Please use this identifier to cite or link to this item: http://hdl.handle.net/1946/5993
In order to study eruption history and magmatic evolution of the subglacial volcanoes, Grímsvötn, Bárdarbunga and Kverkfjöll, tephra layers have been sampled systematically from measured soil profiles around the Vatnajökull ice cap. In total 921 tephra samples have been analysed for major element composition by electron microprobe. Their provenance has been assessed by comparison with the chemical composition of products from each volcanic system as determined in previous studies. The major element compositions fall into three distinctive groups, each one featuring a compositional range reflecting a liquid-line-of descent. Although this grouping reinforces the compositional distinction between the three volcanic systems it also demonstrates that a slight overlap remains between the major element compositions. However, a more distinct grouping is obtained via in-situ trace element analyses by laser-ablation ICP-MS. Collectively, major and trace element compositions allow robust determination of provenance for basaltic tephra having similar major element composition, and consequently the Holocene tephra record around Vatnajökull is improved significantly.
On a regional scale the soil profiles are correlated using key tephra markers. The basaltic tephra between these markers are correlated via tephra stratigraphy and chemical composition. Approximately 70% of the tephra layers originate at Grímsvötn, Bárdarbunga or Kverkfjöll. The eruption frequency, calculated from tephra layer frequency in soil profiles around Vatnajökull, reveals Grímsvötn as the most active volcanic system followed by Bárdarbunga, but Kverkfjöll show episodic activity with repose periods of more than 1000 years. All three volcanic systems had lower eruption frequency 5-2 ka caused by decrease in volcanic activity traced to periodic magma generation and delivery from the mantle rather than changes in environmental factors such as changing ice load. During prehistoric time a 1-3 thousand years age difference is found between peak activities above the mantle plume (Grímsvötn and Bárdarbunga) and peak activities at volcanoes located farther SW on the non-rifting part of the Eastern Volcanic Zone (EVZ; e.g. Katla). This suggests that a significant increase could be expected in volcanism on this part of the EVZ in the future since the highest observed eruption frequency was only 2-1 ka in Grímsvötn and Bárdarbunga.
Magmatic evolution of the three volcanic systems is controlled by crystal fractionation and crustal contamination. Trace element systematic suggests similar source mineralogy beneath the three volcanoes. This allows an assessment of the relative magma source melting at depth: Bárdarbunga above the assumed centre of the Iceland mantle plume produces basalts formed by highest degree of melting whereas the smallest melt fraction is recorded in the Kverkfjöll basalts erupted farther away from the assumed plume centre. A deep magma source appears to have played an important role in the activity of both Grímsvötn and Bárdarbunga, with a sill and dyke complex most active beneath Grímsvötn during a period of highest eruption frequency. This complex evolved into a magma chamber approximately 1000 years ago with correspondingly lower eruption frequency. The compositional variations of basalts from both Grímsvötn and Bárdarbunga are consistent with the presence of active magma chambers in their plumbing systems at depth.