Ash aggregation enhanced by deposition and redistribution of salt on the surface of volcanic ash in eruption plumes

Abstract

Interactions with volcanic gases in eruption plumes produce soluble salt deposits on the surface of volcanic ash. While it has been postulated that saturation-driven precipitation of salts following the dissolution of ash surfaces by condensed acidic liquids is a primary mechanism of salt formation during an eruption, it is only recently that this mechanism has been subjected to detailed study. Here we spray water and HCl droplets into a suspension of salt-doped synthetic glass or volcanic ash particles, and produce aggregates. Deposition of acidic liquid droplets on ash particles promotes dissolution of existing salts and leaches cations from the underlying material surface. The flow of liquid, due to capillary forces, will be directed to particle-particle contact points where subsequent precipitation of salts will cement the aggregate. Our data suggest that volcanically-relevant loads of surface salts can be produced by acid condensation in eruptive settings. Several minor and trace elements mobilised by surface dissolution are biologically relevant; geographic areas with aggregation-mediated ash fallout could be "hotspots" for the post-deposition release of these elements. The role of liquids in re-distributing surface salts and cementing ash aggregates also offers further insight into the mechanisms which preserve well-structured aggregates in some ash deposits.

Figure 1. EDX mapping of soda-lime glass bead aggregates.

(a) Shows a secondary electron (SE) image of the mapping area. (b) Highlights Na and (c) Cl content which, in combination with a), can be identified as NaCl crystals sitting on glass bead surfaces and in connection points of glass beads as a cementing agent.

Figure 2

SEM images of experimental particles after (a) halite doped glass bead materials produced in experimental step 1, (b) halite doped glass bead materials after spraying with deionised water, (c) un-doped glass bead materials after spraying with 12 M HCl, (d) halite doped glass bead materials after spraying with 12 M HCl, (e) surface nodules on halite doped glass bead materials after spraying with 12 M HCl, and (f) halite doped volcanic ash materials after spraying with 12 M HCl.

Figure 3

(a)Liquid droplets of HCl or H₂O spread around the particle as a liquid film, dissolving the NaCl coating and triggering cation exchange with the underlying particle surface. (b) Capillary forces accumulate NaCl-H₂O brine at particle-particle contact points, forming liquid bridges. The liquid layer thickness h, particle radius R and the curvature of radius in the liquid neck a are labelled. (c) Evaporation of the liquids during drying processes leads to precipitation of a solid NaCl bridge and depletes the particle surface in NaCl crystals.

Figure 4

Composite figure displaying (a) Na_s, Al_s, K_s, Ca_s, Mg_s, Mn_s, Si_s from volcanic ash materials sprayed with varying HCl concentrations; (b) selected soluble element concentrations plotted against soluble Na concentrations using data from eleven leachate studies (see Data Repository), normalized to their maxima and offset arbitrarily to be viewable at the same scale, with best-fit power-laws provided to guide the eye and to demonstrate a commonality of slope.