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Piton de la Fournaise is an active shield volcano located in the eastern area of the Réunion Island (Indian Ocean) whose activity is characterized by effusive and explosive episodes with the emission of scarcely differentiated magmas with mostly tholeiitic affinity. The presently active edifice has grown within the Enclos Fouqué caldera, a polylobate plain bounded on its western side by the 80–200 m high Bellecombe vertical cliffs. This escarpment exposes a vertical sequence of 12 lava flows cut by a dike with an age > 5.5 kyrs. In this work, the Bellecombe products were investigated by X-ray fluorescence, Inductively Coupled Plasma Mass Spectroscopy, a Scanning Electron Microscope and X-ray computed microtomography in order to characterize the evolution over time of the magmatic system feeding the eruptive activity prior to the Enclos Fouqué caldera collapse. The results indicate that lava flows share a geochemical affinity with the two main series documented at Piton de la Fournaise, namely, Steady State Basalts (SSB) at the bottom and top of the sequence and Abnormal basalt Group (AbG) with different degrees of differentiation in the central part. The emission of these two different products in both a restricted area and timespan testifies to the dynamic activity of the plumbing system, capable of shifting rapidly from central to eccentric activity in the recent past.

International audience; Basaltic eruptions are commonly associated with lava emissions and relatively weak explosive activities, but they can sometimes produce strong explosive eruptive phases. In April and November 2005, two paroxysmal eruptive events occurred within the summit crater of Karthala basaltic shield volcano (Grande Comore Island, Comoros), which hosted a water lake before each of these events. Both 2005 ash plumes spread across the Comoros Archipelago and heavily impacted the whole Grande Comore Island. Associated deposits on the volcano summit are extremely fine-grained (up to 50 wt% of fine ash

Co-auteur étranger; International audience; Deep-sea submarine eruptions are the least known type of volcanic activity, due to the difficulty of detecting, monitoring, and sampling them. Following an intense seismic crisis in May 2018, a large submarine effusive eruption offshore the island of Mayotte (Indian Ocean) has extruded at least 6.5 km3 of magma to date, making it the largest monitored submarine eruption as well as the largest effusive eruption on Earth since Iceland's 1783 Laki eruption. This volcano is located along a WNW-ESE volcanic ridge, extending from the island of Petite Terre (east side of Mayotte) to about 3,500 m of water depth. We present a detailed petrological and geochemical description of the erupted lavas sampled by the MAYOBS 1, 2, and 4 cruises between May and July 2019 and use these to infer characteristics and changes through time for the whole magmatic system and its dynamics from the source to the surface. These cruises provide an exceptional time-series of bathymetric, textural, petrological, and geochemical data for the 2018-2019 eruptive period, and hence bring an invaluable opportunity to better constrain the evolution of magma storage and transfer processes during a long-lived submarine eruption. Integrating the petrological signatures of dredged lavas with geophysical data, we show that the crystal-poor and gas-rich evolved basanitic magma was stored at mantle depth (>37 km) in a large (≥10 km3) reservoir and that the eruption was tectonically triggered. As the eruption proceeded, a decrease in ascent rate and/or a pathway change resulted in the incorporation of preexisting differentiated magma stored at a shallower level. Magma transfer from the deep mantle reservoir is syn-eruptive, as indicated by transfer times estimated from diffusion in zoned olivine crystals that are much shorter than the total eruption duration. Our petrological model has important hazard implications concerning the rapid and stealthy awakening of a deep gas-rich magma reservoirs that can produce unusually high output rates and long-lived eruption. Sudden tapping of large crystal poor reservoirs may be the trigger mechanism for other rarely witnessed high-volume (>1 km3) effusive events.

International audience; Since 2018, the submarine east flank of Mayotte Island (Comoros archipelago) is the site of a major eruption located at 3.5 km depth bsl on a WNW-ESE volcanic ridge. Samples brought by oceanographic cruises carried out to monitor this seismo-volcanic crisis indicate that this volcanic ridge is built by a bimodal sodic alkaline magmatic series that includes basanites and phonolites. A petrological study of dredged samples allowed us to image the magmatic system feeding the volcanic ridge and to determine the link between basanitic and phonolitic magmas. The magmatic system feeding the volcanic ridge comprises multiple levels of magma storage. Basanitic magmas generated at 80–100 km mantle depth are stored in two or more deep reservoirs (≥ 37 km) and then in shallower basanitic and phonolitic lenses located close to the Moho interface before rising the surface. This study identifies three possible scenarios: (1) the deep basanitic magma rises directly and quickly to the surface from the deep mantle reservoir (as is currently happening 60 km offshore), (2) the basanitic magma stalls in a shallower reservoir near the Moho before resuming its ascent toward the surface and erupting as porphyritic basanite, (3) the basanitic magma stops and evolves to phonolite in these sub-crustal reservoirs. The phonolitic lavas are produced by approximately 80% fractional crystallization (34% clinopyroxene, 30% anorthoclase feldspar, 15.5% magnetite, 12.5% olivine, 5% apatite and 4% ilmenite) of a hydrous basanitic magma at mantle depths (P > 0.6 GPa) under reduced oxygen fugacity (~ FMQ-1). In this third scenario, the phonolitic magma might be reactivated by the arrival of a new batch of deeper basanitic magma.

