Effects of eruption plume processing on volcanic ash iron solubility and speciation: insights from high temperature gas-ash interaction experiments

Maters, Elena;Delmelle, Pierre;Ayris, Paul
(2016) Volcanic Ash as an Active Agent in the Earth System (VA3): Combining models and experiments — Location: Hamburg, Germany (12.September.2016)

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Authors
  • Maters, ElenaUCLouvain
    Author
  • Delmelle, PierreUCLouvain
    Author
  • Ayris, PaulLudwig Maximilians University Munich
    Author
Abstract
Volcanic ash deposition to seawater can alleviate the dissolved iron (Fe) deficiency that limits marine primary production in vast regions of the surface ocean. However, controls on Fe chemistry in ash during its transit between magma source and ocean sink remain poorly understood. Here we experimentally investigate the effect of interactions between ash and gases within the eruption plume on Fe solubility and redox speciation, using a powdered glass as a surrogate for the main constituent of ash, and an advanced gas-ash reactor to simulate in-plume chemical and thermal conditions. Samples were treated for 180 s to various gas (H2O, SO2, HCl) and temperature (300 °C, 700 °C) combinations, and Fe(II) and Fe(III) release were then measured in pH 4 H2SO4 to mimic ash contact with cloud water during subsequent atmospheric transport. Variation in Fe release from samples treated at different temperatures may relate to changes in glass resistance to leaching and dissolution imparted at 300 °C, and changes in Fe mineralogy and redox speciation imparted at 700 °C. The effect of gas atmosphere is inferred to reflect the distinct ways in which gaseous SO2 and HCl adsorption act on cations within the glass, possibly involving Fe2+/O2- co-diffusion and Fe2+/2H+ inter-diffusion, respectively, as well as the different efficiencies at which these mechanisms occur at lower versus higher temperature. Iron enrichment in glass surfaces by cation outfluxing under SO2- and/or HCl-bearing atmospheres, and formation of discrete Fe oxide phases within the glass surface by oxidation may have impacted Fe mobilisation from samples treated at 700 °C. By demonstrating experimentally for the first time a significant influence of high temperature gas-ash interactions on Fe solubility and speciation, our findings lay the ground for future laboratory studies aimed at unravelling volcanic controls on the capacity of ash as a source of soluble Fe to the environment.
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Maters, E., Delmelle, P., & Ayris, P. (2016). Effects of eruption plume processing on volcanic ash iron solubility and speciation: insights from high temperature gas-ash interaction experiments. Volcanic Ash as an Active Agent in the Earth System (VA3): Combining models and experiments, Hamburg, Germany. https://hdl.handle.net/2078.5/182026