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Durham University

Department of Earth Sciences

Research Staff

Publication details for Prof. Ed Llewellin

Gardner, James E., Wadsworth, Fabian B., Llewellin, Edward W., Watkins, James M. & Coumans, Jason P. (2018). Experimental sintering of ash at conduit conditions and implications for the longevity of tuffisites. Bulletin of Volcanology 80(3): 23.

Author(s) from Durham

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 λs ¼ ηR=σ, where η is melt viscosity, 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.