A crystal-scale record of recharge and mixing at Rabaul Volcano (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 densely populated caldera volcano in Papua New Guinea that last erupted a caldera-forming volume ~1,400 years ago and has produced numerous intra-caldera eruptions since. Its erupted compositions have commonly been attributed to fractional crystallisation, and although mafic enclaves point to a role for mafic recharge and magma mixing, no study has yet used the crystal cargo itself to test this across multiple eruptions, and the depth, connectivity and evolution of the magma storage system remain poorly resolved. We present the first systematic, multi-eruption crystal-cargo and textural study at Rabaul, characterising the 1937, 1994 and 2014 eruption products (with re-examination of 2006) within a single classification scheme and combine this with whole-rock mixing modelling, machine-learning clinopyroxene thermobarometry and oxybarometry. Two texturally and compositionally distinct types of mineral clot are identified. Mafic mineral clots, of olivine, calcic plagioclase and magnesian clinopyroxene, are fragments of a basaltic crystal mush. Intermediate mineral clots, of more sodic plagioclase, less magnesian clinopyroxene, orthopyroxene and Ti-magnetite, are fragments of a separate, more evolved andesitic–dacitic mush and are described herein for the first time. Mafic-derived crystals within the intermediate clots show that the intermediate mush inherited material from the basaltic mush. The wider crystal cargo records how these mushes interact: mafic crystals transferred into the resident, evolved magma acquired low-anorthite and low-magnesium overgrowth rims, and developed quench-crystallised crystal clusters, olivine-to-orthopyroxene reactions and reverse-zoned recharge bands, all recording mafic recharge and mixing. Our two-endmember mixing model reproduces every post-caldera eruption as a mixture of a resident dacite and a mafic recharge magma. Clinopyroxene thermobarometry yields crystallisation pressures of 200 ± 50 MPa (50–500 MPa) and temperatures of 1050 ± 50 °C (960–1125 °C). Oxybarometry yields ΔQFM ≈ +1, typical of arc magmas. This systematic classification of the crystal cargo across multiple Rabaul eruptions reveals that the volcano is underlain by at least two distinct crystal mushes of basaltic and andesitic–dacitic composition, that exchange crystals and are repeatedly reactivated by mafic recharge, providing a crystal-scale framework for interpreting the compositions and activity of this hazardous caldera.