Preeruptive P‐T Conditions and H2O Concentration of the Aso‐4 Silicic End‐Member Magma Based on High‐Pressure Experiments

Abstract

AbstractTo constrain the preeruptive physical and chemical conditions (pressure, temperature, H2O content, and fO2) 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; SiO2 = 70.5 wt %). Experimental conditions were as follows: starting material = KJ5665 pumice, pressure = 100–700 MPa, temperature range = 810–950 °C, and injected H2O 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 fO2 of KJ5665 were estimated to be ~900 °C and Nickel‐Nickel oxide (NNO) +1.1 log units, respectively. Run products contain plagioclase (An20–48), 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 % H2O.