Understanding the flow dynamics of a Plagioclase Ultraphyric Basalt- a case study from the Westfjords, Iceland

The volcanic landscapes of Iceland's Westfjords are marked by the intriguing of Plagioclase Ultraphyric Basalts (PUBs), distinctive lava formations characterized by plagioclase contents so high (up to 50-60%) that they approach the crystallinity of a crystal mush. This research investigates the magmatic origin, emplacement mechanisms, and flow dynamics of these unusual lavas through an interdisciplinary approach combining petrographical, geochemical, and rock magnetic analyses. Thermobarometric calculations reveal magma equilibration conditions at mid-crustal depths (11.6±2.5 km) and temperatures of 1204.5±9.7°C. Mineral-melt relationships reveal a mush origin for the macrocrysts cores, with high An-cores for the plagioclases (An85-89) and high Fo-cores for the olivines (Fo83-86), whereas the rims show more evolved rims with compositions ranges of An50-70 for the plagioclases and Fo48-84 for the olivines, indicating a complex crystallization history. Trace element concentrations in the lava matrix are similar to other non-PUB westfjords basalts. Plagioclase trace element compositions are nearly in equilibrium with the composition of the host lava, unlike most modern Icelandic PUBs that typically show plagioclase-melt disequilibrium. Despite calculated viscosities suggesting potentially explosive behaviour (with viscosity values (log η) = 6.2 Pa·s and crystal volume fractions (φ) = 0.59 at 900ºC), field evidence indicates effusive emplacement, explained by several mechanisms including crystal alignment, thermal exchange, and shear thinning behavior. Anisotropy of Magnetic Susceptibility (AMS) measurements reveal flow directions predominantly oriented WNW-ESE, consistent with the location of the Arnarfjörður central volcano. However, scattered AMS distributions in crystal-rich samples suggest that elevated crystal content influenced the flow dynamics.

These findings enhance our understanding of crystal-rich magmatic systems and their emplacement mechanisms, contributing to refined models of magma dynamics. The results highlight the importance of considering more complex emplacement mechanisms when studying crystal-rich magmas that appear to defy traditional rheological models.