Bubble heterogeneities in pumices: Implications for strain localizations and fragmentation

Abstract:

The crystal-poor dense rhyolitic pumices from the Los Espejos fall deposit have a wide variety of sizes (1/16–200 mm) and densities (0.67–1.57 g cm^-3) throughout the deposit. Log-normal bubble size distributions (BSDs) are concave up suggesting an extensive late-stage vesiculation. After separating bubbles with a watershed filter, BSDs are straights, the BSD slope is controlled by the pumice density and the maximum bubble size is one sixth of the obtained from the original BSDs suggesting continuous bubble nucleation and growth and high coalescence. The bubble convexity decreases with bubble size confirming the high coalescence and the bubble aspect ratio vary substantially at all bubble sizes, suggesting multi-scale strain localizations. The bubble number density, size, convexity and aspect ratios were spatially quantified showing that bubble textures vary heterogeneously within pumices. The resulting bubble textural maps were smoothed by applying moving averages in order to constrain bubble textural domains (Figure 1). Large-low convexity bubbles concentrates around quartz phenocrysts and bubble coalescence is leaded by the high bubble nucleation around large bubbles. The main bubble elongation and orientation vectors match with bubble size contours showing that strain localization matches with boundaries between large-bubble domains and small-bubble domains. Thus, bubble heterogeneities have a weakening effect on the rheological behavior of magmas by triggering high-strain-rate shear zones that could lead to brittle fragmentation. This breakage mechanism could explain the occurrence of explosive eruptions with low-vesiculated high-viscosity magmas.

Figure 1: Map of Bubble Number Density for a pumice of 1.54 g cm^-3. Lighter colors represent higher number densities. Domains of high nucleation are clearly concentrated (white areas) whilst domains of low nucleation (black areas) occurs around quartz phenocrysts (blue polygons).