Diverse patterns of ascent, degassing, and eruption of rhyolite magma during the 1.8 ka Taupo eruption, New Zealand: Evidence from clast vesicularity

The 22 km(3) (DRE) 1.8 ka Taupo eruption ejected chemically uniform rhyolite in a wide range of eruptive styles and intensities. The 7 eruptive units include the 'type examples' of phreatoplinian (units 3 and 4) and ultraplinian fall (unit 5) deposits, and low-aspect-ratio ignimbrite (unit 6). Contrasts in bulk vesicularity, vesicle (and microlite) number densities and the size distributions of bubbles (and crystals) in the Taupo ejecta can be linked to the influence of shallow conduit processes on volatile exsolution and gas escape, before and during eruption, rather than changes in pre-eruptive chemistry. Existing work has modeled the individual phases of this complex eruption but not fully explained the abrupt shifts in style/intensity that occur between phases. We link these rapid transitions to changes in vent position, which permitted contrasts in storage, conduit geometry, and magma ascent history. Samples in the study show that coalescence of bubbles was a late-stage process in the pre-fragmentation degassing of even the most rapidly ascending magma, but, in most cases, nucleation of new bubbles continued until close to fragmentation. In the two phases of the Taupo eruption linked to dome or cyptodome formation, we can also recognize an influence of onset of permeability, partial outgassing, limited syn-eruptive crystallization, and bubble collapse. Post-fragmentation expansion or contraction of vesicles was only marked in some pumices within the deposits of the two heat-retentive yet nonwelded pyroclastic density currents (units 5B and 6). We can recognize three different types of history of ascent, bubble nucleation, and degassing for the Taupo eruption. Units 1, 2 3, 5, and 6 involved magma that ascended rapidly under the southern and central portions of the vent system and underwent late-stage closed system, coupled vesiculation without syn-eruptive crystallisation of microlites. The limited contrasts in textures amongst these units reflect only slightly different ascent histories, including slower rise permitting extended bubble coalescence (unit 1), rapid, accelerating ascent (units 2,3 and 5), and exceptionally rapid decompression and ascent at the time of highest mass discharge rates (unit 6). In contrast to all these units, the unit 4 magma also rose rapidly initially but then underwent some degree of limited and shallow storage, permitting variable degrees of prolonged bubble maturation (growth and coalescence), development of permeability and outgassing, under the northern portion of the 10-km-long vent system, probably beginning at the time of eruption of units 1-3 from the southern portion of the fissure. Finally, the magma which formed the late-stage dome (unit 7), remained deeper in the plumbing system for an extended time permitting limited growth of microphenocrysts, (perhaps in response to partial depressurisation occurring during the earlier phases of the eruption), before ascending and continuing to outgas in equilibrium fashion. (C) 2010 Elsevier B.V. All rights reserved.