The post-emplacement hydration and alteration of subglacially erupted obsidian to form perlite

This thesis contains a detailed investigation of volatiles within subglacially erupted rhyolites from Torfajokull and Krafla, Iceland. Thermogravimetric, FTIR and XRD analyses have been combined with field observations and thin section obser- vations to investigate the coupling between fracturing and hydration resulting in the formation of perlite. The dominant volatiles in perlites are shown to be water. Perlites are enriched in water with respect to obsidians and total volatile contents vary between litho- facies. More-perlitised obsidians are found to be enriched in water compared to less-perlitised obsidians. Water is shown to decrease with distance inwards from the margin of lava lobes and the assumed external water source present during formation. Water also decreases inwards from the edges of perlitic beads (away from the perlitic fracture). The variations in water content are shown to be due to changes in water species indicative of external hydration. An inverse relationship exists between total volatile content and the temperature of maximum degassing. Hydration is shown to be stronger when grainsizes are smaller due to a greater surface area to volume ratio. Fracturing and volatile contents are strongly coupled with the extent of hydration related to the locations of fractures. The distance of water enrichment either side of major and perlitic fractures corre- lates well with theoretical diffusion distances of water at