Production and detachment of oxide crystal shells on bubble walls during experimental vesiculation of andesitic magmas

Abstract

The mutual affinity between bubbles and oxide crystals (especially magnetite) is well established and their tendency to remain in contact once they become connected (either by nucleation of one upon the other, or by attachment) has led to models of oxide transport via bubbles in natural melts. However, despite the widespread acceptance of bubble–oxide association, there is little direct textural evidence for these processes. We present results from a series of decompression experiments on andesitic melts, during which aggregates of bubbles and oxides formed because of hydrogen loss through the capsule walls causing oxidation of the melt. Experimental charges were imaged using 3D X-ray computed tomography that revealed complex bubble + oxide aggregates, with small oxide crystals coating part of the outer bubble surfaces in a shell-like morphology. These shells have smooth inner and rugose outer surfaces. Sometimes, additional concentric shells or partial shells can be found around bubbles, in the glass between the bubble wall and another shell. We quantified the volumes of bubbles and oxides and the oxides’ compositions. We measured the surface area where the bubbles and oxides are in contact, thus quantifying their interface in 3D, and used these measurements to investigate the process of oxide shell formation. The complexity of the oxide textures when studied in 3D reveals a range of bubble–oxide interactions, from continuous generation, detachment and disintegration. These processes carry important implications on why such textures seem to have a low preservation potential in natural environments. Nevertheless, we have found natural samples that resemble our experimental results in a range of rock compositions from different geological environments that could have formed either due to rapid oxidation via the fluid phase or by bubbles harvesting different crystals.