Storage, evolution, and mixing in basaltic eruptions from around the Okataina Volcanic Centre, Taupō Volcanic Zone, Aotearoa New Zealand

Abstract

The Okataina Volcanic Centre (OVC) is the most recently active caldera system in the Taupō Volcanic Zone, Aotearoa New Zealand. Although best known for its high rates of explosive rhyolitic volcanism, there are several examples of basaltic to basaltic-andesite contributions to OVC eruptions. These range from minor involvement of basalt in rhyolitic eruptions to the exclusively basaltic 1886 C.E. plinian eruption of Tarawera. To explore the basaltic component supplying this dominantly rhyolitic area, we analyse the textures and compositions (minerals and melt inclusions) of four basaltic eruptions from within and around the OVC that have similar whole rock chemistry, namely: Terrace Rd, Rotomakariri, Rotokawau, and Tarawera. Data from these basaltic deposits provide constraints on the conditions of magma evolution and ascent in the crust prior to eruption, revealing that eruptions sample multiple distinct reservoirs during ascent to the surface. The most abundant basaltic component is generated by cooling-induced crystallisation of a common, oxidised, volatile-rich basaltic melt at various depths within the crust that mixes upon ascent. Despite similar bulk compositions, these four eruptions are texturally distinct from each other as a result of their wide variation in eruption style.