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

DUNBAR NW, KYLE PR

AMERICAN MINERALOGIST
v. 78(#5-6) pp. 612-618 1993
(Title at LANL MAIN.)

Institutions:

Abstract:

Ion and electron microprobe analyses of melt inclusions from the rhyolitic Taupo eruptive sequence of 1800 yr ago indicate that the 35-km3 preeruptive magma was homogeneous with respect to volatile, major, and trace elements at the time of crystallization and presumably at the time of eruption. The early-erupted Hatepe and Taupo plinian tephra both contain melt inclusions with an average of 4.3 wt% H2O. Melt inclusions from an intraplinian ignimbrite contain 3.7 wt% H2O, whereas melt inclusions in Taupo ignimbrite average 3.6 wt%. A sample from the latest-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, respectively. The uniform H2O content of the preeruptive magma suggests that whereas H2O was almost certainly the main force behind this eruption, shifts in eruptive style from plinian to ultraplinian to ignimbrite were not controlled by variations in preeruptive H2O contents. Vent configuration or the ability of the magma to degas passively may have played important roles in determining eruptive style.

Keywords:

EXPLOSIVE VOLCANIC-ERUPTIONS; CALCALKALINE MAGMA CHAMBER; NEW-ZEALAND; MOUNT-MAZAMA; CRATER LAKE; WATER; ZONE; H2O; EVOLUTION; PETROLOGY Return to HOMEPAGE