VISUALIZING THE MAGMA SYSTEM OF THE ROTOITI CALDERA-FORMING ERUPTION, TAUPŌ, NEW ZEALAND, USING GLASS AND GEOBAROMETRY

The Taupō Volcanic Zone (TVZ), located on New Zealand's North Island, is a region of highly active volcanism characterized by large silicic caldera-forming eruptions. The Rotoiti eruption (60 ka) erupted 120 km3 of high-silica rhyolite from the Okataina Volcanic Centre. It was the first caldera-forming event in the TVZ after almost 200 ka of relative quiescence. It is believed to have triggered the nearby Earthquake Flat (EQF) eruption. Our aim is to use geobarometry techniques to calculate pre-eruptive storage pressures of the Rotoiti and Earthquake Flat magma systems.

We collected glass compositions via EDS-SEM of 21 individual pumice samples from the Rotoiti ignimbrite. We also collated 158 Rotoiti glass compositions and 28 Earthquake Flat glass compositions from the literature (Schmitz & Smith 2004, J Petrol, Smith et al., Contrib Mineral Petrol). For a subset of these compositions, we calculated pre-eruptive storage pressures using rhyolite-MELTS geobarometry (Gualda & Ghiorso 2014, Contrib Mineral Petrol).

The Rotoiti event involved two compositionally distinct magmas, T1 (n=72 in our data collation) and T2 (n=7), which are differentiated by their glass K2O content (3.16-3.53 wt.% for T1, and 3.99-4.21 wt.% for T2). T2 samples and Earthquake Flat samples are compositionally very similar. Major-oxide comparison between literature samples and our samples show that ours are of the T1 composition. Here we show the storage pressures for our Rotoiti samples were between 41–124 MPa, with pressures for T1 and T2 samples overlapping. We also calculated storage pressures of 49–192 MPa for the Earthquake Flat eruption. The similar compositions and storage pressures of the Rotoiti T2 magma and Earthquake Flat eruptions support the theory that thermal pulses triggered the eruption of the silicic melt within these magma bodies. Our results reveal that the Rotoiti magma system was composed of two discrete magma bodies, T1 and T2, situated side-by-side at similar depths in the crust, with the T2 magma also the source of the EQF eruption.