A deep insight into a chemically homogeneous banded pumice sample: a role of crystal cargo immiscibility

In this study, a set of representative banded rhyolitic pumice xenoliths collected from the ejecta of a basaltic cinder cone around Göllüdağ volcanic center, Central Anatolia, Türkiye, were characterized through whole-rock and mineral analyses, scanning electron microscopy (SEM), microcomputed tomography (µ-CT), and magnetization studies. The light-and dark-colored bands exhibit similar whole-rock chemistry, mainly distributed in rhyolite composition (71.35–71.66 wt.% SiO₂). The dark pumice band shows high crystallinity (12–14 vol.%) with abundant subhedral-euhedral phenocrystals, microphenocrystals, and microlites, and a glass composition consisting of 71–76 wt.% SiO₂. In contrast, the light pumice band exhibits relatively low crystallinity (1.86–8.08 vol.%) with phenocrystals and/or microphenocrystals, and a slightly higher silica content (74–77 wt.% SiO₂). Both pumice bands display homogeneous distribution patterns with partially deformed and highly coalesced vesicles as observed through µ-CT and SEM studies. Additionally, we determined that the dark bands also contain magnetic minerals, which have a high attenuation coefficient and impart magnetic properties to the bands. The discrepancy in the crystal population between the light and dark pumice bands is attributed to the process occurring within the magma ascending throughout the conduit. We suggest that the response of the crystals involves separation and enrichment within their flow patterns, which prevents homogenization and results in a degree of mechanical immiscibility of the layers.