Experimental sintering of ash at conduit conditions and implications for the longevity of tuffisites

Abstract

Escape of gas from magma in the conduit plays a crucial role in mitigating explosivity. Tuffisite veins—ash-filled cracks that form in and around volcanic conduits—represent important gas escape pathways. Sintering of the ash infill decreases its porosity, eventually forming dense glass that is impermeable to gas. We present an experimental investigation of surface tension-driven sintering and associated densification of rhyolitic ash under shallow conduit conditions. Suites of isothermal (700–800 °C) and isobaric H₂O pressure (20 and 40 MPa) experiments were run for durations of 5–90 min. Obsidian powders with two different size distributions were used: 1–1600 μm (mean size = 89 μm), and 63–400 μm (mean size = 185 μm). All samples evolved similarly through four textural phases: phase 1—loose and cohesion-less particles; phase 2—particles sintered at contacts and surrounded by fully connected tortuous pore space of up to ~40% porosity; phase 3—continuous matrix of partially coalesced particles that contain both isolated spherical vesicles and connected networks of larger, contorted vesicles; phase 4—dense glass with 2–5% fully isolated vesicles that are mainly spherical. Textures evolve faster at higher temperature and higher H₂O pressure. Coarse samples sinter more slowly and contain fewer, larger vesicles when fully sintered. We quantify the sintering progress by measuring porosity as a function of experimental run-time, and find an excellent collapse of data when run-time is normalized by the sintering timescale ${\lambda}_s=\eta \overline{R}/\sigma$, where η is melt viscosity, $\overline{R}$ is mean particle radius, and σ is melt–gas surface tension. Because timescales of diffusive H₂O equilibration are generally fast compared to those of sintering, the relevant melt viscosity is calculated from the solubility H₂O content at experimental temperature and pressure. We use our results to develop a framework for estimating ash sintering rates under shallow conduit conditions, and predict that sintering of ash to dense glass can seal tuffisites in minutes to hours, depending on pressure (i.e., depth), temperature, and ash size.