Geochemical insights into magmatic processes and transitioning eruption styles from Devil's Rock (Ngwala), Ambae, Vanuatu

Abstract

Ambae, the largest basaltic volcano within the New Hebrides Arc, Vanuatu, has displayed variable eruptive behaviour through time, with volcanic activity occurring at both the summit and along the island’s prominent rift zone. Rift volcanism on Ambae is characterised by dry strombolian eruptions, typically producing scoria cones until the rift axis terminates at the sea. Here, phreatomagmatic activity is commonplace. Devil’s Rock on the SW coast of Ambae contains several examples of where eruptive activity transitions from strombolian to phreatomagmatic. Deposits from these eruptions contain abundant glassy olivine and pyroxene hosted melt inclusions, and groundmass textures covering a wide spectrum of crystallinities, thus providing excellent material to study fundamental questions regarding transitions between strombolian and phreatomagmatic volcanism, and how magma is generated and modified beneath Ambae. Melt inclusion geochemistry reveals that magmas generated beneath Ambae are derived from the partial melting of garnet lherzolite and/ or amphibole bearing clinopyroxenites at pressures less than 3 GPa. These melts preserve high Ba/Nd, Ba/La and [La/Sm]N ratios consistent with significant subduction, slab- fluid and slab- sediment inputs respectively. The slab- sediment derived component is particularly high relative to melts generated elsewhere along the New Hebrides Arc, probably owing to the position of Ambae opposite the subduction of the D’Entrecasteaux Zone where sediment accumulation is greatest. Strong correlations between major and trace element ratios in melt inclusions to indices of differentiation indicates Ambae melt compositions dominantly evolve through fractional crystallisation. The extent of fractional crystallisation in some cases is too great to be explained by the partial melting of a single parental source. Mineral- melt disequilibrium, mineral zoning and resorbed cores within olivine and pyroxene phenocrysts suggests that mixing between separate magma batches of varying levels of differentiation is primarily responsible for producing the observed trends in mineral and melt compositions. These magmatic processes are occurring within a shallow (0.6- 2.0 km) magma chamber near current sea level. Similarities in S concentrations between Devil’s Rock melt inclusions and groundmass glass with those erupted from the summit between 2017 and 2018 highlight the possibility that this magmatic reservoir is a common source for summit and rift volcanism on Ambae. The transition between strombolian and phreatomagmatic activity at the Devil’s Rock study site is accompanied by minor changes in volatile abundance, however this variation is unlikely to produce such a dramatic change in the nature of magma fragmentation. Instead, it is more likely that this transition is driven by the presence of an external water source and internal magma properties probably only played a minor role. This idea is supported by changes in groundmass crystallinity and porosity between eruption styles which is thought to relate to how slowly magma cooled post- eruption. Similarly, microscopic groundmass and melt inclusion textures show strong modification post- eruption which is attributed to magma cooling rates and volatile degassing on the surface. This has allowed for the identification of at least four separate eruptive phases at the study site, each detailing greater magma- melt interactions through time.