Control of Magma Plumbing Systems on Long‐Term Eruptive Behavior of Sakurajima Volcano, Japan

We conducted a crystal-size-distribution (CSD) analysis of plagioclase phenocrysts of four historical lavas after the 15th century from Sakurajima volcano, southern Kyushu, Japan. The CSD analyses of type A (lower An content in core) and type B (higher An content in core) phenocrysts show that the slopes and intercepts of the former were nearly constant over five centuries and that those of the latter substantially increase with time. According to the open-system model of CSD under steady-state conditions, the increases in both the slope and intercept indicate that the effective growth rate of type A phenocrysts was nearly constant in a mixed felsic magma chamber (FMC), whereas that of type B phenocrysts systematically decreased in a mafic magma chamber (MMC) over five centuries. The effective growth rate can be connected to the ascent rate, or the supply rate, of magmas from the mantle. We infer that both the supply rate of mantle-derived mafic magma to MMC and the input rate from MMC to FMC increase with time. We found from CSD that this supply rate of mafic magmas correlates with those from geological data (volumes and intervals between eruptions) for large historical eruptions, suggesting that the supply rate from the mantle controls the triggering of eruption through the excess-volume condition.