Disequilibrium during mush evolution in the Bárðarbunga volcanic system, Iceland

The prevalence, durability and physical significance of crystal mushes in crustal magmatic systems is a topic of current interest in igneous petrology. Fragments of mushes brought to the surface by basaltic eruptions provide a snapshot of the temporal evolution of crustal magmatic systems.  Petrographic and geochemical analysis of such fragments give valuable insights into basaltic magma reservoirs, including information about magma storage conditions and possible eruption triggers. A detailed petrological and geochemical study was carried out on gabbroic mush nodules from the Brandur, Fontur and Saxi tuff cones to understand the processes that occur before large fissure eruptions in the Bárðarbunga system, Iceland.

Petrographic studies of the mush nodules, from QEMSCAN images, reveal a bimodal phenocryst population in a glassy vesicular groundmass. Probe analyses confirm the bimodal population consists of a primitive and evolved assemblage. The former is composed of large equant crystals of high-anorthite plagioclase (An~88), high-forsterite olivine (Fo~86) and high Mg# clinopyroxene (Mg#~86) forming an interconnected solid framework. The evolved assemblage consists of low-anorthite plagioclase (An~75), low-forsterite olivine (Fo~77) and low Mg# clinopyroxene (Mg#~79) crystallising in the pore space of the mush framework and on the rims of the primitive macrocrysts. The textures and compositions seen suggest the nodules experienced two stages of crystallisation: primitive macrocrysts crystallised first and were stored in crystal mushes. Then a later event caused a change in PTX conditions and triggered relatively rapid crystallisation in the pore-spaces of the mushes.

The quenched glass in the pore spaces of the nodules has the composition of a basaltic liquid that in chemical equilibrium with the evolved assemblage of crystals. Thermobarometry based on equilibrium between this liquid and the phases indicates that the final stage of crystallisation occurred at pressures of ~2 kbar. A putative interstitial liquid composition was reconstructed under the assumption of closed system growth of the evolved assemblage by using the QEMSCAN pixel maps to add the evolved crystals to the interstitial glass composition. This reconstructed liquid is far from chemical equilibrium with the primitive crystals in the mush framework, indicating that the assumption of simple closed system crystallisation from an initial mush liquid in equilibrium with the primitive solids is not correct. Therefore, the phase mapping and compositional relationship provide constraints on open-system processes in mushes.

The failure of the closed system models to match the observations is significant in two ways. First, the lack of equilibrium between mush liquid and cumulus plagioclase is consistent with the expected sluggish diffusion of NaSi-CaAl in plagioclase. This disequilibrium poses challenges for numerical models of magmatic systems that use the assumption of crystal-melt equilibrium to link temperature, melt fraction and phase compositions.  Second, bubble expansion during pre-eruptive ascent forces mush liquid out of solid framework in the nodules and may provide observational constraints on the physics of multiphase flow in deep magmatic systems.