3D visualisation of nanolite aggregation in basaltic magmas using X-ray ptychography: Implications for magma rheology

Nanoscale crystals, or ‘nanolites’, are becoming increasingly recognised in both experimental products and natural samples of volcanic eruptions, across a range of magma compositions and explosivity. Nanolites can increase magma viscosity and influence eruptive style, due to the rheological impact of the nanoparticle suspension, by inducing chemical and structural changes in the residual melt and by facilitating heterogeneous bubble nucleation. Due to their large surface area, nanolites are also prone to aggregation. However, their morphology, spatial distribution and interaction in 3D has not yet been investigated.
 Here we present a 3D, nanometre-scale visualisation and quantification of nanolites within scoriae of highly explosive basaltic volcanic eruptions, obtained using X-ray ptychography, a nanoscale microscopy technique. We find that titanomagnetite nanolites aggregate, forming elongate, irregular structures in 3D. Compositional heterogeneities are also observed within the matrix glass, as extraction of Fe and Ti from the melt during nanolite crystallisation forms differentiated, Si-rich boundary layers surrounding nanolites with higher viscosity. We support our 3D nanoscale observations with images acquired using SEM and STEM, utilising multi-scale imaging methods to visualise nanolite crystallisation in basaltic magmas. We find that syn-eruptive nanolite crystallisation can increase magma viscosity through their aggregation and impact on the composition of the residual melt, increasing the potential of magma fragmentation during ascent. Our results provide insight into the nanoscale structure of volcanic products and also the driving mechanisms of highly explosive basaltic volcanic eruptions.