LAUSR

Basalt eruptions at Grimsvotn volcano, Iceland, are generally of low intensity; however, occasionally, an order of magnitude larger eruptions occur. In order to discuss the reasons for this difference, the degassing budget of S and Cl and crystallization conditions of the eruptive magma were determined from volatile concentration measured in melt inclusion (MI) and groundmass glass and thermobarometry, respectively. Tephra of two of the largest historical eruptions (2011 and 1873) and two much smaller eruptions (2004 and 1823) were investigated. Sulfur and Cl concentrations are higher in groundmass glass of the smaller eruptions due to incomplete outgassing caused by melt quenching in contact with glacial water. Sulfur concentration and degassing budget correlate with erupted magma volumes. Higher volatile concentrations of MI from the larger eruptions re fl ect important recharge of gas-rich magma from depth. The recharge causes a high-magnitude eruption followed by increased eruption frequency over the following decades. Pressure and temperature estimates of crystallization are obtained through equilibrium clinopyroxene-glass pairs, where crystals adjacent to, and in textural equilibrium with, both groundmass glass and that of MI were measured. An average crystallization pressure of 4 ± 1 kbar corresponding to approximately 15 ± 5 km depth was obtained together with a considerable temperature range, 1065 – 1175°C. Similar crystallization depths are obtained for the basalt of the 2014 – 2015 Barðarbunga rifting event and to a low resistivity layer revealed by magnetotelluric surveys. Therefore, an important magma storage depth is inferred at lower crustal depth above the center of the Iceland mantle plume