McGowan, Ellen and Tuffen, Hugh and James, Michael and Wynn, Peter (2016) Magma emplacement and deformation in rhyolitic dykes : insight into magmatic outgassing. PhD thesis, Lancaster University.
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Abstract
Exposed rhyolitic dykes at eroded volcanoes arguably provide in situ records of conduit processes during rhyolitic eruptions, thus bridging the gap between surface and sub-surface processes. This study involved micro- to macro-scale analysis of the textures and water content within shallow (emplacement depths <500 m) rhyolitic dykes at two Icelandic central volcanoes. It is demonstrated that dyke propagation commenced with the intrusion of gascharged currents that were laden with particles, and that the distribution of intruded particles and degree of magmatic overpressure required for dyke propagation were governed by the country rock permeability and strength, with pre-existing fractures playing a pivotal governing role. During this stage of dyke evolution significant amounts of exsolved gas may have escaped. Furthermore, during later magma emplacement within the dyke interiors, particles that were intruded and deposited during the initial phase were sometimes preserved at the dyke margins, forming dykemarginal external tuffisite veins, which would have been capable of facilitating persistent outgassing during dyke growth. It is further demonstrated that following initial dyke-opening, geochemically homogenous dykes grew via the incremental emplacement of magma, with fluctuations in the shallow-dyke permeability occurring via bubble collapse, and this is deemed to have been critical in dictating pressure within the deeper magma source region and fragmentation. Of further significance, it is also shown that shear deformation was localised during magma emplacement, with localised vesiculation occurring along emplacement boundary layers via viscous heating, which temporarily promoted magma ascent, but with later bubble collapse culminating in brittle failure of bubble-free magma, after shear zone migration. However, in some instances high strain rates during viscous bubble deformation resulted in ductile-brittle transitions, with resultant slip triggering micro-tensile failure of bubbly magma, as the slipped magmatic plug experienced decompression. This tensile failure probably occurred distal to shear zones, where bubbles where relatively isolated. Interlinking of the micro-cracks formed extensive internal tuffisite vein networks, which acted as efficient outgassing pathways, given their access to significant quantities of preexsolved volatiles. The models presented in this thesis are relevant to the conduit processes that take place during rhyolitic eruptions; insight is provided into how rhyolitic magma ascends through the shallow (<500 m deep) crust and also into how the magma deforms during its ascent and into the processes that govern magmatic outgassing.