2:00 PM - 2:15 PM
[SVC50-14] Experimental constraints on pre-eruptive P-T conditions of Aso-4 silicic magma
Keywords:high pressure and high temperature experiments, Aso-4, hydrous melting experiments, magma chamber
KJ5665 pumice had plagioclase, orthopyroxene, magnetite, and ilmenite and trace of hornblende phenocrysts. All frequent distributions of core compositions for these phenocryts were unimodal. Core compositions of plagioclase and orthopyroxene ranged from An30 and Mg#72 to An50 and Mg75#, respectively. Temperature and fO2 were 870~880 °C and FMQ+2, respectively, estimated by thermometer and oxygen barometer using equlibrium between magnetite and ilmenite (Lepage 2003; Andersen and Lindsley 1985). Three hydrous glasses (2.0 to 6.0 wt.% H2O) were synthesized using the powder of KJ5665 for starting materials of high-P and high-T experiments. Melting experiments were performed in temperatures between 810 and 930 °C at 200, 400, and 700 MPa under NNO-buffered condition using IHPVs (SMC8600 and HARM200 installed at Tokyo Institute of Technology and Geological Survey of Japan, AIST, respectively). Plagioclase, orthopyroxene and/or K-feldspar crystallized with small amount of Fe-Ti oxides in the lower H2O content of run products, while orthopyroxene did not crystallize and biotite crystallized in the higher H2O content of run products. At 200 MPa and 900 °C and under low H2O content (~2wt.%) of run products, plagioclase and orthopyroxene which compositions falled within the range of that of phenocrysts crystallized. In the experiments, hornblende which existed rarely as phenocrysts in the KJ5665 pumice did not crystallized. Origin of hornblende phenocrysts need to be carefully considered using various petrological features.
This study was supported by the Secretariat of the Nuclear Regulation Authority, Japan.