To constrain the temporal evolution of the fluid-magma system responsible for the 2010 Eyjafjallajkull eruption (20 March to 20 May, 2010, Southern Iceland), we investigated the volatile, major, trace element, and Sr-Nd-Pb isotopic compositions of bulk lapilli and ash samples representing different stages of the eruption. In addition, we analyzed ash leachates and volcanic plume-derived aerosols sampled over Southern Europe in early May 2010. Available remote-sensing data for the total mass of SO2 liberated in the 2010 eruption, together with data obtained in this study, suggest that the high explosivity of the 2010 sub-plinian Eyjafjallajkull eruption was caused by saturation of the pre-eruptive hybrid trachyandesitic magma with aqueous fluid. We hypothesize that the bulk of the aqueous fluid had been dissolved in the trachydacitic melt at least since the eruption of 1821-1823. The trachydacitic melt was enriched in volatiles, large-ion lithophile elements, and high field strength elements, a composition similar to that of oceanic sediments. The Sr-Nd-Pb isotopic data obtained in this study, combined with existing O isotopic data on bulk 2010 Eyjafjallajkull lapilli and ashes, demonstrate that this melt enrichment was due to its primary source being hydrous oceanic lithosphere entrained in a deep mantle plume rather than shallow hydrothermally altered crust. The hypothesized strong crystal fractionation of plume-derived mafic melts (crystallized fraction >90%) from an enriched mantle source can explain the observed high explosivity of silicic and hybrid Icelandic magmas, especially those of the southeastern volcanic zone (Eyjafjallajkull, Katla, and Hekla).