PROMINENT FLOW BANDING IN BASALTS FROM KVERKFJÖLL VOLCANO, ICELAND

Flow-banded rocks erupted from Kverkfjöll, a subglacial volcano located in the Northern Volcanic Zone of Iceland, present an intriguing case for understanding magma dynamics and behavior. Flow banding is typically associated with highly viscous rhyolitic magmas, but at Kverkfjöll, it is commonly observed in mafic rocks. Viscosity, influenced by factors such as water content, crystal content, silica content, and temperature, plays a significant role in emplacement textures of magmas. Flow banding is conventionally attributed to shear and alignment of grains during high-viscosity magma emplacement. Understanding why Kverkfjöll basalts show flow banding may yield valuable insights to volcanic processes and magma behavior in active rift zones and subglacial volcanoes.

We collected seven samples displaying flow banding from Kverkfjöll. Thin sections were analyzed with electron probe microanalysis (EPMA) using a Cameca SX-Five FE-EPMA at the University of Wisconsin–Madison to obtain high resolution images and determine the chemical composition of the glass. SiO₂ values ranged from 48.8–52.5 wt.% in glasses from most samples. We also analyzed rhyolite glass in one section with SiO₂ values ranging from 69.2–76.1 wt%. In addition, we were able to estimate maximum water contents ranging from 0–2 wt% in basalt glasses and ~3.6–5 wt% in the rhyolite glasses. The data revealed no apparent correlation between water content and SiO₂ content. We also observed and analyzed abundant clinopyroxene and plagioclase crystals and sparse olivine in the basalt samples. With the addition of melt temperature information obtained from future two-oxide geothermometry and SEM-EDS images quantifying crystallinity in these samples, we will calculate a potential range of viscosities to better understand why flow banding features are prominent in these largely basaltic lavas.