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Preeruptive P ‐ T Conditions and H 2 O Concentration of the Aso‐4 Silicic End‐Member Magma Based on High‐Pressure Experiments
Author(s) -
Ushioda Masashi,
Miyagi Isoji,
Suzuki Toshihiro,
Takahashi Eiichi,
Hoshizumi Hideo
Publication year - 2020
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2019jb018481
Subject(s) - phenocryst , pumice , ilmenite , silicic , plagioclase , geology , magma , mineralogy , magnetite , geochemistry , volcano , volcanic rock , quartz , paleontology
To constrain the preeruptive physical and chemical conditions (pressure, temperature, H 2 O content, and fO 2 ) of the most recent and largest eruption (Aso‐4) of Aso volcano, Japan, we compared phase assemblages and chemical compositions of erupted material with those of experimentally determined run charges of the silicic end‐member magma of Aso‐4 (the KJ5665 pumice and MY5774 welded lens from the Aso‐4 pyroclastic flow deposit; SiO 2 = 70.5 wt %). Experimental conditions were as follows: starting material = KJ5665 pumice, pressure = 100–700 MPa, temperature range = 810–950 °C, and injected H 2 O content = 2.9–6.7 wt %. The starting material comprised mainly rhyolitic glass with plagioclase, orthopyroxene, magnetite, and ilmenite phenocrysts. Hornblende phenocrysts are not found on cross sections of the KJ5665 pumice. Core compositions of these phenocrysts are unimodal. Using the magnetite‐ilmenite thermometer, the preeruptive magmatic temperature and fO 2 of KJ5665 were estimated to be ~900 °C and Nickel‐Nickel oxide (NNO) +1.1 log units, respectively. Run products contain plagioclase (An 20–48 ), orthopyroxene (Mg# = 53–72), magnetite, ilmenite, biotite, and K‐feldspar. Hornblende was not observed. Preeruptive conditions of the silicic end‐member magma of the Aso‐4 eruption are estimated to be ~900 °C, ≤ 300 MPa, and ≤ 3 wt % H 2 O.