Reinterpretation of the post 26ka Taupō rhyolitic magmatic system (New Zealand) as deep and vertically extensive based on Isotope thermometry and measured and modeled zircon destinies

Abstract

Taupō volcanic zone, the site of the 26ka Oruanui supereruption, produced ~70km3 of new rhyolites since 11ka, culminating in 50 km3 Taupō eruption 1.8 ka. Major phenocrysts decrease from 4 to 1 vol%, and Oruanui and post-Oruanui ignimbrites all have identical high-d18Omelt values of 7.39±0.1‰ and lack low-d18O values despite overlapping calderas. The D’17O values are -0.07‰, lower than the mantle and indicate source contamination of high-d18O, low-D’17O metasediments, and limited interaction with high-D’17O hydrothermally altered crust. Previously published U-Th-Pb zircon ages demonstrate their diversity spanning 104-105 years for each unit. Zircon crystal size distribution shows a decrease in abundance and the mean size, and some units lack small (<~10 um) zircons suggesting that zircons were both growing and dissolving in the coexisting magma generation areas. Isotope thermometry indicates heating of the system from ~812±35°C to 874±36°C past zircon saturation in 1.8ka eruption. We advocate that a deep vertically continuous and laterally discontinuous silicic magma system at the base of the Taupō rift, rather than a shallow batholith or an evolving mush, drives volcanism at Taupō. To explain the post-Oruanui magma production, rift-base silicic magma origin and moderate (~2 km3/1000years) rhyodacitic magma flux from a growing and heating liquid magma body there creates a sufficient solution for the most recent magmatism.