Experimental constraints on pre-eruptive conditions of Aso-4 silicic end-member magma

In order to constrain the pre-eruptive physical and chemical conditions (pressure, temperature, X_H2O, and fO₂) of the largest eruptive stage of activity at Aso volcano, Japan, we experimentally determined phase assemblages and chemical compositions of the felsic end-member magma of Aso-4 (KJ5665 pumice and MY5774 welded lens from Aso-4 pyroclastic flow deposit: SiO₂ = 70.5 wt.%) using high-pressure and high-temperature gas medium apparatus with rapid-quench capability. Experimental conditions are as follows; starting material is KJ5665 pumice, pressure range is 50-700 MPa, temperature range is 810-950 °C, run duration is about a week, injected H₂O content is1.5-6.7 wt.% (saturated/under-saturated), using composite Ag-Pd, Au-Pd, and Pt capsules with Ni-NiO fO₂ buffering technique. The starting materials contain plagioclase, orthopyroxene, magnetite, and ilmenite phenocrysts with unimodal core compositions of An30-50 (= Ca/(Ca + Na + K)*100) in plagioclase, Mg# 72-75 (= Mg/(Mg + Fe_total)*100) in orthopyroxene, X_Usp = 0.25 ± 0.01 as ulvöspinel content, and X_ilm = 0.76 ± 0.01 as ilmenite content, respectively. Hornblende phenocrysts are rarely found in KJ5665 pumice. Pre-eruptive magmatic temperature and fO₂ were estimated to be 905 ± 15 °C and Fayalite-Magnetite-Quartz (FMQ) +2, respectively, according to magnetite-ilmenite thermometer in felsic magma sample KJ5665. Run product contain plagioclase, orthopyroxene, magnetite, ilmenite, biotite, and K-feldspar. Plagioclase and orthopyroxene composition are An20-48 and Mg# 53-72 in run products. Hornblende was not observed. We corrected possible compositional modification of KJ5665 pumice (due to weathering) using whole-rock Ca/Na ratio of a less altered MY5774 glass lens. The pre-eruptive conditions of the felsic end-member magma of Aso-4 are estimated to be 900 °C in temperature, 200MPa in pressure, and <3 wt.% H₂O in the bulk water content, according to the observed partial matching of the phenocryst assemblage in the natural sample and experimental run charge.