Using high-resolution Pb isotopes to unravel the petrogenesis of Sakurajima volcano, Japan

Abstract

AbstractSakurajima volcano on the rim of Aira caldera erupts daily, threatening the major nearby population centre of Kagoshima. Before 1955, eruptions have typically consisted of intermittent Plinian and effusive activity, but since 1955, frequent Vulcanian explosions have occurred, indicating a change in pre-eruptive processes. High-resolution Pb isotopes are used here to constrain the components, including crustal assimilants, and petrogenetic processes contributing to the composition of both pre and post 1955 magmas. Sakurajima eruptive products have more radiogenic Pb (206Pb/204Pb = 18.40–18.42) than nearby Sumiyoshiike basalts (206Pb/204Pb = 18.24), a proxy for the primitive magma feeding the Sakurajima-Aira system. Sakurajima samples lie along a mixing line between these basalts and locally exposed crustal compositions. Their Pb isotopes are consistent with addition of ~ 5% average crustal melt to the primitive basaltic magma. The narrow range of Pb isotope ratios, despite variable SiO2 contents (56.6–72.3 wt%), suggests that the final erupted magmas are derived from fractional crystallization of a mafic precursory magma displaying relatively consistent levels of crustal contamination. Andesites erupted between 4–3.7 ka and the 1995 eruption are contaminated to a greater extent, indicating that magmas with distinct compositions can feed phases of activity or individual eruptions. Post 1955 andesitic pyroclastics have lower SiO2 and higher MgO than older lavas, yet equivalent Pb isotope ratios. The more mafic composition of post 1955 eruptive products can be attributed to increased throughput of mafic magma to the system.