Investigation of pre-Eruptive Volatile Contents of the TaupoIgnimbrite: Implications for Plainian-Ignimbrite Transitions

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The Taupo eruption of New Zealand (ca. A.D. 186) contains a spectacular example of transition from plinian to ignimbrite eruptive activity. This change is probably due to a combination of decrease in volatile content of the melt and increase in vent size at the time of eruption. The volatile content of the magma which produced the precursor plinian tephras is well known. This project will determine the volatile content and volatile distribution within the ignimbrite-producing magma by analyzing melt inclusions in phenocrysts, in order to assess the influence of the volatile component in a plinian-ignimbrite transition. Using ion and electron microprobe techniques it is planned to determine H2O, f, and Cl in melt inclusions throughout the ignimbrite. Electron microprobe analyses of major elements will help assess if inclusions have undergone alteration. Finally, trace element analyses of bulk rock from different stratigraphic horizons will show if volatile and trace elements were coupled or decoupled in this magmatic system.