Recyclage et dégazage du soufre dans une zone de subduction : Aperçus à partir des compositions isotopiques de soufre et des éléments volatils des inclusions magmatiques piégées dans les olivines des volcans de l'île de Kyushu, au Japon

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/Y10 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