Mafic Explosive Volcanism at Llaima Volcano

Mafic eruptions, which are typically effusive to mildly explosive, can produce much stronger explosive 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, inhibiting violent explosive activity. In contrast, if the permeability is sufficiently low to retain the gas phase within the magma during ascent, bubble overpressure may drive magma fragmentation. Rapid ascent rates may induce disequilibrium crystallization, increasing viscosity and explosivity, and have consequences for the geometry of the vesicle network. Quantitative vesicle texture analyses are commonly measured in 2D. However, 2D vesicle analyses do not provide sufficient information about the internal vesicle structures for permeability analysis. 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 obtain 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. We use an analytical technique to estimate permeability and tortuosity by combing empirical relationships and pyroclasts vesicle textures. We identified two populations of vesicles: (1) a convoluted connected vesicle network produced by extensive coalescence of smaller vesicles (> 99% of pore space), and (2) a population of very small and completely isolated vesicles (< 1% of porosity network). Bubble numbe2r density measurements are 1-29×10 3 bubbles per mm 3 , implying an average decompression rate of 1.4 MPa/s under heterogeneous nucleation. 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 m 2 . 3D vesicle textures evidence rapid ascent rates that induced high disequilibrium, promoting rapid syn-eruptive crystallization of microlites and late vesiculation. We propose that the increase in viscosity due to crystallization and vesiculation, combined with rapid ascent, inhibited outgassing and increased bubble overpressures, leading to explosive fragmentation. We estimated that a bubble overpressure greater than 5.2 MPa could have been sufficient to fragment the Curacautín magma. Other mafic explosive eruptions report similar disequilibrium conditions induced by rapid ascent rate, implying that syn-eruptive disequilibrium may control the explosivity of mafic eruptions more generally.