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Dynamic Petrology and Rheology of Ascending Magma During Lava Dome Eruptions: Effusive–Explosive Activity

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Dynamic Petrology and Rheology of Ascending Magma During Lava Dome Eruptions: Effusive–Explosive Activity Dynamic petrology and rheology of ascending magma during lava dome eruptions: Effusive–explosive activity Thesis submitted in accordance with the requirements of the University of Liverpool for the degree of Doctor of Philosophy by Paul Anthony Wallace May 2019 Declaration of authorship I, Paul Anthony Wallace, declare that this thesis entitled “Dynamic petrology and rheology of ascending magma during lava dome eruptions: Effusive–explosive activity” and the work presented in it is my own. I confirm that: This thesis was completed as part of a research degree at the University of Liverpool; The material contained in this thesis has not been presented, nor is currently being presented, either wholly or in parts, for any other degree or qualifications; Where I have consulted published studies, this have been clearly referenced; Where the work was part of a collaborative effort, I have made clear what others have done and what I have contributed myself; Parts of this thesis have been published or in preparation for publication as: o Wallace, P.A., Kendrick, J.E., Ashworth, J.D., Miwa, T., Coats, R., De Angelis, S.H., Mariani, E., Utley, J.E.P., Biggin, A., Kendrick, R., Nakada, S., Matsushima, T., and Lavallée, Y. (published in Journal of Petrology). Petrological architecture of a magmatic shear zone: A multidisciplinary investigation of strain localisation during magma ascent at Unzen Volcano, Japan. o Wallace, P.A., De Angelis, S.H., Hornby, A.J., Kendrick, J.E., von Aulock, F.W., Clesham, S., Hughes, A., Utley, J.E.P., Hirose, T., Dingwell, D.B., and Lavallée, Y. (published in Geochimica et Cosmochimica Acta). Frictional melt homogenisation during fault slip: Geochemical, textural and rheological fingerprints. o Wallace, P.A., Lamb, O.D., De Angelis, S., Kendrick, J.E., Hornby, A.J., Díaz-Moreno, A., von Aulock, F.W., Lamur, A., Utley, J.E.P., Rietbrock, A., Chigna, G., and Lavallée, Y. (in preparation for Earth and Planetary Science Letters). Integrated constraints on explosive eruption intensification at Santiaguito dome complex, Guatemala. Signed: Paul Anthony Wallace I Thesis Abstract Lava dome eruptions are one of the most hazardous forms of intermediate volcanic activity, with their ability to switch from benign effusive to catastrophic explosive eruption style with little precursory warning. Such behaviour can be attributed to a complex interplay between deep and shallow magmatic processes which alter the rheological, physical and chemical state of the magma, and its propensity to erupt. A comprehensive understanding of such processes is thus paramount for deciphering causes for different eruptive behaviours. This thesis aims to address three distinct processes, all of which are deemed a common phenomenon at lava dome volcanoes and can have an integral role in magma ascent dynamics in shallow volcanic conduits: (1) magma shearing, (2) fault friction and (3) magma recharge. During the ascent of high-viscosity magma, crystallisation and volatile exsolution favours a non-Newtonian response, which promotes localisation of stress and strain near the conduit margins, leading to the formation of discrete shear zones. The impact and role of strain localisation during magma ascent was investigated through a detailed petrological survey of a shear zone developed at the margins of the 1994–1995 lava spine at Unzen volcano (Japan). A multidisciplinary approach combining field observations, microtextures, crystallography and magnetics was deployed, which was supplemented by the first high-temperature, high- velocity rotary shear experiment to simulate the processes involved during shear. I show that crystals can effectively monitor stress conditions during magma ascent through the viscous– brittle transition, with viscous remobilisation, crystal plastically and comminution all systematically increasing with shear intensity at the conduit margins. I suggest evidence for a thermo-mechanical response due to strain localisation (shear/frictional heating) which triggered disequilibrium conditions leading to mineral reactions, altered rock magnetic properties and compacted the original porous network. The range of deformation textures recorded can have a profound effect on magma outgassing and the style of surface activity. Strain localisation can lead to seismogenic magma failure proceeded by faulting and slip along fracture planes. In particular, frictional sliding near the conduit margins can cause localised melting (by frictional heating) and impose an important rheological control on slip dynamics, acting as a lubricant or viscous brake that controls magma ascent in the upper