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Mineralogical studies and sulfur isotopic characteristics of volcanic ash from phreatomagmatic eruptions -Case studies of Aso volcano, Japan-

Aoi Hirata¹, Tsukasa Ohba¹

Abstract

Sulfur-bearing minerals from volcanic eruptions are commonly found in volcanic ash. Analyzing these sulfur minerals provides valuable insights into the processes of eruptions. This study aims to determine the origin of sulfur-bearing minerals from volcanic ash and to understand the condition of the hydrothermal system during each activity. We examined the volcanic ash from Aso 2021 eruptions, Oct-14th-2021 and Oct-20th-2021. Ash samples collected on the day of the eruptions were analyzed. We prepared bulk samples for XRD analysis, chemical extraction, and sulfur isotope analysis, and untreated samples for polished sections for SEM-EDS analysis. For sulfur isotopic analysis, we performed sequential extraction to separate sulfur from different minerals: water-soluble sulfur, native sulfur, calcium sulfate (gypsum and anhydrite), alunite group minerals, and pyrite. Coarse ash grains in ash aggregates observed by microscope are primarily volcanic glass shards and hydrothermally altered rock fragments. The Oct-14th-2021 bulk sample contained natroalunite, alunite, pyrite, gypsum, anhydrite and plagioclase from XRD analysis. The Oct-20th-2021 bulk sample contained cristobalite, in addition to the minerals mentioned in the Oct-14th-2021 sample. The isotopically fractionated values indicate that sulfide minerals and sulfate minerals from the Oct-14th-2021 sample were in equilibrium in a hydrothermal system. The changing δ34S suggests that the ratio of minerals directly condensed from magmatic gas between the two eruptions increased and fluctuated.