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Mineralogy and formation mechanisms of the aerosol particles emitted by a permanent lava lake at Erta'Ale volcano, Danakil depression, Ethiopia.

Cedric Botter. Christoph Neuruhrer, Bernard Grobéty

Abstract

In a campaign 2015 aerosol and gases were sampled using in house-developed passive sampler allowing sampöing 2 -- 3m above the lake, filter packs (FP), and an in-house developed Portable Gas Analyzer System (PGAS) above the lake during quiet and turbulent periods as well as from fumaroles. The chemistry, mineralogy and morphology of particles were investigated using Computer-Controlled SEM and TEM coupled with EDS. Thermodynamic equilibrium calculations (HSC Chemistry) were done to simulate the speciation of the magmatic gas mixtures and the condensation of the latter into aerosol liquids and solids. The analysis of the PGAS data and the results of the FP samples analysis show a gas composition dominated by CO2 and SO2, typical for open-vent volcanoes. The very quick sulfation and the preferential evoparation of potassium above the lake determines the mineralogy of the aerosol particles deposited from the emitted high temperature volcanic gas. The observed evolution of the system, well reproduced by the by HSC calculations, shows a reaction pathway going through the formation of intermediate gaseous hydrogen sulfate and oxysulfur chlorides from initial gaseous alkali chloride and hydroxide species, followed by the production of potassium rich alkali sulfate particles. The fumaroles however, release rather sodium rich sulfate particles pointing to a gas source deeper in the volcanic edifice.