[S-VC28] 火山の熱水系

[SVC28-P11] マグマ水蒸気噴火火山灰に含まれる硫黄鉱物の起源および風化過程

*平田 碧¹、大場 司¹ (1.秋田大学国際資源学研究科)

キーワード：熱水系、阿蘇火山、火山灰、熱水変質岩片、風化

Mineralogy and geochemistry of volcanic ash can provide valuable insights into volcanic systems' physical and chemical conditions. Volcanic ash from phreatomagmatic eruptions contains sulfur-bearing minerals. Mineralogy and sulfur isotopic compositions can help understand their origins and underground processes before, during, and after the eruption. This study aims to determine the origin of sulfur-bearing minerals from their mineralogy and sulfur isotopic compositions and the influence of weathering on the sulfur-bearing minerals after deposition. We examined the volcanic ash from the Aso 2021 eruption ash. The ash consisted of 1-mm-size ash aggregates comprising coarse and fine grains when it fell onto the ground on October 20th, 2021. In six days, the volcanic ash had lost its original occurrence, conjugating into larger grains or massive sheets. The occurrence after one year was a layer forming on the surface. Ash samples collected on the day of the eruption, six days later, and one year later were analyzed. We prepared bulk samples and their fine fractions smaller than 63μm 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 with water, acetone, HCl, NaOH, and HNO₃ to separate sulfur from different minerals: water-soluble sulfur, native sulfur, calcium sulfate (gypsum and anhydrite), alunite group minerals, and pyrite. The Oct-20th-2021 bulk sample contained natroalunite, alunite, pyrite, native sulfur, gypsum, anhydrite, and cristobalite. The X-ray intensity of anhydrite decreased in later sampled bulk ash. Gypsum occurs only in the six-day later bulk sample. The X-ray intensity of native sulfur decreased from the six-day to one-year-later fine-grained samples. Coarse ash grains in ash aggregates are primarily volcanic glass shards and hydrothermally altered rock fragments. The coarse fraction occasionally contains aggregates of anhydrite and pyrite. Fine fractions of ash aggregates consist of native sulfur, anhydrite, gypsum, pyrite, natroalunite, alunite, and cristobalite. Sulfur-bearing minerals occur as isolated fine crystals and fill pores within the coarse grains. In chronological order, δ³⁴S values of water-soluble sulfur were +9.8‰, +11.8‰, and +9.3‰, those of calcium sulfate were +11.4‰, +11.0‰, +11.1‰, those of alunite group minerals were -0.6‰, +4.3‰, +6.9‰, and those of pyrite were -6.4‰, -5.9‰, and -1.9‰, respectively. Native sulfur showed a constant value at -6.5‰. Fine fractions were obtained from the 6-day-later and 1-year-later samples, and no calcium sulfate occurred in the 1-year-later sample. +9.3‰, -6.5‰, +11.1‰, +7.0‰, -6.8‰. Water soluble sulfur from the one-year-later sample has different δ³⁴S (-4.9‰) from the bulk samples, whereas the 6-day-later sample has a similar value (+9.3‰). Native sulfur showed little change (+6.5‰ 6 days later, +6.4‰ one year later). Calcium sulfate in a fine fraction from the 6-day-later sample has nearly the same value (+11.1‰) as the bulk sample. The δ34S of alunite in fine fraction has a slightly higher value (+7.0‰ 6-days later and +10.4‰ one year later) than those of bulk samples. Pyrite in fine fraction slightly changed from -6.8‰ to -4.1‰. The water-soluble sulfur of the early two samples had δ³⁴S similar to sulfate minerals from hydrothermal origin. In contrast, those from the 1-year-later sample are close to those of pyrite, indicating pyrite oxidation. The isotopically fractionated values indicate that native sulfur and pyrite were in equilibrium with calcium sulfate minerals in a hydrothermal system. The earliest sample contained abundant anhydrite with high δ³⁴S values, indicating that they were derived from a hydrothermal system. The anhydrite had partly hydrated to form gypsum in 6 days without any change in δ³⁴S. The δ³⁴S value of HCl-leached sulfur in the 1-year-later sample is similar to those of sulfide minerals and native sulfur, indicating oxidation of native sulfur and pyrite on the surface after deposition. We assumed that gypsum and anhydrite were the source of the HCl-leached sulfur, although they were not detected with XRD analysis. We interpret that the earliest sample contained alunite minerals from two different origins: one through the direct condensation from magmatic gas and the other from existing hydrothermal systems. The variation in δ³⁴S of alunite group minerals was due to the disappearance of fine condensate grains from magmatic gas during weathering.