Short-lived radioactive disequilibria in freshly erupted volcanic rocks and their implications for the preeruption history of a magma

Abstract

Radioactive disequilibria between Ra and Th short- and intermediate-lived isotopes belonging to the 232Th and 238U decay series have been measured in lavas and bombs erupted since 1971 at Etna and Stromboli volcanoes. They indicate that Ra was enriched in relation to Th in the erupted fraction of the magmas by processes of magma formation, differentiation, and eruption. Equations relative to one-stage, twostage, and continuous Ra enrichment were derived. Application of these equations to Etna and Stromboli data indicates either two (or more) stages or an exponentially growing Ra over Th enrichment. All models require a late event of Ra enrichment occurring at, or shortly before, the eruption. By the two–stage model the first major event of Ra enrichment was dated at 30–40 years for both volcanoes. The exponential growth model furnishes the age of the beginning of Ra enrichment within the erupted fraction of the magmas, which is some hundred years for Etna and 500–700 years for Stromboli. Although the volcanological meaning of the obtained dates is still uncertain, it is noteworthy that the same models of magmatic histories apply to two volcanoes which are very different in terms of both chemical composition of magmas and activities.