The opening phase of the 2010 summit eruption of Eyjafjallajökull volcano, Iceland: contributions from morpho-textural and geochemical characterization of tephra

The 2010 summit eruption of Eyjafjallajökull volcano (Iceland) started on 14 April when the eruption melted its way through the ice-filled summit caldera. We focus on the opening phase of the event (first 15-17 hours), which represents a unique opportunity to investigate triggering mechanisms and pre-eruptive dynamics. Tephra samples were collected from the pristine fallout deposits ~4.5 km east from the crater. On the whole, the juvenile components along the eruptive sequence show a large morphological and textural variability. The peculiarity of the products of the opening phase is the presence of abundant (40 vol.%) blocky, dense, obsidian-like, microlite-poor clasts. These features suggest important quenching effects of the first body intruded at the base of the ice cover immediately before the eruption onset. Major and trace element analyses on groundmass glass of this phase highlight the occurrence of clasts with a larger compositional variability than those of the products from the following phases. Similarly, 87Sr/86Sr ratios determined by microdrilling on single ash clasts revealed the presence of groundmass glass with large isotopic variability, ranging from the typical values recorded in Icelandic magma, up to much higher values (87Sr/86Sr = 0.70668), never found in Icelandic volcanics. Conversely, plagioclase crystals from the same phase of activity have low 87Sr/86Sr values. These compositional and isotopic data suggest that the very first erupted magma assimilated hydrothermally altered volcanic/epiclastic rocks with high Sr isotopic ratio, possibly in the immediately pre-eruptive phase. The results confirm the importance of studying the products of the opening phases of explosive eruptions in order to get a complete understanding of pre-eruptive conditions.