Temporal variations in storage conditions of the post-collapse rhyolites, Valles Caldera, New Mexico (USA)

Abstract

We present a petrologic study of the post-collapse rhyolites from Valles Caldera (New Mexico, USA) to evaluate changes in magmatic storage conditions following the eruption of the Upper Bandelier Tuff (UBT). We assess likely phenocrysts through textures, comparison with experiments, and rhyolite-MELTS outputs, and apply thermobarometers, oxygen barometers, and hygrometers to these minerals. The post-collapse rhyolites have average pre-eruptive temperatures ranging from 742–824 °C, average pre-eruptive oxygen fugacities ranging from –0.1 to +1.3 ΔNNO, and average pre-eruptive H2O contents ranging from 4.4–6.0 wt% H2O. Average storage pressures range from 130–196 MPa for all rhyolites except the youngest, which has a range of 157–376 MPa. The post-collapse rhyolites are more oxidizing than the UBT and have similar pre-eruptive pressures. The change in oxidation state between the rhyolites and UBT, despite similar storage pressures, can be explained if the UBT is generated from a unique, reduced source but accumulated at depths similar to the post-collapse rhyolites.