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Understanding the pre-eruptive volatile contents in magmas is critical to charactering the magmatic plumbying systems that feed acative volcanoes, and is key to volcano monitoring and volcanic hazard assessment. Silicate melt inclusions (MIs) hosted in primitive minerals are a powerful tool to definite parental melt volatile contents, and to track the volatile degassing path upon magma ascent and decompression.Here, we apply different analyses (Raman Spectroscopy, Nano SIMS, Electron microprobe, Laser Ablation ICPMS) for the characterisation of major and trace elements and volatiles in silicate melt inclusions entrapped in minerals from recently erupted tephra by Fogo Volcano in Cape Verde archipelago, one of the most active intraplate volcanic systems on Earth.Our aims are to (i) characterise the pressure-dependent magma compositional changes taking place during magma storage and ascent, (ii) model magmatic degassing and (iii) constrain the magmatic source, and the rates/modes of magma ascent prior and during eruption.Seventeen MIs hosted in twelve olivine phenocrysts (Fo79-85) were examined from tephra samples of two distinct periods of the last 10 ky of activity of the volcano. In detail, we studied basanitic (SiO2 ~42 wt.%, MgO ~4.8 wt. %) and alkali-rich (Na2O + K2O = 6.9 wt.%) tephra samples of São Jorge (early Holocene activity, ~10 ka) and of the most recent eruptions (1951 and 2014/15).Results reveal high concentrations of incompatible trace elements (e.g.,˜70 ppm Nb) and dissolved volatiles ( ˜2.1 wt.% H2O and ≥1 wt.% CO2) in the parental (un-degassed) magma. We use different H2O-CO2 solubility models to estimate MI entrapment pressures along the magma plumbing system. The deepest entrapment pressures of ˜1000-1400 MPa (corresponding to ˜ 30-46 km) are recorded in Holocene products, while the inclusions from the recent eruptions indicate shallower entrapment pressures of ˜ 350-1100 MPa (˜ 11-35km). These entrapment pressure data, combined with previous independent barometric results, demonstrate a relatively deep (30-40 km) magma source for Fogo eruptions. Our results are the first to unambiguously demonstrate the CO2-rich nature of alkali-rich mafic melts feeding intraplate volcanism at Cape Verde.

International audience; Geochemical heterogeneities observed in the mantle are usually attributed to recycling of oceanic lithosphere through subduction. However, it remains hotly debated where recycled material stagnates, and how quickly it can be liberated back to surface. This knowledge gap hinders our understanding of mantle circulation and the chemical evolution of the Earth. Here we address these questions using a combination of geochronology and geochemistry from South China Sea (SCS) seamounts. The Shixingbei seamount lavas formed during active seafloor spreading at c. 19.1 Ma show limited geochemical variability, whereas the Zhenbei-Huangyan seamount chain formed during the post-spreading stage at c. 7.8 Ma and displays a wide range of compositions. However, melt inclusions in olivine and plagioclase from the Zhenbei-Huangyan basalts show considerably greater isotopic variability than seen in the whole rock compositions of both the SCS syn- and post-spreading lavas. A previously unidentified third mantle source component (FOZO) revealed by olivine-hosted melt inclusions along with both depleted (DMM) and enriched (EMII) mantle components is required in the source region to explain the observed isotopic and chemical variability. On the basis of our results, the age of the recycled ocean crust and sediments in this region are estimated to be c. 120 - 350 Ma. We infer that these enriched components in the SCS lavas come from the mantle transition zone. Variations in mantle source heterogeneity coupled with melting process control spatial-temporal (spreading vs. post-spreading stage) geochemical variations of lavas from the SCS and surrounding areas. Together with the results from published studies, we propose that marginal basins are one of the major locations on Earth where oceanic and/or continental lithosphere is transferred into the upper mantle and transition zone, representing an important source of upper mantle heterogeneity. We provide a simple conceptual model linking plate subduction and upper mantle heterogeneity and the volcanism in the SCS and surrounding areas.

