Relationship between eruption volume and neodymium isotopic composition at Unzen volcano

SILICA-rich lavas, erupted at island-arc or continental volcanoes, are often produced by a complex process involving the assimilation of crust into a crystallizing, mantle-derived basaltic magma1. The different strontium, neodymium and oxygen isotopic compositions of mantle-derived magmas and continental crust provide a powerful method for tracing the different contributions to continental silicic magmas, and for understanding the parameters controlling the composition and volume of erupted magma1–4. In the large rhyolite eruptive centres of the western United States, the largest-volume, explosive rhyolite eruptions have more mantle-like Nd isotope ratios than other silicic lavas from the same centre2–4, a relationship that has been interpreted as reflecting increased influx of mantle-derived basaltic magma to a crustal magma chamber before large-volume eruptions1. Here we report isotope data for lavas from Unzen volcano, which suggest a similar relationship: the Nd isotope composition is more mantle-like in three larger-volume dacite eruptions (>0.1 km3) than in one small-volume (0.02 km3) eruption. We accordingly suggest that, in small-volume systems like Unzen, where the timescales for magma-chamber evolution are of the order of decades, isotope data such as those presented here might be used in volcanic hazard evaluation.