Sr isotope evidence for short magma residence time for the 20th century activity at Stromboli volcano, Italy.

Stromboli is the type locality of continuous and moderately explosive volcanic activity. Monitoring temporal variations in the composition of the material erupted allows constraints to be made on the magma chamber dynamics and volcanic hazard. Here we present an Sr isotope survey of scoriae and lavas erupted from Stromboli volcano during this century. The material erupted is transitional between shoshonite and High-K basalt, with relatively constant major and trace element composition. This implies no substantial physical separation of minerals during crystallization (ca. 50 vol% minerals in both lavas and scoriae) and hence a relatively homogeneous reservoir. 87Sr/86Sr values are constant from 1900 to ca. 1980, then, beginning prior to the major lava flow eruption of December 1985, there is a smooth decrease. The Sr isotope decrease records the arrival of a new feeding magma, and allows estimation of the magma residence time and the volume of the reservoir beneath Stromboli volcano. Our results, along with a critical assessment of magma flux estimates, are best reconciled with steady state conditions, and establish the existence of a relatively small, ∼0.3 to 0.04 km3, reservoir with a magma residence time (τ) of ca. 19 years. Monitoring the temporal variation of isotopic ratios in active volcanoes appears a successful tool in forecasting some major volcanic eruptions.