A Petrologic Insight into Transitioning Eruption Styles from the Devil's Rock Region, Ambae, Vanuatu

Abstract

Ambae Island is the largest volcano in the New Hebrides Arc with recent eruptive activity occurring primarily at the summit and along the island's rift zone. The Devil's Rock area forms a prominent outcrop on the SW coast. Eruptive deposits here are derived from both strombolian and phreatomagmatic eruptions that contain a similar olivine- and clinopyroxene-rich juvenile basaltic component. This study focuses on a particular transition from strombolian to phreatomagmatic activity to understand if the change in eruption style is a function of magmatic processes or properties (e.g. different composition, ascent rate, degassing history) or if it is driven purely by external factors (e.g. magma-water interaction and/or vent migration). Melts from the strombolian to phreatomagmatic phase record the same melt compositions and volatile contents, suggesting the same magma batch is involved throughout the eruption. More broadly, similarities in H2O, CO2 and S concentrations between olivine- and pyroxene-hosted melt inclusions from Devil's Rock melt inclusions and those erupted during the 2017-2018 summit eruptions may indicate that a long-term shared magmatic reservoir exists beneath Ambae. Physical characteristics of juvenile tephra including groundmass crystallinity and porosity are combined with melt inclusion compositions to better understand the degassing and crystallisation history and melt evolution of this volcanic system across the transitioning eruptive sequence. Groundmass crystallisation is variable and negatively correlated with connected porosity of erupted scoria reflecting mixing of materials at the vent and inclusion of dense clasts from conduit margins. A direct comparison of crystallinities between strombolian and phreatomagmatic phases reveals higher crystallinity in the strombolian deposits, which is reflective of post-fragmentation crystallisation of clasts. This is particularly evident in the proximal strombolian materials. Qualitative crystallisation textures of melt inclusions are used in a similar fashion to groundmass crystallinities to assess the relative timing of cooling. These trends mirror those of the groundmass and suggest longer cooling times and more effective degassing for samples of the transitional materials. Based on our analysis of deposits at Devil's Rock, the transition from a strombolian to a phreatomagmatic eruption style was likely driven by groundwater or seawater incursion into the shallow conduit, close to modern-day sea level. Overall, these results suggest a dynamic system where different magmatic cooling histories for strombolian versus phreatomagmatic eruptive phases are reflected in changing groundmass crystallinity. This highlights the propensity for transitions in eruption style over seemingly short time intervals and significantly enhancing eruption explosivity.