Abstract
Volcanic ash and pyroclasts can weld when deposited hot by pyroclastic density currents, in near-vent fall deposits, or in fractures in volcano interiors. Welding progressively decreases the permeability of the particle packs, influencing a range of magmatic and volcanic processes, including magma outgassing, which is an important control on eruption dynamics. Consequently, there is a need for a quantitative model for permeability evolution during welding of ash and pyroclasts under the range of conditions encountered in nature. Here we present in situ experiments in which hydrous, crystal-free, glassy pyroclasts are imaged via x-ray tomography during welding at high temperature. For each 3D dataset acquired, we determine the porosity, Darcian gas permeability, specific surface area, and pore connectivity. We find that all of these quantities decrease as a critical percolation threshold is approached. We develop a constitutive mathematical model for the evolution of permeability in welding volcanic systems based on percolation theory, and validate the model against our experimental data. Importantly, our model accounts for polydispersivity of the grainsize in the particle pack, the pressures acting on the pack, and changes in particle viscosity arising from degassing of dissolved H2O during welding. Our model is theoretically grounded and has no fitting parameters, hence it should be valid across all magma compositions. The model can be used to predict whether a cooling pyroclast pack will have sufficient time to weld and to degas, the scenarios under which a final deposit will retain a permeable network, the timescales over which sealing occurs, and whether a welded deposit will have disequilibrium or equilibrium H2O content. A user-friendly implementation of the model is provided.
Highlights
Magma may fragment during ascent, forming volcanic ash and pyroclasts, which are subsequently transported in a gas phase
While the dynamics of welding is important in controlling the bulk dynamics in all of these scenarios (Wadsworth et al, 2019), we focus on tuffisites and the sedimentation of ignimbrites as the two scenarios where permeability evolution may be most important for understanding outgassing and pressure evolution
We identify three dynamic regimes captured by the capillary Peclet number, (1) at Pc ≪ 1, deposits degas rapidly and weld to preserve equilibrium low H2O concentration, (2) at Pc ≫ 1, deposits weld rapidly to preserve high nearinitial disequilibrium H2O concentration, and (3) at intermediate Pc = 1, welding produces partially degassed deposits
Summary
Magma may fragment during ascent, forming volcanic ash and pyroclasts, which are subsequently transported in a gas phase. Welding is important because it leads to substantial changes in the physical properties of the deposit, which transitions progressively from particulate to coherent (Branney and Kokelaar, 1992), from mechanically weak to mechanically strong (Kolzenburg et al, 2012; Vasseur et al, 2013), and from permeable to impermeable (Colombier et al, 2017; Farquharson et al, 2017; Wadsworth et al, 2017b; Kolzenburg et al, 2019; Heap et al, 2019). The effect on permeability is of particular interest because it has been proposed that closure of outgassing pathways through tuffisites (Farquharson et al, 2017; Heap et al, 2019) and clastic vent-filling deposits (Quane et al, 2009) can cause vent plugging and build-up of gas pressure, leading to explosive eruption
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