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Pedro Valdivia-Munoz Thesis Final

May 3 @ 3:00 pm - 5:00 pm MDT

Pedro Valdivia

MAFIC PLINIAN VOLCANISM AT LLAIMA VOLCANO: 3D X-RAY MICROTOMOGRAPHY RECONSTRUCTION OF PYROCLASTS TO CONSTRAIN SHALLOW CONDUIT PROCESSES

Mafic eruptions, which are typically effusive to mildly explosive, can produce much stronger Plinian-style eruptions. Eruption style is determined by the ability of gas to escape through the permeable network. If the permeability is sufficiently high to reduce vesicle overpressure during ascent, the volatiles may escape from the magma, limiting the chance of violent explosive activity. In contrast, if the permeability is sufficiently low to maintain the gas phase trapped within the magma during ascent, the resulted bubble overpressure may drive magma fragmentation. Studies on crystallization kinetics in mafic magmas suggest that rapid ascent rates may induce disequilibrium crystallization, increasing viscosity and explosivity, but the implications on the geometry of the vesicle network are often overlooked. Here we use synchrotron X-ray computed microtomography of 10 pyroclasts from the 12.6 ka mafic Curacautín Ignimbrite (Llaima Volcano, Chile) to reconstruct and quantify pyroclast textures in three dimensions. Our goal is to compute accurate 3D measurements of porosity, bubble interconnectivity, bubble number density, and geometrical properties of the porous media to investigate the role of magma degassing processes at mafic explosive eruptions. Then we propose an analytical technique to estimate permeability and tortuosity by combing empirical relationships and pyroclasts vesicle textures. There are two populations of vesicles: (1) a convoluted connected vesicle network produced by extensive coalescence of smaller vesicles (>99% of porosity network), and (2) a population of very small and completely isolated vesicles (<1% of porosity network). Bubble number density measurements are 1-29×103 bubbles per mm3, implying an average decompression rate of 1.37 MPa/s under heterogeneous nucleation. Tortuosity was calculated using the MATLAB application TauFactor, which quantifies the apparent decrease in diffusive transport resulting from convolutions of the flow paths through porous media. We computed tortuosities factor between 1.89 and 4.4, with higher values in the less vesicular samples. Permeability ranges are between 3×10-13 and 6.27×10-12 m2. 3D vesicle textures evidence rapid ascent rates that induced high disequilibrium conditions promoting rapid syn-eruptive crystallization of microlites and late vesiculation. This change in rheology, combined with rapid ascent, inhibited outgassing increasing bubble overpressures that led to fragmentation of the magma. We estimated that a bubble overpressure greater than 5.2 MPa could have been sufficient to fragment the Curacautín magma. Similar conditions are reported for other mafic explosive eruptions, implying that syn-eruptive disequilibrium controls the explosivity of mafic eruptions.

Advisor: Brittany Brand

Co-Advisor: Michael Manga, Dorsey Wanless

When: May 3, 2021
Time: 3:00 PM
Where: Zoom Meeting ID: 991 2225 8219