Constraints from plagioclase on the post-glacial magma storage and ascent beneath the Barðarbunga volcanic system

Abstract:

Geochemical parameters and textural features of macrocryst (>1000 µm) plagioclase 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 Barðarbunga-Veiðivötn 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, producing voluminous, far-reaching lava flows from both its North and South fissures. The Tungnárhraun is a series of large volume (>1 km³) lava flows, linked to the southwest-reaching Veiðivötn fissure, which span nearly the entire Holocene period. 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. These 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 Barðardalur lavas (northern and southern extents of the volcanic system) to assess magma storage and movement through time in Iceland’s most productive fissure system.