The Oruanui and Taupo Magma Bodies (central Taupo Volcanic Zone, NZ): Evolution and Longevity as recorded in Pumice Textures

Abstract:

The central Taupo Volcanic Zone (TVZ: New Zealand) has produced numerous large-volume high-silica rhyolite eruptions in the past 1.6 Ma. In this study, we focus on two such eruptions, derived from the same area: the 26.5 ka Oruanui- the most recent supereruption in earth’s history- and the 1.8 ka Taupo- the most recent large, rhyolitic eruption from the TVZ. In order to understand the evolution of these crystal-poor magma bodies and constrain their pre-eruptive residence times in the crust, we use x-ray tomography and energy dispersive spectroscopy (EDS) to qualitatively and quantitatively characterize pumice textures in both Oruanui and Taupo. We obtain crystal size distributions (CSDs) using x-ray tomography for three groups of phases: quartz+feldspar, magnetite+ilmenite, and zircon. Quartz+feldspar size distributions from both eruptions show a shallow-sloped log-linear section describing large crystals (>~200 µm), which we interpret as a pre-eruptive crystal population that grew over time under low supersaturation. Magnetite+ilmenite and zircon size distributions are also log-linear, but have fewer crystals and a smaller maximum crystal size (<200 µm). Taupo quartz+feldspar size distributions are kinked at ~200 µm and show a steep-sloped section describing small crystals (<200 µm), which we interpret to represent a crystal population that nucleated and grew during eruptive decompression. In contrast, Oruanui distributions are not kinked. This difference is supported by EDS compositional maps that show a small (20-50 µm) crystal population in Taupo samples, but not in Oruanui samples. This is consistent with rhyolite-MELTS geobarometry results, which indicate the Taupo magma resided at higher pressures (i.e. greater depth) than did the Oruanui magma. As such, Taupo magma had a longer path to eruption and thus more time for crystal nucleation and growth during decompression. Timescales calculated from CSD slopes of the pre-eruptive crystal populations of magnetite, zircon, and quartz+feldspar indicate residence times on the order of decades to centuries, and at most millennia, for both systems. Timescales calculated for the small crystal population (i.e. the onset of eruptive decompression) indicate a maximum timescale of several years.