3D insights into the 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 are porous yet cohesive frameworks of crystals interspersed with interstitial melts that form the plumbing systems beneath many active volcanoes. Gabbroic nodules from Gígöldur, central Iceland, provide a unique window into the structure and degassing behaviour of mushy magma reservoirs - localised crystal mush domains beneath Icelandic volcanoes. These plagioclase-rich nodules formed through crystallisation, accumulation, and ascent within a stratified mush system, with their mafic components crystallising within interstitial spaces between a framework constructed from predominantly large plagioclase grains. The high vesicularity (21–30 vol.\%) and extensive vesicle connectivity (>93\%) of these nodules identified from XCT observations confirm that a free vapour phase was present at the surface. Saturation pressure estimates (2.75–3.75 kbar) indicate the melts in the nodules were volatile-saturated under their conditions of final storage ($\sim$ 2.3 $\pm$ 0.5 kbar, $\sim$ 7.8 $\pm$ 1.6 km). Despite significant degassing, the rigid crystal framework and interconnected vesicle networks within the nodules preserve their structural integrity during ascent, as volatiles escape through these vesicle channels. However, where such channels are absent or disrupted, nodules may disaggregate into individual crystals in the final stages of ascent, potentially supplying the crystal cargo of plagioclase-ultraphyric basalts that erupt at Gígöldur. The nodules sample the buoyant, plagioclase-rich roof cumulates of a stratified mushy magma reservoir. These findings highlight the role of volatiles in mush dynamics, the evolution of stratified magma reservoirs, and the transport of crystalline material in Icelandic volcanic systems.