Petrological insights into magma storage and evolution at Rabaul Caldera, Papua New Guinea

Melina Höhn, Margaret E. Hartley, John Dikaung, Ima Itikarai, Kila Mulina, Steve Saunders, Mikhail Sindang, Brendan T. McCormick Kilbride

Rabaul is a caldera volcano in Papua New Guinea. Its most recent caldera-forming eruption occurred ~1400 years ago, with numerous intra-caldera eruptions since. Erupted whole rock compositions are commonly attributed to fractional crystallisation along a single liquid line of descent, but mafic mineral clots indicate mafic recharge and magma mixing also influence whole rock compositions. Geophysical studies identified two low-velocity zones at ~0.5–4 km and 9–15 km beneath the caldera, suggesting a shallow reservoir for evolved magmas and a deeper mafic storage zone. However, uncertainties remain regarding depths, connectivity and temporal evolution of Rabaul’s magma storage zones. We present petrological and thermobarometric analyses of products from the 1937, 1994 and 2014 eruptions at Rabaul. Samples contain two distinct mineral clot types. Mafic mineral clots comprise olivine, high-anorthite plagioclase and high-Mg# clinopyroxene that are out of equilibrium with surrounding matrix glass. Textures include rapid growth rims, olivine replacement by orthopyroxene and reversely zoned clinopyroxene. We interpret these mafic mineral clots as cumulate fragments mobilised from a deep mafic mush by mafic recharge. Intermediate mineral clots contain low-anorthite plagioclase, low-Mg# clinopyroxene, orthopyroxene and Ti-magnetite, and occur as macrocrysts in equilibrium with matrix glasses (Mg# 30–40). We interpret these intermediate mineral clots as fragments from a shallower, more evolved mush stored. Binary mixing models demonstrate that eruption products can be reproduced by mixing a mafic magma similar to 1937 basaltic enclaves with a dacitic magma resembling the last caldera-forming deposit. Clinopyroxene-only and clinopyroxene–liquid thermobarometry yield crystallisation pressures of 0.5–5 kbar (~2–20 km) and temperatures of 960–1125 °C, but the models lack the vertical resolution to resolve storage zones separated by ~5 km. Combining geophysical, geochemical and thermobarometric data we propose a refined model of magma storage beneath Rabaul, in which magmas are stored in a complex, vertically extensive magma plumbing system with a heterogeneous but interconnected mush reservoir.