| Summary: | Doktorsverkefnið er unnið í sameiningu við Háskóla Íslands og Université Blaise Pascal, Clermont-Ferrand, Frakklandi. 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 ...
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