Mineral and glass chemical composition of volcanic ash from the eruptions of Shinmoedake, Kirishima volcano on July 2-4, 2025

1. Background and Purpose

Since the resumption of eruptive activity at Shinmoedake on June 22, 2025, the volcano has continuously emitted large amounts of sulfur dioxide (SO₂), suggesting a deep-seated magma supply. However, as of August 21, no high-temperature magmatic products such as pumice or lava flows had been observed.

This study aimed to characterize the magma by analyzing fresh vesicular glass (DG) particles and dense lithic (GL) fragments from volcanic ash collected July 2–4, 2025, and to estimate magma temperature using groundmass glass compositions.

2. Methodology

Ash samples collected during specific eruption windows on July 2, 3, and 4, 2025, were analyzed after cleaning and sieving (125–250 µm). The particles were examined using an Electron Probe Microanalyzer (EPMA) for chemical analysis and microstructural observation. Thermodynamic analysis was also performed using Rhyolite-MELTS.

3. Key Findings

• Particle Characteristics: The vesicular glass (DG) particles are fresh, with no signs of weathering or hydrothermal alteration. While they appear glassy under an optical microscope, SEM observations reveal that they contain numerous microcrystals (plagioclase and orthopyroxene), with glass accounting for no more than half of the volume.
• Magmatic Continuity: The chemical composition of the groundmass glass in the 2025 DG particles is remarkably similar to the products of the 1716, 2011, and 2018 eruptions. This suggests strong continuity in the magma supply system of Shinmoedake.
• Temperature Estimates:
  • Pre-crystallization (Liquidus): Estimated at approximately 1, 050–1,090°C.
  • During Crystallization: Estimated at approximately 830–990°C based on SiO₂ and K₂ O concentrations.
  • At Eruption: The temperature is thought to have dropped to around 500°C due to interaction with groundwater.

4. Conclusion

The 2025 DG particles are identified as magmatic material. Their compositional similarity to past eruptions indicates a consistent magma source. The estimated temperatures (830–990°C during crystallization) are consistent with those observed in historical eruptions, such as those in 1716, 2011, and 2018.