Mafic Plinian Volcanism at Llaima Volcano (Chile): 3D reconstruction of pyroclasts to constrain shallow conduit processes

Abstract:

Mafic eruptions, which are typically effusive to mildly explosive, can produce much stronger Plinian-style eruptions. Eruption style is determined by the ability of magma to fragment, in turn controlled by volatile exsolution and escape and the evolving magma rheology. Here we use synchrotron X-ray computed microtomography of 43 pyroclasts from the 12.6 ka mafic Curacautin ignimbrite (Llaima Volcano, Chile) to reconstruct and quantify pyroclast textures in three dimensions. Our goal is to compute 3D measurements of vesicularity, permeability, tortuosity, connectivity, bubble number density, specific surface area and bubble size distributions in order to understand the influence of porous network on magma permeability and eruptive behavior. Also, two-dimensional (2D) vesicularities are contrasted with 3D measurements to describe the internal variabilities of the samples. There are two populations of vesicles: (1) a convoluted connected vesicle network produced by extensive coalescence of smaller vesicles (>98% of porosity network), and (2) a population of very small and completely isolated vesicles (<2% of porosity network). Vesicle number density measurements are between 10^2 and 10^5 bubbles per mm^2. 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 1.8 and 6, with higher values in the less vesicular samples. Permeabilities ranges are between 10^-12 and 10^-9 m^2, and have a positive correlation with vesicularities. Comparing our findings to the literature, such high values of vesicle number density indicates homogeneous nucleation and rapid decompression rate at shallow depths. Further, high tortuosity factors and low permeability indicate an inefficient coalescence that likely resulted in overpressurization of the system, driving the explosive nature of the eruption.