Behavior of volatiles in arc volcanism : geochemical and petrologic evidence from active volcanoes in Indonesia

Large amounts of material are recycled along subduction zones by uprising magmas, of which volcanoes are the surface expression. This thesis focuses on the behavior of volatiles elements (S, Cl, H) during these recycling processes. The study area is the Indonesian arc system, which hosts many active volcanoes. Application of whole-rock and in situ analytical techniques on material collected from nine volcanoes provided a large data set including the chemical and stable isotope compositions of bulk lavas, and chemical compositions of minerals, melt inclusions and soluble material adsorbed on ashes from eruption plumes. Evidence is presented that sulfur is enriched in the sub-arc mantle by slab-derived additions. Magma sources in the mantle wedge are metasomatized by material enriched in heavy sulfur, most likely in the form of hydrous fluids, that is derived from subducted sediments rather than from subducted altered oceanic crust. Among the chalcophile elements, lead, and possibly copper, must have been added as well, whereas cobalt and nickel are largely controlled by the composition of the pre-subduction mantle. Furthermore, it is demonstrated that large sulfur fluxes at the onset of the 1982-83 eruptions of Galunggung volcano were derived from degassing of non-erupted deep-seated magma, whereas most of the chlorine must have been released directly from erupting magma. The presence the rare mineral zirconolite in vesicle fillings in the lavas of Lewotolo volcano provides strong evidence that high field-strength elements (e.g., Nb, Zr, Ti), rare earth elements (e.g., Ce, La, Nd) and actinides (Th, U) can be mobilized during fluid-driven processes in fluorine-rich shallow magmatic environments. This finding lends support to the hypothesis that HFSE, REE and actinide-bearing ore deposits in island arcs are preferentially formed around potassic magma bodies