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Basalt eruptions at Grímsvötn 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 reflect 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 Bárðarbunga 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. Clinopyroxene crystallization of Grímsvötn basalt indicates mid‐to‐lower crustal magma storage above the Iceland mantle plumeDifferent eruption regimes are due to variable recharge of deeper‐seated and gas‐rich magma rechargeSulfur concentration of matrix glass and exsolution budget correlate with known magma volumes

Sr/86Sr (0.70322) and δ18O ( ∼2.9‰), whereas significantly lower and higher values, respectively, are found in samples from the Bárdarbunga volcanic system (0.70307 and 3.8‰). These results strongly indicate that the Gjálp magma originated from the Grímsvötn magma system. The 1996 magma is of an intermediate composition, representing a basaltic icelandite formed by 50% fractional crystallization of a tholeiite magma similar in composition to that expelled by the 1998 Grímsvötn eruption. The differentiation that produced the Gjálp magma may have taken place in a subsidiary magma chamber that last erupted in 1938 and would be located directly beneath the 1996 eruption site. This chamber was ruptured when a tectonic fracture propagated southward from Bárdarbunga central volcano, as indicated by the seismicity that preceded the eruption. Our geochemical results are therefore not in agreement with lateral magma migration feeding the 1996 Gjálp eruption. Moreover, the results clearly demonstrate that isotope ratios are excellent tracers for deciphering pathways of magma migration and permit a clear delineation of the volcanic systems beneath Vatnajökull ice sheet.