Exploring fine to extremely fine ash componentry and morpho-textures to unravel plug disruption and physical segregation processes within dilute PDCs

Abstract:

By combining automated SEM/EDX single-particle analysis and μmX-CT we obtained robust chemical and morpho-textural classification of several thousands of particles within the 250-63 mm and the 63-32 mm size ranges. By studying these particles in a stratigraphic and sedimentary context, we found in Azufral volcano (Colombia) a natural example of nearly simultaneous magmatic and phreatomagmatic fragmentation mechanisms, finely disrupting a silicic plug.

Once digitally discriminated, the parametrization of morphological (area-based) and textural (perimeter-based) roughness, together with a Principal Component Analysis, facilitates the classification of juvenile glassy ash particles. Co-existing variable morphologies and microtextures of juvenile particles at nearly constant chemical composition in the same stratigraphic level, confirms that the magma physical state at the time of fragmentation was largely heterogeneous. Also, the mmX-CT observations allows confirming the strong interdependencies between external morphologies and particle inner microtextures. Each glassy ash type could be linked to a shallow conduit region, following published density “stratified” plug models. Most of the fragmented juvenile material derived from a variably degassed transition zone between dense and vesicular regions, subjected to vesicle collapse and evolving into highly interconnected paths. Upon magma rising, the dense regions likely cracked, allowing water injection and providing the contact surface required for effective phreatomagmatic interaction while enhancing ongoing magmatic processes. Non-systematic variations in the relative contribution of each of the fragmentation mechanisms might dictate variations in the resulting particle:gas ratio of the pyroclastic mixture, subsequently transported/deposited in successive dilute PDCs. Physical fractionation processes related to primary and secondary fragmentation, elutriation and interaction with topography were identified, even at the very fine to extremely fine ash fractions scales.

Our model implies a challenging call for monitoring research to detect the timescales for plug density-stratification, plug shearing and cracking, and the injection of the external water, leading to multiple base-surges through crater lakes.