Evolution of the Tungnárhraun mafic lavas: Insights into magma storage and ascent beneath Barðarbunga volcanic system
Open Access
- Author:
- Oborn, Collin
- Graduate Program:
- Geosciences
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- October 13, 2020
- Committee Members:
- Tanya Furman, Thesis Advisor/Co-Advisor
Peter Christopher La Femina, Committee Member
Andrew James Smye, Committee Member
Andrew Arnold Nyblade, Program Head/Chair - Keywords:
- Iceland
Geochemistry
Petrology
PUBs
Bárðarbunga
Tungnárhraun - Abstract:
- Geochemical analyses of bulk lavas and textural features of macrocryst (>1000 µm) plagioclase feldspar provide insights into the timing and structure of magma storage and ascent beneath active volcanic systems. Data from these crystals are used to infer crystallization P and T, provenance, and crustal residence times, and therefore, constrain processes of magma evolution. The Bárðarbunga volcanic system is distinct within Iceland for its consistent production of plagioclase porphyritic basalts since the last glacial maximum (~12 ka). This system is also the most productive in Iceland over the Holocene, generating voluminous, far-reaching lava flows from both its northern and southern fissures. We link the Tungnárhraun, a series of large volume (>1 km3) lava flows which span nearly the entire Holocene period, to the southern Bárðarbunga fissure. Several of these flows approach plagioclase ultraphyric basalts (PUBs) with 10-20 vol% plagioclase. The macrocrysts are characterized by high anorthite cores (>An80), which, like most PUBs, are too primitive to have grown from the host lava. The cores are surrounded by An55-80 rims which have intergrowth textures with the groundmass, suggesting chemical equilibrium. The chemistry of the Tungnárhraun define three eruptive episodes: early and late periods of evolved lavas (~5.5-6 wt.% MgO) with <10 vol.% plagioclase and a middle period of more primitive lavas (~8 wt.% MgO) with 14-20 vol% plagioclase. These features suggest variation in magma storage and ascent. We explore the evolution of the Tungnárhraun through a series of plagioclase and liquid thermometers and barometers and examine their relationship to calculated host melt compositions. This project combines our extensive dataset with published data from the Tungnárhraun sequence and the Bárðardalur lavas (northern and southern extents of the volcanic system) to assess magma storage and movement through time in Iceland’s most productive fissure system.