Insights into the 3D structure and behaviour of Icelandic crystal mushes from gabbroic nodules

RAHUL SUBBARAMAN , Margaret Hartley, Jonathan Fellowes, Margherita Polacci, Barbara Bonechi, Lucia Pappalardo, Gianmarco Buono, David Axford Neave 

Crystal mushes – porous yet cohesive frameworks of crystals interspersed with interstitial melts – form the plumbing systems of active volcanoes. In Iceland, magmatic plumbing systems are inferred to be constructed from vertically stacked mushy magma reservoirs separated by subsolidus rock. Gabbroic nodules from Gígöldur in central Iceland provide a rare window into the structure, evolution, and degassing behaviour of upper crustal (6–10 km) mushy magma reservoirs. These plagioclase-rich nodules preserve recycled high-An plagioclase cores, rim overgrowths, and interstitial mafic crystallisation, reflecting in situ modification within the final storage reservoir and roof cumulate formation via density-driven segregation. Oscillatory zoning and variable rim patterns indicate that crystals experienced non-steady-state growth and were incorporated into clusters at different times. Compositional similarity between carrier and interstitial melts demonstrates that the final stages of mush evolution occurred in relatively homogeneous melts. Volatile saturation pressures of ~2.0–2.8 kbar, together with high vesicularity (21–30 vol.%) and extensive connectivity (>93%), are consistent with the presence of a CO₂-rich vapour phase within the mush. Nodules are erupted when a rigid crystal framework and a well-connected bubble network, which enables effective degassing, preserve their integrity. Where bubble networks fail to form, degassing disrupts the framework – even if initially rigid – causing disaggregation into plagioclase-phyric crystal cargoes, akin to plagioclase-ultraphyric basalts erupted in diverse settings. Nodules record the dynamics of stratified mushy magma reservoirs, the influence of volatiles on mush behaviour, and the transport of crystalline material in active volcanic systems, providing new insights into how crystal mushes shape the storage and mobilisation of magma in the Earth’s crust.