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A glimpse into the magma dynamics beneath a large caldera; erupted magma mush fragments in the Rotoiti ignimbrite, New Zealand

Elizabeth Teeter1, Sarah Smithies2, Rachel Beane3, Guilherme Gualda1, Darren Gravley2

  • Affiliations: 1Department of Earth and Environmental Sciences, Vanderbilt University, Nashville, USA; 2School of Earth and Environment, University of Canterbury, Canterbury, NZ; 3Department of Earth and Oceanographic Sciences, Bowdoin College, Brunswick, USA

  • Presentation type: Poster

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

  • Poster Board Number: 214

  • Programme No: 1.9.21

  • Theme 1 > Session 9

Abstract

Magma mush fragments -- crystal-rich, glass bearing clasts potentially genetically related to the erupted magmas -- can provide important perspective on mush processes in active magmatic systems. Granitoid clasts from the Rotoiti Ignimbrite, Taupō Volcanic Zone (TVZ), include magma mush fragments that can provide insights on magmatic systems of one of the most active areas of rhyolitic volcanism in the world. This study focuses on a large (35 cm) layered clast found within a lag-breccia of the 64 ka Rotoiti Ignimbrite, one of the largest eruptions from the Ōkataina Volcanic Center (TVZ). This fragment is layered and can be divided into 5 zones. Backscattered Electron (BSE), Cathodoluminescence (CL), and Energy Dispersive Spectrometry (EDS) techniques highlight the textural and mineralogical differences between the zones. In this study, we focus on a fine-grained, biotite granite zone with microcrystalline patches and sparse granophyric texture. Quartz grains range from euhedral to subhedral, with complex zoning patterns that are visible in both BSE and CL. Some feldspar grains are zoned, others are not. The microcrystalline patches make up less than 10% of the sample and they are surrounded by subhedral grains (both quartz and feldspar). The interstitial texture and distribution of the microcrystalline patches indicate that this fragment is representative of a magma mush with <10% trapped melt. The average compositions of this microcrystalline material range from 72-77% SiO2 and 4-6% K2O and do not correspond with Rotoiti eruptive products; this mush fragment may illuminate older eruptive sequences of the TVZ.