Nous avons étudié des harzburgites à spinelles échantillonnées au volcan andésitique Avachinsky (Sud du Kamchatka, Russie). Ces roches contiennent des inclusions vitreuses dans des spinelles et sont recoupées par des veines pyroxénitiques. L’étude des inclusions vitreuses dans les spinelles révèle deux générations de liquides: (1) un liquide formé à haute température (>1200°C) dans l’asthénosphère et (2) un liquide formé à plus basse température (>900°C) dans la lithosphère mantellique. Ce travail permet de mesurer directement la composition en éléments majeurs et en trace des liquides produits par fusion partielle dans le manteau sous-arc. Les veines webstéritiques recoupant les xénolithes sont formées par des boninites riches en Ca. Ces liquides peuvent être issus de la fusion partielle d’une harzburgite hybridisée par des fluides alumino-silicatés dérivant de la plaque Pacifique plongeante. Les boninites riches en Ca peuvent être produites dans un arc mature et ne pas être des indicateurs géodynamiques fiables mais des précurseurs de certains produits éruptifs du volcanisme andésitique. L’étude des veines orthopyroxénitiques recoupant les xénolithes révèle les processus de modification des liquides produits et percolants dans le manteau sous-arc. Ces processus se caractérisent par une combinaison de fractionnement et ré-équilibration partielle du liquide percolant avec l’encaissant péridotitique. Le processus global affecte des liquides avec des signatures géochimiques variées et se caractérise par la formation de spectres de terres rares en forme de U coexistant avec des anomalies positives en Zr-Hf. La signature en éléments en trace des boninites pauvres en Ca peut être le résultat de ce processus. Le développement méthodologique de l’imagerie tridimensionnelle par microscopie multiphotonique, appliquée aux sulfures disséminés dans les xénolithes, montre que cette technique permet d’obtenir une résolution sub-micronique avec un minimum de mise en oeuvre. L’étude de la signature en éléments du groupe du platine des sulfures des veines orthopyroxénitiques montre que cette signature est aussi fortement modifiée lors de l’interaction du liquide percolant avec un liquide sulfuré interstitiel, résidant dans la roche encaissante. L’étude du F et du Cl dans les minéraux des xénolithes nous permet d’établir les premiers coefficients de partage pour ces deux éléments, applicables à la fusion et à la cristallisation dans le manteau sous-arc. Les mesures de H2O, CO2, S et halogènes dans les inclusions vitreuses montrent qu’une part importante du recyclage de ces éléments est réalisé au travers des liquides de relative basse température (>900°C) dans le manteau lithosphérique sous-arc / We have studied spinel harzburgites sampled at the andesitic volcano Avachinsky (South of Kamchatka, Russia). These rocks contain spinel-hosted glassy inclusions and are crosscut by pyroxenitic veins. The study of glassy inclusions in spinels reveals two types of liquids: (1) one formed at high temperature (>1200°C) in the asthenosphere and (2) one formed at lower temperature (>900°C) in the lithospheric sub-arc mantle. This work allows to measure directly the composition in major and trace elements of liquids produced by partial melting in the sub-arc mantle. Websteritic veins crosscutting the xenoliths are formed by Ca-rich boninites. These liquids can be produced by partial melting of an harzburgite, hybridized by alumino-silicated fluids deriving from the sinking Pacific plate. Ca-rich boninites can be produced in a mature arc, may not be reliable geodynamic indicators and rather precursors of some eruptive products of the andesitic volcanism. The study of orthopyroxenitic veins crosscutting the xenoliths reveals the processes of modification of liquids produced and percolating in the sub-arc mantle. These processes are characterized by a combination of fractionnation and partial re-equilibration of the percolating liquid with the host peridotite. The overall process affects liquids with various geochemical signatures and is characterized by the formation of U-shaped rare-earth elements spectra coexisting with Zr-Hf spykes. The trace element signature of Ca-poor boninites can result from this process. The methodological development of three-dimensional imaging by multiphoton microscopy applied to disseminated sulfides in xenoliths shows that this technique allows to obtain a sub-micronic resolution with a minimum of implementation. The study of the platinum group elements signature of sulphides in orthopyroxenitic veins reveals that this signature is also strongly modified during the interaction of the percolating liquid with an interstitial sulfide liquid located in the host harzburgite. The study of F and Cl in minerals from the xenoliths allows us to establish the first partitioning coefficients for these two elements, suitable for partial melting and crystallization in the sub-arc mantle. H2O, CO2, S and halogens measurements in the glassy inclusions reveal that an important part of the recycling of these elements is made through the relative lowtemperature (>900°C) liquids in the lithospheric sub-arc mantle

Fluid-mobile elements (FMEs) such as B, Cs, As, Li and Tl can mobilize readily under low P-T conditions (0.2-0.5 GPa). This makes them effective geochemical tracers that can be used as a way of tracking fluid-rock exchanges at the shallow depths encountered in the earliest stages of subduction. The Izu-Bonin-Mariana (IBM) subduction system is unique in that it preserves a record of the sequences produced from the onset of subduction through the development of arc magmatism. International Ocean Discovery Program (IODP) Expedition 352 recovered >800m of boninite core material from the earliest IBM magmatic events. Select boninitic glasses from these IODP 352 cores, found mostly as selvages on the rinds of pillow lavas and as clasts within hyaloclastites, were examined via EPMA and laser ablation ICP-MS techniques. The boninite glasses analyzed were separated into two categories – low-silica boninite (LSB) and high-silica boninites (HSB), based on the bulk chemistry and mineralogy of the lithostratigraphic locations from which the glass samples occur in the drill core. LSB are the earlier erupted boninite series, which show both greater variation in extent of differentiation and reflect less depleted mantle sources than HSB. Boron concentrations in the Expedition 352 boninite glasses analyzed range from 0.08 to 12.91 ppm, arsenic contents vary from 0.15 to 3.26 ppm, and cesium varies from 0.01 to 0.91 ppm. Lithium concentrations in the boninites range from 1 to 18.35 ppm while Tl concentrations vary from 10 to 155 ppb. FME concentrations trend toward higher values in HSB than in LSB. Low-Si boninites appear to form via simple mixing of depleted mantle source and an FME enriched fluid endmember, which mobilizes B, As, Cs, (Tl) and Li very early in the subduction process. Coupled with inputs from upwelling mantle, this FME-rich fluid triggers fluid-fluxed boninite melting. The high-Si boninites reflect the addition of a subduction component with a higher Ba/La ratio than that of the depleted mantle; this higher ratio more closely resembles that of Mariana cherts from altered Pacific crust. Thus, the high-Si boninites are consistent with the fluid-fluxed melting of a highly depleted, harzburgitic mantle source and reflect inputs of two distinguishable slab-derived components, one that is sedimentary in nature and another that is FME-enriched. This model for melting that is more similar to the melting regime of modern arcs and reflects the transition from early extension-related melting into that of a “normal” subduction system.