A model for the kinetics of high temperature reactions between polydisperse volcanic ash and SO2 gas
(2020) The American Mineralogist (Print) : an international journal of earth and planetary materials — (2020)
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- Rapid calcium diffusion occurs in rhyolitic volcanic ash particles exposed to hot SO2 atmospheres. Such chemical transport is especially important immediately following fragmentation, during proximal transport in eruption plumes and during percolative transport through a permeable volcanic edifice. Here we analyze published results of experiments designed to constrain the kinetics of this process. The experiments involved crushed natural rhyolitic glass particles tumbled in SO2-bearing atmospheres at a wide range of relevant temperatures. We find that the particle-gas reaction is fed by calcium diffusion from the bulk to the particle surfaces where calcium-bearing salt crystals grow. The calcium flux is charge-compensated by local iron oxidation state changes. This process results in time-dependent concentrations of surface calcium that are leachable in aqueous solutions. Those leachate concentrations represent a proxy for the diffusive flux of Ca2+ out of the particle to form the surface crystal deposits. We formulate a mathematical framework to convolve the starting particle size distributions with the solution to Fickian 1-dimensional diffusion in a sphere, to find a weighted polydisperse result. Using this framework, we minimize for a temperature-dependent calcium diffusivity and compare our results with published calcium diffusivity data. We demonstrate that calcium diffusivity in rhyolite can be decomposed into two regimes: (1) a high temperature regime in which the diffusivity is given by the Eyring equation, and (2) a low-temperature regime more relevant to rhyolite volcanism and these gas-ash reactions. As a further test of our model, we compare the output against spatially-resolved data for calcium chemical gradients in experimental particles. We conclude by suggesting how this framework could be used to make quantitative predictions of sulfur budgets and iron oxidation during rhyolitic eruptions.
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Fabian B. Wadsworth, Vasseur Jérémie, Casas Ana Silvia, Delmelle, P., Hess Kai-Uwe, Ayris Paul M., & Dingwell Donald B. (2020). A model for the kinetics of high temperature reactions between polydisperse volcanic ash and SO2 gas. The American Mineralogist (Print) : an international journal of earth and planetary materials. Published. https://doi.org/10.2138/am-2021-7691 (Original work published 2020)