Skip to content

Glass-bearing mush fragments reveal the plutonic-volcanic connection in the Taupō Volcanic Zone, New Zealand

Sarah L. Smithies 1, Elizabeth C. Teeter2, Guilherme A. R. Gualda2, Rachel Beane3, Christine M. Reimer3,4, Natasha Haft3, Caroline Rogers5,6

  • Affiliations: 1School of Earth and Environment, University of Canterbury, Christchurch, New Zealand; 2Department of Earth and Environmental Sciences, Vanderbilt University, TN, USA, 3Earth and Oceanographic Science, Bowdoin College, ME, USA; 4now at Department of Earth, Environmental, and Planetary Sciences, Rice University, TX, USA; 5Geosciences Department, Skidmore College, NY, USA; 6now at Department of Geological Sciences, University of Missouri, MO, USA

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 215

  • Programme No: 1.9.22

  • Theme 1 > Session 9

Abstract

Fragments of mush erupted during large, caldera-forming eruptions give us a unique window into the plutonic systems that generate huge volumes of eruptible, rhyolitic magma. Here, we examine glass-bearing, crystal-rich mush fragments from the Taupō Volcanic Zone, New Zealand. Studied clasts were entrained in the 240 ka Ohakuri eruption, which evacuated 150 km3 of high-silica rhyolite. Previous work revealed that eruptible Ohakuri magma was stored in at least two shallow magma bodies at <4 km depth, and was extracted from a heterogeneous, vertically extensive mush system at 4-13 km depth. We combine new insights from the mush fragments with this model of the Ohakuri magma system. Detailed textural analysis of the mush fragments was performed using optical and scanning electron microscopy. In situ compositions were collected with SEM-EDS and LA-ICPMS. Whole-rock compositions were collected by XRF. Intensive parameters were estimated with rhyolite-MELTS geobarometry. The mush fragments are hornblende gabbro, dominated by partially resorbed hornblende and euhedral plagioclase. Glass is interstitial, vesicular, and microlite-free. These textures suggest that the interstitial glass is trapped residual melt that quenched during eruption. Whole-rock compositions of the mush fragments are low SiO2 (50 wt.%), overlapping in major elements with basalts from the TVZ. Glass is high-silica rhyolite (77.5 wt.% SiO2), overlapping with the whole-rock composition of Ohakuri pumice. Geobarometry indicates that the mush fragments equilibrated shallow in the magma plumbing system (<4 km). Together, this suggests that rhyolites can be generated at very shallow depths from a single fractionation step of basalt to rhyolite.