Sulfur is a minor element in magmas but one of the major volatile elements released in volcanic systems, from the magma to the fluid phase upon ascent. Not only are sulfur gasses potentially toxic for humans and plants, they are also involved in causing drastic climate changes after major volcanic eruptions. Therefore, studies are carried out by the geoscience community to assess the magmatic sulfur flux by looking at the sulfur content and isotopes in erupted products, with the ultimate aim of improving understanding of the sulfur cycle in subduction zones. Kyushu Island in Japan hosts 25 volcanoes, among which 11 are active and represent a natural hazard for the local population. It is perhaps the most suitable site for the study of the sulfur cycle for its availability of recent volcanic deposits and its many highly monitored volcanoes. We investigated sulfur and sulfur isotope compositions of the magma source of Kyushu Island arc using olivine-hosted melt inclusions in mafic tephras and lavas, from eight volcanoes (nine Holocene samples) going from Northern Kyushu with Oninomi, Yufu, Kuju, and Aso, to Southern Kyushu volcanoes such as Kirishima-Ohachidake, Kirishima-Shinmoedake, Sumiyoshi-ike, and Kaimondake, and one back arc volcano, Fukue-Onidake. We measured major, trace and volatile elements and S isotopes (δ34S) in melt inclusions. Magma composition recorded in the inclusions ranges from basalt to andesite (SiO2 ranging from 40.3 to 60.7 wt%). For each edifice, we identified the least degassed and least differentiated compositions based on volatile and trace element systematics and selected the melt inclusions closest to their primitive melts. Comparing these primitive magmas, Sr/Y underlines a compositional dichotomy between volcanoes from northern (Sr/Y > 20) and southern Kyushu (Sr/Y

co-auteur étranger; International audience; Melt inclusions are often used to infer melting processes or to determine source magmas that are usually overprinted in bulk rocks due to late stage mixing or near surface contamination. Here we present the first investigation of oxygen (O) isotope equilibrium between melt inclusions and their host olivines from arc samples. Olivines in all but one sample record either magma mixing or fractional crystallization. All six melt inclusions from Vulcano, 83% of seven from Sukumoyama, 44% of 21 from St Vincent, 37% of four from Iwate, and 21% of 13 from Aoba are not in isotopic equilibrium with their olivine host, despite the other major elements being in apparent equilibrium. A detailed study of some of the olivines shows that only a small volume around the melt inclusions is in equilibrium with its host. This strongly suggests that in these olivines melt inclusions are trapped in partly recrystallized olivines, highlighting the importance of magma mixing and crystal recycling in the magmatic plumbing system of these volcanoes. Oxygen isotope fractionation between melt inclusions and their host olivines, as well as phosphorus-δ18O systematics, could be used to better understand the formation of melt inclusions and crystal history. It could also provide valuable information to help characterize the magmatic plumbing system that the inclusions and their olivine hosts formed in (e.g., crystal rich-mush versus crystal poor melt lenses).

Sulfur is a volatile element that outgasses from magmas. Its role in volcanology and mantle geochemistry, therefore, needs to be assessed through primitive magmas, to exclude the influence of magma evolution process and degassing. In my thesis, the primary goal is to find these primitive magmas, and characterize their volatile element concentrations and sulfur isotopic compositions. My research covers two important processes of the sulfur cycle in a subduction zone. 1) First, the transport of material from the subducting plate (slab) to the mantle wedge, because the nature of the sulfur dissolved in the fluids escaping from the slab, as well as its speciation and sulfur isotopic composition, remain poorly constrained. 2) Second is the degassing of primitive magma during its ascent, prior to eruption at arc volcanoes, because this process is critical in understanding the observed global discrepancies between total sulfur masses discharged and dissolved sulfur in magmas estimated by petrology, commonly referred to as "excess degassing of sulfur". Melt inclusions of Aso volcano (Japan) allows to identify the primitive mafic magma responsible for the deep volatile element flux in a mature volcano, providing petrological constraints on (1) the depths of the magma reservoirs, as well as (2) the origin of the observed volcanic gases. In this study, I analyzed melt inclusions and phenocrysts of Holocene basaltic eruptions, and reported their concentrations of major and volatile elements. The samples showed clear evidences of magma mixing, such as reverse mineral zonation, and highly variable mineral and glass compositions. I found that the pre-eruptive storage depths were 2 km and 4 km below the edifice, for the Strombolian eruption and the sub-Plinian eruption, respectively. The most volatile-rich primitive magma, which is one of the magmas participating in the mixing, originates from a deeper level (>10 km). I determined the initial volatile element concentrations of the primitive magma: >4.68 wt% H2O, 400 - 750 ppm CO2, 3750 ppm S, 716 ppm Cl and 324 ppm F. I then studied melt inclusions trapped in olivines from nine volcanoes on Kyushu