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

Joshua R Brown,Masato Iguchi,Rex N Taylor

doi:10.1007/s00445-020-1371-0

Joshua R Brown, Masato Iguchi + Show 1 more

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https://doi.org/10.1007/s00445-020-1371-0

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Abstract

Sakurajima 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.

Highlights

Subduction zone magmas develop by addition of material from the downgoing slab to the mantle wedge (e.g. Armstrong 1971; Hawkesworth et al 1993; Elliott 2004; Straub et al 2010), with the resulting melts interacting with crustal lithologies in the overriding plate before eruption along the arc (e.g. Harmon et al 1984; Hildreth and Moorbath 1988; Davidson et al 2005; Ducea et al 2015)
Arc magmas often show temporal variation in radiogenic isotopic composition, both along arc and at individual volcanic centres, which can be related to changing contributions of mantle, slab and crustal components in their petrogenesis (e.g. Elburg and Foden 1998; Gómez-Tuena et al 2003; Hanyu et al 2006; Labanieh et al 2010; Straub et al 2015; Ishizuka et al 2015)
We utilize basalts erupted in the vicinity of Sakurajima volcano as a proxy for primitive magma compositions feeding the system

Summary

Introduction

Subduction zone magmas develop by addition of material from the downgoing slab to the mantle wedge (e.g. Armstrong 1971; Hawkesworth et al 1993; Elliott 2004; Straub et al 2010), with the resulting melts interacting with crustal lithologies in the overriding plate before eruption along the arc (e.g. Harmon et al 1984; Hildreth and Moorbath 1988; Davidson et al 2005; Ducea et al 2015). Dupré and Allègre 1983; Taylor and Maclennan 1985; Asmerom and Jacobsen 1993; Plank and Langmuir 1998; Workman and Hart 2005; Straub et al 2010), but subtle changes in radiogenic isotopes have the potential to provide new information on the development and dynamics of sub-arc magma reservoirs. Arc magmas often show temporal variation in radiogenic isotopic composition, both along arc and at individual volcanic centres, which can be related to changing contributions of mantle, slab and crustal components in their petrogenesis We utilize high-resolution Pb isotope analysis, complemented by major and trace element data, to probe short timescale changes in petrogenetic processes and magma-crust interaction at Sakurajima-Aira caldera volcano, of the Ryukyu arc in southern Japan

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