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

• Published in: Bulletin of volcanology 2000-01, Vol.61 (7), p.468-476
• Main Authors: SIGMARSSON, O, KARLSSON, H. R, LARSEN, G
• Format: Article
• Language: English
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Isotope geochemistry; Isotope geochemistry. Geochronology
• ISSN: 0258-8900; 1432-0819
• DOI: 10.1007/PL00008912

Sr/ super(86)Sr (0.70322) and delta super(18)O ( 2.9ppt), whereas significantly lower and higher values, respectively, are found in samples from the Bardarbunga volcanic system (0.70307 and 3.8ppt). 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 Bardarbunga 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 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.