What controlled the explosivity and sulfur release of the largest recent explosive eruption on Earth?

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Intermediate alkaline magmas (e.g. trachyandesite) have been responsible for some of the most explosive and highest sulfur emitting eruptions in the Holocene, in contrast to the rhyolitic magmas that we typically consider as producing Earths largest eruptions (due to their viscous, gas-rich nature). Such trachyandesite magmas are also commonly much more enriched in sulfur compared to their rhyolitic counterparts, and their potential for global climatic impact is thus far greater. A major puzzle with these events is why these alkaline intermediate magmas erupt so explosively, in spite of their low viscosity, and how they can accumulate such high S concentrations. The answer to this apparent inconsistency must lie in either: how the magma is stored, its ascent speed and style, and how the volatiles behave during storage and ascent. To assess this, I will focus on the Plinian 1257 Samalas eruption from Rinjani volcano (Indonesia), which has only recently been associated with the highest sulfur peak in ice core records in the past 2000 years and the consequential Little Ice Age. Phase equilibrium and decompression experiments will be conducted on the products from this particularly explosive eruption to determine its bubble nucleation and degassing behaviour, as well as pre-eruption storage and ascent conditions. Alongside this, sulfur addition experiments will aim to quantify sulfur partitioning between the melt, fluid and mineral phases during pre-eruptive storage and, novelly during ascent. Our proposed experiments will provide a timely and new insight into how sulfur behaves in trachyandesite magmas and thus how eruptions (such as the Samalas) can be capable of such high S fluxes.