2:45 PM - 3:00 PM
[SVC39-03] Pre-eruptive magmatic processes leading to Aso-4 caldera-forming eruption
Keywords:Pre-eruptive magmatic processes, caldera-forming eruption, Aso volcano, Omine pyroclastic cone, Aso-ABCD tephra
Despite similar compositional trends observed in bulk rock chemistry, melt inclusions from Aso-ABCD tephra, those from Omine cone, and those from Aso-4 main eruptions show compositional trends distinct from all the others. Silica content of melt inclusions varies from 70-72 wt.% for Aso-ABCD, 67-70 wt.% for Omine, and 73-74 wt.% for the earliest subunit of Aso-4, followed by 71-74 wt.% for the next and voluminous subunit. Melt inclusions from Aso-ABCD tephra show similar composition to 123 ka Aso-3 tephra. Plagioclase also shows distinct compositions between Aso-ABCD, Omine, and Aso-4. Orthopyroxene and clinopyroxene often show a disequilibrium relationship in terms of Fe-Mg exchange.
When all the data are considered, a remnant of Aso-3 magma reservoir was possibly still active 9000 years before Aso-4 eruption. Magma reservoir for Omine was similar to Aso-4, however different compositional trend indicate a subsidiary reservoir to Aso-4 main one. Eruption of Aso-4 started from the top of most silica-rich part of a main reservoir, followed by mixing products of layers from a compositionally zoned large reservoir.