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 centr al volcano, beneath the Vatnajokull ice sheet in Iceland, was generated by a subglacial eruption at Gjalp midway between Bardarbunga and Grimsvotn cen tral 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 f rom 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 Vatnajokul l ice sheet have indistinguishable isotope compositions. Gjalp and Grimsvot n 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, res pectively? are found in samples from the Bgrdarbunga volcanic system (0.703 07 and 3.8 parts per thousand). These results strongly indicate that the Gj alp magma originated from the Grimsvotn magma system. The 1996 magma is of an intermediate composition, representing a basaltic icelandite formed by 5 0% fractional crystallization of a tholeiite magma similar in composition t o that expelled by the 1998 Grimsvotn eruption. The differentiation that pr oduced the Gjalp magma may have taken place in a subsidiary magma chamber t hat last erupted in 1938 and would be located directly beneath the 1996 eru ption site. This chamber was ruptured when a tectonic fracture propagated s outhward from Bgrdarbunga central volcano, as indicated by the seismicity t hat preceded the eruption. Our geochemical res;lts are therefore not in agr eement with lateral magma migration feeding the 1996 Gjalp eruption. Moreov er, the results clearly demonstrate that isotope ratios are excellent trace rs for deciphering pathways of magma migration and permit a clear delineati on of the volcanic systems beneath Vatnajokull ice sheet.