Magmatic volatile budgets of the 2014 Tavurvur eruption at Rabaul Caldera, Papua New Guinea

Rabaul is a caldera volcano on New Britain Island, Papua New Guinea, whose active cone Tavurvur ranks seventh globally for long-term SO2 and CO2 emissions. It is unknown why Rabaul is such a strong emitter of volcanic gases. Magma mixing between basaltic and dacitic magmas is envisioned to play a fundamental role in driving eruptions at Rabaul, but the compositions of mafic recharge magmas and their contribution to ongoing gas emissions are poorly constrained. We analysed 87 plagioclase-, clinopyroxene- and olivine-hosted melt inclusions from 2014 Tavurvur volcanic bombs for major, trace and volatile elements (H2O, CO2, SO2, Cl, F), corrected for post-entrapment crystallisation, and combined these data with satellite-derived SO2 retrievals (OMI, IASI) and plume-trajectory modelling. Olivine- and high-Mg# clinopyroxene-hosted melt inclusions record basaltic compositions likely representing the mafic recharge magmas of the volcanic system. Low An plagioclase- and low-Mg# clinopyroxene-hosted melt inclusions are andesitic to dacitic in composition which closely match dacites erupted at Rabaul (Rabaul Pyroclastics) and likely record the composition of magma resident in Rabaul’s shallow plumbing system. Reverse fractional crystallization and trace element constraints imply primary volatile contents of ~1.1–1.9 wt% H2O, 1400–2600 ppm SO2, ~392 ppm Cl and ~59 ppm F, indicating Rabaul magmas are not anomalously volatile-rich compared to those generated in other subduction zones with an intermediate slab thermal parameter. Our analyses of OMI and IASI satellite data yield an SO2 mass of ~17–19 kt for the 2014 eruption, while petrological estimates derived from our melt inclusion analyses give ~21 kt. We attribute the modest 2014 SO2 release to low erupted magma volumes and extensive pre-eruptive degassing of the shallow reservoir between 2006–2010, and note that volatile-rich mafic magma was volumetrically minor in the erupted material. Our results reconcile melt-inclusion and satellite records and constrain the roles of magma mixing and prior degassing in governing Rabaul’s gas output.