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 cru st into a crystallizing, mantle-derived basaltic magma1. The different strontium, neodymium and oxygen isotopic compositions of mantle-deriv ed magmas and continental crust provide a powerful method for tracing the different contributions to continental silicic magmas, and for und erstanding the parameters controlling the composition and volume of er upted magma1-4. In the large rhyolite eruptive centres of the western United States, the largest-volume, explosive rhyolite eruptions have m ore mantle-like Nd isotope ratios than other silicic lavas from the sa me centre2-4, a relationship that has been interpreted as reflecting i ncreased influx of mantle-derived basaltic magma to a crustal magma ch amber 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 t he timescales for magma-chamber evolution are of the order of decades, isotope data such as those presented here might be used in volcanic h azard evaluation.