Unraveling the complex magma dynamics during the Eyjafjallajökull 2010 eruption by high-resolution geochemistry of volcanic ash

The April-May 2010 eruption of Eyjafjallajökull (Iceland) volcano was characterized by a change in eruptive style and a large compositional variability of erupted products. Lava deposits of the initial phase consist of evolved Fe-Ti-basalt ( 47 wt% SiO2), whereas fallout deposits of the explosive phases are characterized by a wider compositional spread. We present new EMPA and LA-ICP-MS analyses on groundmass glasses of ash particles erupted between May 18th and 22nd in 2010, i.e. the last days of the eruption. The glasses define two well-separated groups; a basalt composition ranging from 49.98 to 51.76wt% SiO2 and a second group of trachyandesitic and rhyolitic compositions between 57.13 and 70.38 wt% SiO2. Introduction of basalt to a resident silicic melt increased that compositional diversity in the chamber. The ash particles contain plagioclase (An81-10), clinopyroxene (Mg#=0.44-0.71), olivine (Fo83-42) and magnetite. Several plagioclase and clinopyroxene crystals exhibit resorbed cores and rims suggesting disequilibrium caused by recharge of a hotter magma. Additionally, certain clinopyroxene reveals reverse zoning (core: Mg# = 0.53-0.60, rim: Mg# = 0.59-0.71) what can be taken as evidence for mixing between a younger, less evolved magma, and one with a more evolved composition. Here, we unravel the genesis of the compositional variance using petrological, mineralogical and geochemical implications. Magma mixing modeling and element concentration frequency diagrams indicate that incomplete magma mixing between a trachyandesite magma and a trachyte-rhyolite component is the probable main process forming the great compositional variability observed in the erupted products. The rhyolite composition probably represents the residual melt of the AD 1821-23 eruption of Eyjafjallajökull. We suggest that different magmas generated and emplaced separately prior to the eruption have been mobilized by a new intrusion in 2010, and have mingled and mixed with each other during eruption.