LACK OF VOLATILE GRADIENT IN THE TAUPO PLINIAN-IGNIMBRITE TRANSITION - EVIDENCE FROM MELT INCLUSION ANALYSIS

Ion and electron microprobe analyses of melt inclusions from the rhyol itic Taupo eruptive sequence of 1800 yr ago indicate that the 35-km3 p reeruptive magma was homogeneous with respect to volatile, major, and trace elements at the time of crystallization and presumably at the ti me of eruption. The early-erupted Hatepe and Taupo plinian tephra both contain melt inclusions with an average of 4.3 wt% H2O. Melt inclusio ns from an intraplinian ignimbrite contain 3.7 wt% H2O, whereas melt i nclusions in Taupo ignimbrite average 3.6 wt%. A sample from the lates t-erupted part of the eruptive sequence has melt inclusions containing 4.4 wt% H2O. The Cl and F content of melt inclusions from all samples range between approximately 1700 and 1900 ppm, and 400 and 600 ppm, r espectively. The uniform H2O content of the preeruptive magma suggests that whereas H2O was almost certainly the main force behind this erup tion, shifts in eruptive style from plinian to ultraplinian to ignimbr ite were not controlled by variations in preeruptive H2O contents. Ven t configuration or the ability of the magma to degas passively may hav e played important roles in determining eruptive style.