桝水原火砕流堆積物における岩石学的および地球化学的研究

抄録・要旨

Daisen Volcano is located in the San-in district, southwest Japan. The volcano is a large composite cone with several amalgamated eruptive centres which are Hiruzen, Karasugasen, and Misen. A thick apron of pyroclastic material surrounds the entire volcanic complex. The erupted material from Daisen volcano is divided into an Older group and a Younger group. The Masumizuhara pyroclastic flow deposits are sourced from the Misen lava dome and are part of the Younger group. Pyroclastic rocks were analysed by X-ray fluorescence spectrometry (XRF) and all samples are classified as adakitic, medium-K dacites. Mineral and glass composition was determined by electron probe microanalyzer (EPMA) to evaluate magma storage conditions and to characterize the magmatic system of Mt. Daisen during its youngest eruptive cycle. Pressure, temperature, water content and oxygen fugacity range from 126 to 201 MPa, 843 to 864 ˚C, 5.0 to 7.7 wt % and +1.6 to +2.0 log units above the nickel-nickel oxide (NNO) buffer, respectively.