conduit. Frictional melting is a non-equilibrium process, involving the selective melting of mineral phases and the softening of a glass at the glass transition. I present a detailed, systematic experimental investigation demonstrating the importance of host-rock mineralogy on the mechanical, geochemical, textural and rheological evolution of a frictional melt. The presence of hydrous minerals, such as amphibole, have the most influential role due to their lower melting point, generating heterogeneous mafic melts with lower viscosities that promote slip. In contrast, when amphibole is absent, frictional melts are more compositionally homogenous with higher viscosities resulting in higher shear resistance during slip. Understanding frictional properties with respect to mineralogy may aid in constraining flow behaviour in the shallow volcanic conduit that can impact activity at the surface. Regular, small-to-moderate gas-and-ash explosions at Santiaguito dome complex (Guatemala) have previously been attributed to shear and frictional processes. However, in 2015–2016, eruptive activity intensified to larger, less frequent ash-rich explosive activity. I present a unique, multiparametric dataset that integrates petrological and geochemical signatures of the eruptive products with a rich geophysical dataset to assess the cause for a shift in eruption style. I provide evidence for a magma recharge event, resulting in changes in ash componentry, mingling textures, bulk compositional changes, alteration of groundmass microlite textures that occur concordantly with a deeper explosion source in the conduit revealed by seismicity. The integrated observations imply a switch from shallow shear-driven fragmentation to deeper gas overpressure fragmentation, intensifying local hazards. These observations not only aid in understanding Santiaguito’s ongoing dynamic activity, but may also assist in deciphering complex shifts in eruption style at many active volcanoes. II Contents DECLARATION OF AUTHORSHIP ................................................................................................ I THESIS ABSTRACT .......................................................................................................................... II LIST OF FIGURES ......................................................................................................................... VII LIST OF TABLES ............................................................................................................................ IX ACKNOWLEDGEMENTS ................................................................................................................ X CHAPTER 1 INTRODUCTION ................................................................................................... 1 1.1. MOTIVATION ....................................................................................................................... 1 1.2. LAVA DOME ERUPTIONS ....................................................................................................... 3 1.3. PETROLOGICAL CONSTRAINTS ON MAGMA STORAGE AND ASCENT ...................................... 6 1.4. MAGMA RHEOLOGY ........................................................................................................... 10 1.5. STRAIN LOCALISATION, MAGMA SHEARING AND FRICTION ................................................ 13 1.6. AIMS OF THE THESIS ........................................................................................................... 20 1.7. THESIS STRUCTURE ............................................................................................................ 21 1.8. STATUS OF PAPERS AND CO-AUTHOR CONTRIBUTIONS ....................................................... 22 CHAPTER 2 PETROLOGICAL ARCHITECTURE OF A MAGMATIC SHEAR ZONE: A MULTIDISCIPLINARY INVESTIGATION OF STRAIN LOCALISATION DURING MAGMA ASCENT AT UNZEN VOLCANO, JAPAN ................................ 25 ABSTRACT ........................................................................................................................................ 25 2.1. INTRODUCTION .................................................................................................................. 26 2.1.1. Overview of the 1991–1995 dome eruption ................................................................. 30 2.1.2. Magma feeding the 1991–1995 eruption ...................................................................... 31 2.2. METHODS .......................................................................................................................... 31 2.2.1. Sample collection ......................................................................................................... 31 2.2.2. Geochemistry ............................................................................................................... 32 2.2.3. Electron Microprobe
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