International audience; 25 It is commonly accepted that effusive activity emplaces the main emitted magmatic volume in basaltic 26 shield volcanoes. At Piton de La Fournaise (La Réunion Island, France), eruptive activity occurs mostly within the 27 non-populated Enclos Fouqué caldera and generally does not pose any risk to the population. However, historical 28 observations, recent monitoring data and field work on tephra deposits suggest that some eruptions have produced 29 unusual and unexpected explosive phases. A comprehensive sampling of tephra from major historical and recent 30 eruptions on this volcano allowed us to perform systematic componentry, grain size, chemical and morphological 31 analyses in order to characterize the eruptive dynamics involved in these explosive basaltic eruptions. This 32 integrative approach reveals highly variable characteristics of the studied tephra reflecting different fragmentation 33 efficiencies and multiple associated mechanisms. Primary ductile and partially brittle fragmentation of various 34 intensity of juvenile magma emitted during Hawaiian fountaining or mild Strombolian explosions were identified 35 as the most common fragmentation mechanisms, in particular during the 2014-2018 dominantly effusive eruptive 36 sequence. In parallel, we distinguished more efficient short-lived fragmentation events related to (i) plug 37 pressurization and brittle fragmentation enhanced by syn-eruptive crystallization, (ii) magma/water interactions, 38 (iii) rare phreatic explosions and (iv) secondary fragmentation producing fine ash during caldera collapse. We 39 conclude that textural, geochemical and morphological analysis make it possible to identify and characterize the 40 variability in eruptive processes, with the grain size and the componentry of the ash particles being probably the 41 most important parameters to quantify both the efficiency and the nature of the fragmentation. 42 43

Crystal nucleation and growth have been investigated approximately 100 K above the glass transition for 12 liquids in the system CaO–Al 2 O 3 –SiO 2 . Nucleation takes place heterogeneously at the gas–liquid interface, yielding various crystal structures, namely, those of yoshiokaite, gehlenite, larnite, anorthite and wollastonite. Characterization by Raman spectroscopy, transmission electron microscopy and electron microprobe indicate that these phases are highly disordered, poorly crystallized and generally non-stoichiometric. Even for incongruent crystallization, crystals have the same Si/Al ratio as their parent melts but variable CaO contents. These features may be rationalized in terms of the contrasting mobility of network-forming cations, which practically vanishes at the glass transition, and that of calcium which remains significant. The fact that the mobility of network-modifying cations controls the structure and composition of nucleating phases implies that liquid viscosity (controlled by the mobility of network formers, i.e. the frequency of Si–O and Al–O bond breaking) is not an appropriate scaling parameter for crystal nucleation. In turn, this may provide an explanation for the discrepancies, reaching many orders of magnitude, between experimentally determined nucleation rates and those predicted from classical and non-classical nucleation theories. An additional factor contributing to such discrepancies is that thermodynamic data obtained on stoichiometric phases stable at liquidus temperatures are not appropriate for estimating the thermodynamic barrier to nucleation of the metastable crystals that actually form at large degrees of supercooling.

As measured in air above the glass transition range, the viscosity of an alkali basalt increases markedly with time by about two orders of magnitude in 12 h. This effect is essentially physical and due to the presence of microcrystals although partial crystallization of the melt into spinel and an SiO2-poor pyroxene leads to a considerable enrichment in silica of the residual liquid. Partial crystallization depends strongly on the initial redox state of samples in that the presence of ferrous iron is required for spinel crystals to form and for pyroxene to nucleate and grow around them. Other measurements show that the viscosity of the crystal-free liquid decreases slightly with increasing ratios r=Fe2+/∑Fe because the differences between samples with r=0.16 and 0.83 amount to about 1.5 and 0.3 log-units at 950 and 1400 K, respectively. Comparisons of the viscosities of the residual liquid matrix and of the initially crystal-free basalt show that physical effects caused by the presence of microcrystals begin to be observed at a low crystal fraction of 5 vol%. Finally, a model of viscosity calculation is developed for the melts which reproduces all data obtained in this work to better than 10%.

Hematite reduction in the presence of pyrite and CaO led to the formation of anhydrite and magnetite at 30 MPa and 723 K under hydrothermal conditions in a sealed gold container. For the first time, hydrothermal magnetite production was monitored magnetically at the run conditions. Sample magnetic susceptibility was found to follow an Avrami kinetic law with k = 1.5 10 −5 /s and n = 2.07. Application of up to two external field reversals in the course of the magnetite production reaction shows that partial Chemical Remanent Magnetizations (CRM) are recorded by the sample. The intensity of each partial CRM acquired within a given time interval (under a given field intensity) is proportional to the amount of magnetite produced during that time interval. Alternating magnetic field demagnetization failed at separating these partial CRMs, which are believed to superpose in the ( V , Hc ) diagram of Neel. Additional TEM, SEM, and magnetic characterizations suggest that continuous nucleation and fast magnetite growth (to a maximum size of ca. 10 μm), compared to the field polarity duration, can account for the evolution of the sample CRM with time. This study demonstrates that one should be cautious when using natural CRM apparent polarity to infer timing of the corresponding chemical event.

Fragments of unaltered andesite found at all levels in the deposits of the catastrophic flank-failure, directed-blast eruption of the Soufriere of Guadeloupe in 3100 bp are thought to be remnants of the cryptodome. They were observed in analytical transmission electron microscopy for clues to the evolution of the intrusion prior to the eruption. Several features that could potentially be used as temperature markers were identified, among which the angle between microexsolutions of magnetite in augite phenocrysts was used to find an upper boundary of the temperature of the intrusion before the eruption: 600–700°C. Calculation of the time a dyke or sphere-shaped intrusion may have taken to cool from the emplacement temperature down to the temperature of exsolution of the magnetite leads to an estimate of the time between emplacement of the intrusion and the eruption, which could not have been less than a few tens of years. It therefore seems probable that the emplacement of the magmatic intrusion was not the immediate cause of the flank destabilization and catastrophic eruption of the Soufriere in 3100 bp.