cela, cette étude combine des observations in situ, des quantifications physiques et texturales des roches, ainsi que des mesures et calculs de viscosité.Cette thèse repose sur une analyse texturale détaillée des vésicules qui met en évidence une variation de texture allant de lave très poreuse (moyenne de 35 %) à de la lave dense (14 %). Ceci résulte d'un dégazage de plus en plus efficace au cours du temps suite à une variation des vitesses d'ascension du magma et du taux d'effusion. En parallèle, l'étude réalisée sur les cristaux (taille, forme et pourcentage) confirme l'absence de phénocristaux dans les laves de Fani Maoré. Cependant, deux populations de cristaux sont mises en évidence : les micro-phénocristaux (>100 µm) et microlites (< 100 µm). D'après les formes squelettiques des micro-phénocristaux, ces derniers se seraient formés lors de l'ascension du magma. En revanche, les microlites observés dans la matrice auraient cristallisés après la mise en place des coulées de lave. Les données texturales, les analyses géochimiques, et les mesures de viscosité ont ensuite été intégrées à des modèles de viscosité à trois phases pour (i) quantifier l'effet des cristaux et des bulles sur la viscosité, et (ii) explorer les variations rhéologiques spatio-temporelles au cours de cette éruption sous-marine. Les résultats démontrent qu'un changement rhéologique s'est produit, conduisant à une évolution de la dynamique de mise en place des coulées de lave.Cette thèse fournie donc des contraintes critiques sur (i) les processus de dégazage et de cristallisation dans le temps et l'espace, et (ii) l'évolution temporelle de la rhéologie de la lave qui a eu un impact sur l'emplacement des coulées au cours de cette éruption sous-marine majeure. Enfin, l'intérêt scientifique de cette étude met en avant la nécessité de caractériser un tel site éruptif car, dans le futur, des éruptions pourraient à nouveau se produire le long de la chaîne volcanique sous-marine de Mayotte.

Following an unprecedented seismic activity that started in May 2018, a new volcanic edifice, Fani Maoré, was constructed on the ocean floor 50 km east of the island of Mayotte (Indian Ocean). This volcano is the latest addition to a submarine volcanic chain characterized by an alkaline basanite-to-phonolite magmatic differentiation trend. Here, we performed viscosity measurements on five silicate melts representative of the East-Mayotte Volcanic Chain compositional trend: two basanites from Fani Maoré, one tephri-phonolite and two phonolites from different parts of the volcanic chain. A concentric cylinder viscometer was employed at super-liquidus conditions between 1500 K and 1855 K, and a creep apparatus was used for measuring the viscosity of the undercooled melts close to the glass transition temperature in the air. At super-liquidus temperatures, basanites have the lowest viscosity (0.11 to 0.99 log10 Pa⸱s), phonolites the highest (1.75 to 3.10 log10 Pa⸱s), while the viscosity of the tephri-phonolite falls in between (0.89 - 1.97 log10 Pa⸱s). Viscosity measurements at undercooled temperatures have only been performed for one phonolite melt because Raman spectroscopy showed nanolites within the basanite and tephri-phonolite glass samples. The phonolite has a viscosity of 10.19 to 12.30 log10 Pa⸱s at 1058 to 986 K. Comparison with existing empirical models revealed discrepancies up to 2.0 log units with our experimental measurements. This emphasizes (i) the lack of data falling along the alkaline basanite-to-phonolite magmatic differentiation trend to calibrate empirical models, and (ii) the complexity of modeling the variations in viscosity as a function of temperature and chemical composition for alkaline magmas. The presented new measurements indicate that, at eruptive temperatures between 1050 °C and 1150 °C, the anhydrous, crystal- and bubble-free basanite melt is very fluid with a viscosity around 2.6 log10 Pa⸱s whereas the anhydrous phonolite crystal- and bubble-free melt at eruptive temperatures ranging from 800 to 1000 °C has a viscosity around 6 - 10 log10 Pa⸱s. These new viscosity measurements are essential to define eruptive models and to better understand the storage, transport and ascent dynamics of Comoros Archipelago magmas, and of alkaline magmas in general, from the source to the surface.