Geothermobarometry of the 2010 Eyjafjallajökull eruption: New constraints on Icelandic magma plumbing systems

The 2010 Eyjafjallajkull eruption in Iceland produced mildly alkaline basalt that was emitted during the initial flank eruptive phase, whereas tephra predominately of benmorite composition was erupted during the second explosive phase from the summit of the volcano. These latter magmas show pervasive magma mingling between basalts and silicic magma. Glass and coexisting equilibrium mineral analyses have been used to define pressure-temperature crystallization paths for the eruption based on melt, clinopyroxene-melt and plagioclase-melt thermobarometry. Temperature calculations show that the early basaltic eruptions from the flank eruption have magmatic temperatures of around 1170°C (±25°C) and a narrow temperature range (<30°C) at any given depth. In contrast, benmoritic products crystallized at lower temperatures (1000-1060°C). Pressure estimates yield an average pressure of 5.6-6.4 kbar (±1.5 kbar) for the basaltic tephra and variable but lower pressures for the benmoritic samples ranging down to 0.6 kbar. The mafic magma mainly crystallized in the deeper crust (16-18 km), whereas mingled magma from the summit eruption crystallized at more shallow crustal levels (2-5 km) suggesting multistage magma ascent. Magmatic water concentrations were estimated with plagioclase-melt hygrometry. The maximum average water content of 1.8 wt % H₂O, obtained in one of the summit samples, is in agreement with melt inclusion observations. Water concentration of this or lower levels is demonstrated to only have limited effect on the pressure-temperature calculations.