title: Spatial analysis of Mount St. Helens tephra leachate compositions: implications for future sampling strategies.
abstract: Tephra particles in physically and chemically evolving volcanic plumes and clouds carry soluble sulphate and halide salts to the Earth's surface, ultimately depositing volcanogenic compounds into terrestrial or aquatic environments. Upon leaching of tephra in water, these salts dissolve rapidly. Previous studies have investigated the spatial and temporal variability of tephra leachate compositions during an eruption in order to gain insight into the mechanisms of gas-tephra interaction which emplace those salts. However, the leachate datasets analysed are typically small and may poorly represent the natural variability and complexity of tephra deposits. Here, we have conducted a retrospective analysis of published leachate analyses from the 18 May 1980 eruption of Mount St. Helens, Washington, analysing the spatial structure of the concentrations and relative abundances of soluble Ca, Cl, Na and S across the deposits. We have identified two spatial features: (1) concentrated tephra leachate compositions in blast deposits to the north of the volcano and (2) low S/Cl and Na/Cl ratios around the Washington-Idaho border. By reference to the bulk chemistry and granulometry of the deposit and to current knowledge of gas-tephra interactions, we suggest that the proximal enrichments are the product of pre-eruptive gas uptake during cryptodome emplacement. We speculate that the low S/Cl and Na/Cl ratios reflect a combination of compositional dependences on high-temperature SO2uptake and preferential HCl uptake by hydrometeor-tephra aggregates, manifested in terrestrial deposits by tephra sedimentation and fallout patterns. However, despite our interrogation of the most exhaustive tephra leachate dataset available, it has become clear in this effort that more detailed insights into gas-tephra interaction mechanisms are prevented by the prevalent poor temporal and spatial representativeness of the collated data and the limited characterisation of the tephra deposits. Future leachate studies should aim to extensively sample across tephra deposit limits whilst simultaneously characterising deposit stratigraphy and tephra chemistry, mineralogy and granulometry, taking steps to ensure the quality and comparability of collected leachate datasets.
authors.affiliation: Department of Earth and Environmental Sciences, Ludwig Maximilian University, Thereisenstrasse 41/III, 80333 Munich, Bavaria Germany.
authors.forename: P M
authors.initials: PM
authors.lastname: Ayris
authors.affiliation: Earth and Life Institute, Université Catholique de Louvain, Croix du Sud 2, bte L7.05.10, 1348 Louvain-la-Neuve, Wallonia Belgium.
authors.forename: P
authors.initials: P
authors.lastname: Delmelle
authors.affiliation: Earth and Life Institute, Université Catholique de Louvain, Croix du Sud 2, bte L7.05.10, 1348 Louvain-la-Neuve, Wallonia Belgium.
authors.forename: B
authors.initials: B
authors.lastname: Pereira
authors.affiliation: Earth and Life Institute, Université Catholique de Louvain, Croix du Sud 2, bte L7.05.10, 1348 Louvain-la-Neuve, Wallonia Belgium.
authors.forename: E C
authors.initials: EC
authors.lastname: Maters
authors.affiliation: Department of Earth and Environmental Sciences, Ludwig Maximilian University, Thereisenstrasse 41/III, 80333 Munich, Bavaria Germany.
authors.forename: D E
authors.initials: DE
authors.lastname: Damby
authors.affiliation: Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Blindern, Postbox 1028, 0315 Oslo, Norway ; Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 USA.
authors.forename: A J
authors.initials: AJ
authors.lastname: Durant
authors.affiliation: Department of Earth and Environmental Sciences, Ludwig Maximilian University, Thereisenstrasse 41/III, 80333 Munich, Bavaria Germany.
authors.forename: D B
authors.initials: DB
authors.lastname: Dingwell
keywords: Leaching; Mount St. Helens; Tephra