Microstructures reveal mechanical compaction and volatile-rich liquid migration in the layered alkaline Ilímaussaq complex, South Greenland

Crystal–melt separation is a fundamental process in magmatic differentiation that has implications for volcanic eruptions and ore deposit formation. However, the mechanical processes governing how crystals and melt separate, which are important for the timescales of melt segregation, are debated. Geochemical and theoretical studies commonly cite viscous compaction, i.e. gravitationally or intrinsic stress-driven deformation of the crystal pile, as a key mechanism of crystal–melt separation. Critically, viscous compaction should produce a microstructural record of internal crystal deformation and crystal alignment that allows us to diagnose its role, if any, in crystal–melt separation. Here, we provide novel textural and microstructural data from the Ilímaussaq complex, a layered alkaline intrusion in South Greenland, and explore whether, and if so through what processes, crystal–melt separation within the crystal pile. Stratigraphically variable disequilibrium and nucleation microstructures within our samples record the vertical migration of interstitial volatile-rich liquid through igneous layers. We observe some mineral bending associated with asymmetrical crystal growth, but no evidence of extensive viscous grain deformation, indicating that viscous compaction was not a significant mechanism in crystal–melt separation for Ilímaussaq. Instead, we suggest that mechanical compaction via crystal rearrangement, accompanied by volatile-rich liquid migration, primarily facilitates crystal–melt separation in intrusions with chemically evolved compositions. Our study shows that post-cumulus crystal repacking and volatile-rich liquid movement impact primary igneous layering in terms of microstructures, modal abundances of main and minor phases as well as phase compositions, therefore potentially controlling the distribution and concentration of rare earth element deposits within layered alkaline intrusions.