Presentation Information
[R7P-02]Relationship between the grain size of plagioclase constituting volcanic ash from Sakurajima Volcano and its structural state estimated by powder X-ray diffraction experiments
*Tomoaki MATSUI1 (1. Kagoshima Univ. Educ.)
Keywords:
volcanic ash,plagioclase,grain size,Sakurajima Volcano,structural state
Sakurajima volcano exhibited a temporary decline in eruption frequency in the early 2000s, followed by renewed activity at the Showa crater in 2006 and a rapid increase to more than 1000 eruptions per year in the early 2010s. Recent activity has stabilized at approximately 200-300 eruptions annually. To investigate mineralogical indicators sensitive to such variations in volcanic activity, we analyzed the structural state of plagioclase contained in volcanic ash collected from Sakurajima. Volcanic ash samples were obtained during July-August 2022 at a site located 5 km ENE of the Minamidake summit crater. Grain-size fractions were separated, plagioclase was isolated using magnetic and heavy-liquid techniques, and structural states were estimated from powder X-ray diffraction patterns following Scheidegger (1973). 0392-0393 plots reveal systematic differences among grain-size fractions: plagioclase in the 250–500 03BCm fraction exhibits a slightly more disordered Al/Si arrangement, corresponding to a higher-temperature structural state, compared with finer fractions. These results indicate that plagioclase structural state varies with grain size, reflecting differences in the fragmentation and thermal histories of phenocrysts during eruption. Continued accumulation of structural state data of plagioclase, integrated with geophysical time-series observations, may provide a new framework for monitoring temporal changes in volcanic activity at Sakurajima.