Rhyolite-MELTS and the storage and extraction of large-volume crystal-poor rhyolitic melts at the Taupō Volcanic Center: a reply to Wilson et al. (2021)

Abstract

In our recent work on the large-to-super-sized Oruanui and Taupō eruptions, we integrated information from crystal textures and glass compositions to compare the storage, longevity, and eruption of these two systems. Wilson et al. (Contrib Mineral Petrol 175:48, 2021) suggest that there are issues in our data quality, methods, and interpretations that make our results and interpretations moot. We consider these issues further, exploring three main points: the quality of our data, the use of rhyolite-MELTS in modeling magmatic temperatures, and the applicability of the rhyolite-MELTS geobarometer to the Taupō Volcanic Center. New error analysis shows that our glass compositions are robust and that the uncertainty on our rhyolite-MELTS pressures is smaller than we previously stated. We show that rhyolite-MELTS appropriately models the quartz–plagioclase saturation surface as a function of pressure and that temperature estimates from rhyolite-MELTS are a strong function of water activity, which reconciles discrepancies between liquidus temperatures used in our decompression simulations and those from mineral geothermometers without requiring crystallization at lower pressures. We find that phase-equilibria favors the conclusion that both the source and the erupted Taupō magma itself were quartz-saturated. New analysis also adds additional detail to our original conclusions: rhyolite-MELTS pressures determined for a range of $f_{O_2}$ indicate that the erupted Taupō magma was extracted from a source crystallizing plagioclase + orthopyroxene ± quartz over a limited range of pressures (~ 300–350 MPa), resided at a similar pressure for a short duration, and finally erupted from that depth with little-to-no residence at shallow depths. In contrast, results for the Oruanui magma suggest that it was extracted from a similarly relatively oxidized source with a plagioclase + orthopyroxene + quartz phenocryst assemblage, but extraction occurred over a larger range of pressures, and the extracted magma did incur shallow residence. Our analysis reinforces our conviction that our data quality and methods are robust, and our new results support our previous suggestion that the characteristics of the Oruanui system are consistent with a thermally mature crust, while those of the Taupō system resemble those of the early stages of a previous TVZ flare-up.