The 1996 and 1998 subglacial eruptions beneath the Vatnajokull ice sheet in Iceland: contrasting geochemical and geophysical inferences on magma migration

The spectacular 1996 jokulhlaup from the subglacial lake at Grimsvotn central volcano, beneath the Vatnajokull ice sheet in Iceland, was generated by a subglacial eruption at Gjalp midway between Bardarbunga and Grimsvotn central volcanoes. This eruption was preceded by a 24-h earthquake swarm that originated at Bardarbunga and migrated 20 km southward toward the eruption site. To test the hypothesis that a horizontal dyke fed the 1996 eruption from Bardarbunga volcano, we measured major and trace element abundances and O, Sr, and Nd isotope compositions in the 1996 volcanic rocks and selected samples from the Bgrdarbunga, Grimsvotn, and Oraefajokull volcanic systems. Lava flows and tephra from a given volcanic system beneath the Vatnajokull ice sheet have indistinguishable isotope compositions. Gjalp and Grimsvotn products have identical Sr-87/Sr-86 (0.70322) and delta(18)O (similar to 2.9 parts per thousand), whereas significantly lower and higher values, respectively? are found in samples from the Bgrdarbunga volcanic system (0.70307 and 3.8 parts per thousand). These results strongly indicate that the Gjalp magma originated from the Grimsvotn 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 Grimsvotn eruption. The differentiation that produced the Gjalp 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 Bgrdarbunga central volcano, as indicated by the seismicity that preceded the eruption. Our geochemical res;lts are therefore not in agreement with lateral magma migration feeding the 1996 Gjalp 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 Vatnajokull ice sheet.