Island, Japan to understand the systematics of sulfur behavior in the primary magmas along this arc segment. I measured major, trace, and volatile elements as well as sulfur isotopes in olivine melt inclusions from tephra of Oninomi, Yufu, Kuju, and Aso, to southern Kyushu with Kirishima (2 cones), Sumiyoshi-ike, and Kaimon, and one back-arc volcano, Fukue. For each edifice, I identified the least degassed compositions and selected melt inclusions from the primary magma. Sr/Y ratios highlight a compositional dichotomy between volcanoes in northern Kyushu (Sr/Y>20) and southern Kyushu (Sr/Y, Le soufre est un élément volatile qui dégaze des magmas. Son rôle en volcanologie et en géochimie du manteau doit donc être évalué à partir des magmas primitifs pour exclure l'influence du processus d'évolution du magma et du dégazage. Dans ma thèse, l'objectif principal est de trouver les magmas primitifs, et de caractériser leurs concentrations en éléments volatils et leurs compositions isotopiques en soufre. Mes recherches portent sur deux processus importants du cycle du soufre dans une zone de subduction. 1) Le premier est le transport de matériel de la plaque plongeante vers le coin mantellique, car la nature du soufre dissout dans les fluides s'échappant de cette dernière, ainsi que sa spéciation et sa composition isotopique, restent mal connues. 2) Le second est le dégazage du magma primitif au cours de son ascension, avant l'éruption au niveau des volcans d’arc, car ce processus est essentiel pour comprendre les écarts mondiaux observés entre les masses totales de soufre évacuées et le soufre dissout dans les magmas estimés par la pétrologie, communément appelé "excès de soufre dégazé".Les inclusions magmatiques du volcan Aso (Japon) permettent d'identifier le magma mafique primitif responsable du flux profond d'éléments volatils dans ce volcan mature, fournissant des contraintes pétrologiques sur (1) les profondeurs des réservoirs magmatiques, ainsi que sur (2) l'origine des gaz volcaniques observés. Dans cette étude, j'ai analysé des inclusions magmatiques et des phénocristaux provenant d'éruptions basaltiques holocènes, et j'ai rapporté leurs concentrations en éléments majeurs et volatils. Les échantillons présentaient des signes évidents de mélange magmatique, comme une zonation minérale inversée, et des compositions minérales et vitreuses très variables. J'ai déterminé des profondeurs de stockage pré-éruptif de 2 km et 4 km sous l'édifice, pour l'éruption strombolienne et l'éruption sub-plinienne, respectivement. Le magma primitif le plus riche en éléments volatils, qui est l'un des magmas participant au mélange, provient d'un niveau plus profond (>10 km). J'ai déterminé les concentrations initiales en éléments volatils du magma primitif : >4,68 % en poids de H2O, 400 - 750 ppm de CO2, 3750 ppm de S, 716 ppm de Cl et 324 ppm de F.J'ai ensuite étudié les inclusions magmatiques piégées dans les olivines de neuf volcans de l'île de Kyushu, au Japon, afin de comprendre la systématique du comportement du soufre dans les magmas primaires le long de ce segment d'arc. J'ai mesuré les éléments majeurs, traces et volatils ainsi que les isotopes du soufre dans les inclusions magmatiques d'olivines provenant des téphras d'Oninomi, Yufu, Kuju et Aso, du sud de Kyushu avec Kirishima (2 cônes), Sumiyoshi-ike et Kaimon, et d'un volcan de l'arrière-arc, Fukue. Pour chaque édifice, j'ai identifié les compositions les moins dégazées et sélectionné les inclusions magmatiques du magma primaire. Les rapports Sr/Y mettent en évidence une dichotomie de composition entre les volcans du nord de Kyushu (Sr/Y>20) et du sud de Kyushu (Sr/Y

Arc magmas inherit its composition from three reservoirs: the mantle wedge, the arc crust and the subducted slab. The contribution of the mantle and slab components is constrained from the study of primitive magmas, which is the magma yet to interact extensively with the crust. Determining the slab component signature is particularly important, because the nature of the petrological and geochemical signature is an indicator of the thermal state occurring along subduction zones. For instance, primitive volcanic rocks that are enriched in fluid-mobile elements (e.g. Ba, Pb, B) are related to cold subduction zones where the slab component is expected to be an aqueous fluid. Hotter subduction zones favors the occurrence of higher-temperature fluids or hydrous siliceous melts which are characterized by a geochemical signature that is enriched in fluid-immobile elements (e.g. La, Th). The geochemical signature of the slab component (either fluid, melt or something between this dichotomy) is ultimately transferred to arc magmas through mantle metasomatism and subsequent partial melting. Some high fluid-immobile element signatures of arc magmas can also be acquired in the crust by assimilation, although, in general this is a feature observed in evolved rocks. In Ecuador, some lavas are enriched in fluid-immobile elements and the origin of this geochemical signature is discussed between whether it is imparted by the slab or acquired in the continental crust. Both scenarios have good basis. On one hand, the subduction of a young oceanic crust in the Northern Ecuador (50 km) where they could acquire such geochemical signature. The aim of my work was to analyze the composition of melt inclusions hosted in rare high-forsterite (Fo>80) olivines from high magnesium basaltic andesite and andesite lavas and tephras to assure the study of the most primitive magmas from the Ecuadorian volcanic arc. All melt inclusions were crystalized, therefore, I experimentally heated them to reverse post-entrapment crystallization. After homogenization, I analyzed major, trace and volatile elements in glassy melt inclusions and their compositions were corrected for post-entrapment modifications. At the beginning, I studied olivine-hosted melt inclusions from Puñalica and Sangay volcanoes located in the southern termination of the Andean Northern Volcanic Zone. Sangay and Puñalica melt inclusions were silica-poor and resulted from partial melting of silica-poor lithologies (i.e. amphibole-bearing clinopyroxenes). Also, Puñalica melt inclusions showed high chlorine content (or high Cl/F), a feature linked with the presence under Puñalica volcano of the highly altered oceanic crust associated with Grijalva fracture zone (GFZ). GFZ is an oceanic fracture that separates a young oceanic crust (31 Ma). In the second part, I enlarged the study of olivine-hosted melt inclusion from volcanoes located in Northern Ecuador in order to cover the Ecuadorian volcanic arc from north to south. The analysis of this unique melt inclusions dataset showed an along-arc compositional variation of melt inclusions that could not be explained by crustal assimilation. However, the observed variation indicated the presence of a compositional change of the slab component. The cause of this change was explained by a change of the thermal regime along the subduction zone due to the different age of the slab. Despite the limited occurrence of high-forsterite olivine at the surface, this study shows that the trace element content of the most primitive melts from Ecuador vary considerably and that such variations depend mainly on the composition of the slab component, Les magmas d'arcs héritent leur composition de trois réservoirs: la péridotite du coin mantellique, la croûte et la plaque plongeante. La contribution du manteau et de la plaque plongeante est déterminée par l'étude des magmas primitifs avant qu'ils interagissent fortement avec la croûte. La détermination de la signature de la plaque plongeante est importante car d’une part elle apporte les principales caractéristiques géochimiques des magmas d’arc, et d’autre part, elle peut apporter des contraintes sur le régime thermique présent le long des zones de subduction. Par exemple, les roches volcaniques primitives qui sont enrichies en éléments les plus facilement mobilisés par des fluides (par exemple Ba, Pb, B) sont liées à des zones de subduction « froides » où le composant de la plaque plongeante est censé être un fluide aqueux. Les zones de subduction plus « chaudes » favorisent l'apparition de fluides à température plus élevée ou des liquides silicatés qui sont caractérisés par une signature géochimique enrichie en éléments fluides-immobiles (par exemple La, Th). La signature géochimique du composant de la plaque plongeante (soit un fluide aqueux, soit un liquide silicaté ou plutôt quelque chose entre les deux) est finalement transférée aux magmas d'arcs par le métasomatisme du manteau et la fusion partielle qui s'ensuit. Une partie de la signature des éléments fluides-immobiles des magmas d'arcs peut également être acquise dans la croûte par assimilation de cette dernière, cependant cette caractéristique est généralement observée dans les roches plus évoluées. En Équateur, certaines laves sont enrichies en éléments fluides-immobiles et l'origine de cette signature géochimique demeure discutée. Est-ce qu’elle est transmise par le composant de la plaque plongeante ou acquise dans la croûte continentale ? Les deux scénarios reposent sur les arguments suivants : d'une part, la subduction d'une croûte océanique jeune dans le nord de l'Équateur (50 km) où ils pourraient acquérir une telle signature géochimique.L'objectif de mon travail était d'analyser la composition des inclusions vitreuses piègées dans des olivines à haute teneur en forstérite (Fo>80) provenant des laves et téphras de compositions andésite-basaltiques et andesitiques à haute teneur en magnésium. Ce choix permettrait d’assurer l'étude des magmas les plus primitifs de l'arc volcanique équatorien. Toutes les inclusions vitreuses étaient cristallisées, c'est pourquoi je les ai re-homogénéisées expérimentalement pour inverser la cristallisation post-piégeage. Après l’homogénéisation, j'ai analysé les éléments majeurs, traces et volatils dans les inclusions vitreuses et leurs compositions ont été corrigées pour tenir compte des modifications post-piégeage. J’ai tout d’abord étudié les inclusions vitreuses piégées dans des olivines des volcans Puñalica et Sangay, situés dans la terminaison sud de la zone volcanique des Andes du Nord. Les inclusions vitreuses du Sangay et du Puñalica étaient pauvres en silice et résultaient de la fusion partielle de lithologies tels que des clinopyroxènes contenant des amphiboles. De plus, les inclusions de Puñalica présentaient une teneur élevée en chlore (ou Cl/F), une caractéristique liée à la présence sous le volcan Puñalica de la croûte océanique très altérée associée à la zone de fracture de Grijalva (GFZ). La GFZ est une structure océanique qui sépare une croûte océanique jeune (31 Ma).Ensuite, j'ai élargi l'étude des inclusions vitreuses piégées dans des olivines provenant des volcans situés au Nord de l'Équateur afin de couvrir l'arc volcanique équatorien du Nord au Sud. (...)